The manipulation of neutral particles...The manipulation of neutral particles* Steven Chu Stanford...

The manipulation of neutral particles*

Steven Chu

Stanford University, Departments of Physics and Applied Physics, Stanford,California 94305-4060

[S0034-6861(98)01003-4]

The written version of my lecture is a personal ac-count of the development of laser cooling and trapping.Rather than give a balanced history of the field, I choseto present a personal glimpse of how my colleagues andI created our path of research.

I joined Bell Laboratories in the fall of 1978 afterworking with Eugene Commins as a graduate studentand post-doc at Berkeley on a parity nonconservationexperiment in atomic physics (Conti et al., 1979; see alsoChu, Commins, and Conti, 1977). Bell Labs was a re-searcher’s paradise. Our management supplied us withfunding, shielded us from bureaucracy, and urged us todo the best science possible. The cramped labs and of-fice cubicles forced us to rub shoulders with each other.Animated discussions frequently interrupted seminars,and casual conversations in the cafeteria would some-times mark the beginning of a new collaboration.

In my first years at Bell Labs, I wrote an internalmemo on the prospects for x-ray microscopy andworked on an experiment investigating energy transferin ruby with Hyatt Gibbs and Sam McCall as a means ofstudying Anderson Localization (Chu, Gibbs, McCall,and Passner, 1980; Chu, Gibbs, and McCall, 1981). Thiswork led us to consider the possibility of Mott or Ander-son transitions in other exciton systems such as GaP:Nwith picosecond laser techniques (Hu, Chu, and Gibbs,1980). During this work, I accidentally discovered thatpicosecond pulses propagate with the group velocity,even when the velocity exceeds the speed of light orbecomes negative (Chu and Wong, 1982).

While I was learning about excitons and how to buildpicosecond lasers, I began to work with Allan Mills, theworld’s expert on positrons and positronium. We beganto discuss the possibility of working together while I wasstill at Berkeley, but did not actually begin the experi-ment until 1979. After three long and sometimes frus-trating years, a long time by Bell Labs standards, wefinally succeeded in exciting and measuring the 1S-2Senergy interval in positronium. (Chu and Mills, 1982;Chu, Mills, and Hall, 1984).

MOVING TO HOLMDEL AND WARMING UP TO LASERCOOLING

My entry into the field of laser cooling and trappingwas stimulated by my move from Murray Hill, New Jer-

*The 1997 Nobel Prize in Physics was shared by Steven Chu,Claude N. Cohen-Tannoudji, and William D. Phillips. This lec-ture is the text of Professor Chu’s address on the occasion ofthe award.

Reviews of Modern Physics, Vol. 70, No. 3, July 1998 0034-6861/9

sey, to head the Quantum Electronics Research Depart-ment at the Holmdel branch in the fall of 1983. Duringconversations with Art Ashkin, an office neighbor atHolmdel, I began to learn about his dream to trap atomswith light. He found an increasingly attentive listenerand began to feed me copies of his reprints. That fall Iwas also joined by my new post-doc, Leo Hollberg.When I hired him, I was planning to construct an elec-tron energy-loss spectrometer based on threshold ion-ization of a beam of atoms with a picosecond laser. Wehoped to improve the energy resolution compared toexisting spectrometers by at least an order of magnitudeand then use our spectrometer to study molecular adsor-bates on surfaces with optical resolution and electronsensitivity. However, Leo was trained as an atomicphysicist and was also developing an interest in the pos-sibility of manipulating atoms with light.

Leo and I spontaneously decided to drive to Massa-chusetts to attend a workshop on the trapping of ionsand atoms organized by David Pritchard at MIT. I wasignorant of the subject and lacked the primitive intuitionthat is essential to add something new to a field. As anexample of my profound lack of understanding, I foundmyself wondering about the dispersive nature of the ‘‘di-pole force.’’ The force is attractive when the frequencyof light is tuned below the resonance, repulsive whentuned above the resonance, and vanishes when tuneddirectly on the atomic resonance. Looking back on theseearly fumblings, I am embarrassed by how long it tookme to recognize that the effect can be explained byfreshman physics. On the other hand, I was not alone inmy lack of intuition. When I asked a Bell Labs colleagueabout this effect, he answered, ‘‘Only Jim Gordon reallyunderstands the dipole force!’’

By 1980, the forces that light could exert on matterwere well understood.1 Maxwell’s calculation of the mo-mentum flux density of light (Maxwell, 1897) and thelaboratory observation of light pressure on macroscopicobjects by Lebedev (1901) and by Nichols and Hull(1903) provided the first quantitative understanding ofhow light could exert forces on material objects. Ein-stein (1917) pointed out the quantum nature of thisforce: an atom that absorbs a photon of energy hn willreceive a momentum impulse hn/c along the directionof the incoming photon pin . If the atom emits a photonwith momentum pout , the atom will recoil in the oppo-site direction. Thus the atom experiences a net momen-tum change Dpatom5pin–pout due to this incoherent scat-

1A more complete account of this early history can be foundin Minogin and Letokhov, 1987.

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686 Steven Chu: Manipulation of neutral particles

tering process. In 1930, Frisch (1933) observed thedeflection of an atomic beam with light from a sodiumresonance lamp where the average change in momen-tum was due to the scattering of one photon.

Since the scattered photon has no preferred direction,the net effect is due to the absorbed photons, resultingin scattering force, Fscatt5Npin , where N is the numberof photons scattered per second. Typical scattering ratesfor atoms excited by a laser tuned to a strong resonanceline are on the order of 107 to 108/sec. As an example,the velocity of a sodium atom changes by 3 cm/sec perabsorbed photon. The scattering force can be 105 timesthe gravitation acceleration on Earth, feeble comparedto electromagnetic forces on charged particles, butstronger than any other long-range force that affectsneutral particles.

There is another type of force based on the lensing(i.e., coherent scattering) of photons. A lens alters thedistribution of momentum of a light field, and by New-ton’s third law, the lens must experience a reaction forceequal and opposite to the rate of momentum change onthe light field. For example, a positive lens will be drawntowards regions of high light intensity as shown in Fig.1.2 In the case of an atom, the amount of lensing is cal-culated by adding the amplitude of the incident lightfield with the dipole field generated by the atomic elec-trons driven by the incident field.

2Detailed calculation of lensing in the Mie scattering range(where the wavelength of the light is less than the diameter ofthe particle) can be found in a number of publications. See, forexample, Ashkin, 1992.

FIG. 1. A photograph of a 10-mm glass sphere trapped in wa-ter with green light from an argon laser coming from above.The picture is a fluorescence image taken using a green-blocking, red-transmitting filter. The exiting (refracted) raysshow a notable decrease in beam angles relative to the incidentrays. The increased forward momentum of the light results inan upward force on the glass bead needed to balance thedownward scattering force. The stria in the forward-scatteredlight is a common Mie-scattering ring pattern (courtesy A.Ashkin).

Rev. Mod. Phys., Vol. 70, No. 3, July 1998

This reaction force is also called the ‘‘dipole force.’’The oscillating electric field E of the light induces a di-pole moment p on the particle. If the induced dipolemoment is in phase with E, the interaction energy2p•E is lower in high-field regions. If the induced di-pole moment is out of phase with the driving field, theparticle’s energy is increased in the electric field and theparticle will feel a force ejecting it out of the field. If wemodel the atom or particle as a damped harmonic oscil-lator, the sign change of the dipole force is easy to un-derstand. An oscillator driven below its natural resonantfrequency responds in phase with the driving field, whilean oscillator driven above its natural frequency oscillatesout of phase with the driving force. Exactly on reso-nance, the oscillator is 90 degrees out of phase andp•E50.

The dipole force was first discussed by Askar’yan(1962) in connection with plasmas as well as neutral at-oms. The possibility of trapping atoms with this forcewas considered by Letokhov (1968) who suggested thatatoms might be confined along one dimension in thenodes or antinodes of a standing wave of light tuned farfrom an atomic transition. In 1970, Arthur Ashkin hadsucceeded in trapping micron-sized particles with a pairof opposing, focused beams of laser light, as shown inFig. 2. Confinement along the axial direction was due tothe scattering force: a displacement towards either of thefocal points of the light would result in an imbalance ofscattered light that would push the particle back to thecenter of the trap. Along the radial direction, the out-wardly directed scattering force could be overcome bythe attractive dipole force. In the following years, otherstable particle-trapping geometries were demonstratedby Ashkin (1980), and in 1978 he proposed the firstthree-dimensional traps for atoms (Ashkin, 1978). In thesame year, with John Bjorkholm and Richard Freeman,he demonstrated the dipole force by focusing an atomicbeam using a focused laser beam (Bjorkholm, Freeman,Ashkin, and Pearson, 1978).

Despite this progress, experimental work at Bell labsstopped a year later because of two major obstacles totrapping. First, the trapping forces generated by intensefocused laser beams are feeble. Atoms at room tempera-ture would have an average energy 3

2 kBT; 12mv2, orders

of magnitude greater than could be confined by the pro-posed traps. A cold source of atoms with sufficientlyhigh flux did not exist and a trap with large volume was

FIG. 2. A schematic diagram of the first particle trap used byAshkin. Confinement in the axial direction is due to an imbal-ance of the scattering forces between the left and right propa-gating beams. Confinement in the radial direction results fromthe induced dipole force, which must overcome the outwardlydirected scattering force.

687Steven Chu: Manipulation of neutral particles

needed to maximize the number of atoms that could betrapped. Second, the relatively large-volume optical trapmade from opposing light beams was found to have se-rious heating problems. An atom could absorb a photonfrom one beam and be stimulated back to the initialstate by a photon from the opposing beam. In this pro-cess, it would receive two photon impulses in the samedirection. However, the same atom could have been ex-cited and stimulated by the two beams in the reverseorder, resulting in a net impulse in the other direction.Since the order of absorption and stimulated emission israndom, this process would increase the random velocityof the atom and they would quickly heat and boil out ofthe trap. This heating effect was rigorously calculated byJim Gordon with Ashkin for a two-level atom (Gordonand Ashkin, 1980).

TAKING THE PLUNGE INTO THE COLD

My first idea to solve the trap loading problem wasmodest but it got me to think seriously about trappingatoms. I proposed to make a cold source of atoms bydepositing sodium atoms into a rare-gas matrix of neon(Chu, 1984). By heating the cryogenic surface support-ing this matrix of atoms with a pulsed laser, I thought itshould be possible to ‘‘puff’’ the neon and sodium atomsinto a vapor with a temperature of a few tens of kelvin.Once a vapor, a reasonable fraction of the sodium wouldbecome isolated atoms and the puffed source wouldcontain the full Maxwell-Boltzmann distribution of at-oms, including the very slowest atoms. In a conventionalatomic beam, the slowest atoms are knocked out of theway by faster moving atoms overtaking them. In apuffed source, the surface could be quickly heated andcooled so that there would be no fast atoms comingfrom behind. An added advantage was that the sourcewould turn off quickly and completely, so that the de-tection of even a few trapped atoms would be possible.

Soon after my passage from interested bystander toparticipant, I realized that the route to trapping wasthrough laser cooling with counter-propagating beamsof light. If the laser beams were tuned below the atomicresonance, a moving atom would Doppler-shift thebeam opposing its motion closer to resonance and shiftthe beam co-propagating with the motion away fromresonance. Thus, after averaging over many impulses ofmomentum from both beams, the atom would experi-ence a net force opposing its motion. In the limit wherethe atoms were moving slowly enough so that the differ-ence in the absorption due to the Doppler effect waslinearly proportional to the velocity, this force wouldresult in viscous damping, F52av. This elegant ideawas proposed by Hansch and Schawlow in 1975 (Hanschand Schawlow, 1975). A related cooling scheme was pro-posed by Wineland and Dehmelt in the same year(Wineland and Itano, 1975).

An estimate of the equilibrium temperature is ob-tained by equating the cooling rate in the absence ofheating with the heating rate in the absence of cooling,dWheating /dt5dWcooling /dt52F•v. The heating rate is


due to the random kicks an atom receives by randomlyscattering photons from counterpropagating beams thatsurround the atoms (Wineland and Itano, 1979; Gordonand Ashkin, 1980). The momentum grows as a randomwalk in momentum space, so the average random mo-mentum p would increase as

dWheating

dt5

d

dt S p2

2M D5N~pr!

2

2M,

where pr is the momentum recoil due to each photonand N is the number of photon kicks per second. Byequating the heating rate to the cooling rate, one cancalculate an equilibrium temperature as a function ofthe laser intensity, the linewidth of the transition, andthe detuning of the laser from resonance. The minimumequilibrium temperature kBTmin5\G/2, where G is thelinewidth of the transition, was predicted to occur at lowintensities and a detuning Dn5G/2 where the Dopplershift asymmetry was a maximum. In the limit of lowintensity, all of the laser beams would act independentlyand the heating complications that would result fromstimulated transitions between opposing laser beamscould be ignored.

Not only would the light cool the atoms, it would alsoconfine them. The laser cooling scheme was analogousto the Brownian motion of a dust particle immersed inwater. The particle experiences a viscous drag force andthe confinement time in a region of space could be esti-mated based on another result in elementary physics:the mean square displacement ^x2& after a time Dt de-scribed by a random walk, ^x2&52Dt , where the diffu-sion constant is given by the Einstein relation D5kBT/a . For atoms moving with velocities v such thatk•v,G , the force would act as a viscous damping forceF52av . By surrounding the atoms with six beamspropagating along the 6x , y, and z directions, we couldconstruct a sea of photons that would act like an excep-tionally viscous fluid: an ‘‘optical molasses.’’ 3 If the lightintensity was kept low, the atoms would quickly cool totemperatures approaching Tmin . Once cooled, theywould remain confined in a centimeter region of spacefor times as long as a fraction of a second.

At this point, Leo and I shelved our plans to build theelectron spectrometer and devoted our energies to mak-ing the optical molasses work. We rapidly constructedthe puffing source of sodium needed to load the opticalmolasses. To simplify matters, we began with a pellet ofsodium heated at room temperature. Rather than dealwith the complications of a rare-gas matrix, Leo and Idecided to increase the number of cold atoms by slowingatoms from the puff source before attempting the opti-cal molasses experiment. There were several early ex-

3I had wanted our paper to be titled ‘‘Demonstration of op-tical molasses.’’ John Bjorkholm was a purist and felt that thephrase was specialized jargon at its worst. We compromisedand omitted the phrase from the title but introduced it in thetext of the paper.


periments that slowed atomic beams with laser light,4

but sodium atoms had to be slowed to velocities on theorder of 200–300 cm/sec (essentially stopped!) before anatom trap could be loaded. Two groups achieved thismilestone in late 1984: a group at the National Bureau ofStandards in Gaithersburg, Maryland, led by Bill Phillipsusing a tapered magnetic field (Prodan et al., 1985) andanother NBS group in Boulder, Colorado, led by JanHall (Ertmer et al., 1985). We decided to copy the tech-nique of Ertmer et al. (1985), and use an electro-opticgenerator to produce a frequency-shifted sideband. Thefrequency-shifted light is directed against the atomscoming off the sodium surface, and as the atoms slow,the frequency is changed in order to keep the light inresonance with the Doppler shifting atoms.

Leo was better at electronics than I and assumed theresponsibility of the radio-frequency part of the project,while I set out to build a wideband, transmission lineelectro-optic modulator. One of the advantages of work-ing at Bell Laboratories was that one could often find aneeded expert consultant within the Labs. Much of theelectro-optic modulator development was pioneered atthe Labs in Holmdel in the 1960s and we were still theleaders of the field in 1983. I learned about makingelectro-optic modulators by reading the book written bya colleague, Ivan Kaminow (1974). I enlisted Larry Buhlto cut and polish the LiTaO3 crystal for the modulator.Rod Alferness taught me about microwave impedancematching and provided the SMA ‘‘launchers’’ needed tomatch Leo’s electronics with my parallel-plate transmis-sion line modulator. One month after we decided to pre-cool the atoms with a frequency-swept laser beam, wehad a functioning, wideband gigahertz electro-opticmodulator and driver and could begin to precool theatoms from our puffing source.

In the early spring of 1984, Leo and I started with acompletely bare optical table, no vacuum chamber, andno modulator. Later that spring, John Bjorkholm, whohad previously demonstrated the dipole force by focus-ing an atomic beam, joined our experiment. In the earlysummer, I recruited Alex Cable, a fresh graduate fromRutgers. Officially he was hired as my ‘‘technician’’: un-officially, he became a super-graduate student. In lessthan one year, we submitted our optical molasses paper(Chu, Holberg, et al. 1985).5 The two papers reportingthe stopping of atomic beams (Ertmer et al., 1985;Prodan et al., 1985) were published one month earlier.

The apparatus we built to demonstrate optical molas-ses is shown in Figs. 3(a) and 3(b). We had an ultrahigh-vacuum chamber, but did not want to be hampered by

4For a comprehensive discussion of the work up to 1985, seePhillips et al. (1985).

5The components of the experiment were assembled fromparts of previous experiments: the cw dye laser needed for theoptical molasses and the pulsed YAG laser were previouslyused in a dye laser oscillator/amplifier system in a positroniumspectroscopy experiment. A surplus vacuum chamber in a de-velopment section of Bell Laboratories became our molasseschamber.


long bake-out times to achieve good vacuum. Instead,we built a cryo-shield painted with Aquadag, a graphite-based substance. When cooled to liquid-nitrogen tem-peratures, the shield became a very effective sorptionpump: we could open the vacuum chamber one day andbe running by the next day. Fast turnaround time hasalways been important to me. Mistakes are unavoidable,so I always wanted an apparatus that would allow mis-takes to be corrected as rapidly as possible.

The first signals of atoms confined in optical molassesshowed confinement times of a few tens of milliseconds,but shortly afterwards we improved the storage time byover an order of magnitude. Surprisingly, it took us aweek after achieving molasses to look inside the vacuumcan with our eyes instead of with a photomultiplier tube.When we finally did, we were rewarded with the sightshown in Fig. 4.

In this early work, the laser beams were aligned to beas closely counter-propagating as we could manage. Ayear later, we stumbled onto a misalignment configura-tion that produced another order of magnitude increasein the storage time. This so-called ‘‘super-molasses’’alignment of our beams also created a compression ofthe atoms into a region of space on the order of 2 mmdiameter from an initial spread of 1 cm. We were neverable to understand this phenomenon and, after a num-ber of attempts, published a brief summary of these re-sults in conference proceedings (Chu, Prentiss, et al.,1988; Shevy, Weiss, and Chu, 1989).

In our first molasses work, we realized that the tradi-tional method of measuring the temperature by measur-ing the Doppler broadening of an atomic resonance linewould not work for the low temperatures we hoped toachieve. Instead we introduced a time-of-flight tech-nique to directly measure the velocity distribution of theatoms. After allowing the atoms in the molasses to cometo equilibrium, we turned off the light for a variableamount of time. The fast atoms escaped ballistically dur-ing this time while the slower atoms were recaptured bythe molasses. This method allowed us to directly mea-sure the velocity distribution. Our first measurementsshowed a temperature of 185 mK, slightly lower than theminimum temperature allowed by the theory of Dopplercooling. We then made the cardinal mistake of experi-mental physics: instead of listening to Nature, we wereoverly influenced by theoretical expectations. By includ-ing a fudge factor to account for the way atoms filled themolasses region, we were able to bring our measurementinto accord with our expectations.

ON TO OPTICAL TRAPPING

Once we demonstrated optical molasses, we began toexplore ways to achieve our original goal of opticallytrapping atoms. As a point of reference, Bill Phillips andhis collaborators had reported the magnetic trapping ofsodium atoms (Migdal et al., 1985) two weeks before ouroptical molasses paper came out. Although the 1/e stor-age of the molasses confinement in our first experiment


FIG. 3. (a) (Color) A photograph of the apparatus used to demonstrate optical molasses and the first optical trap for atoms. Thephotograph is a double exposure made by photographing the apparatus under normal lighting conditions and then photographingthe laser beams by moving a white card along the beam path in a darkened room. The 10-Hz pulsed laser used to evaporate thesodium pellet (doubled YAG at 532 nm) appears as dots of light. (b) Art Ashkin and the author in front of the apparatus in 1986,shortly after the first optical trapping experiment was completed.



FIG. 4. (Color) A photograph of sodium atoms confined in our optical molasses experiment. The atoms were precooled by acounterpropagating laser before entering the region of six crossed laser beams.

was a respectable t;0.36 sec, optical molasses does notprovide a restoring force that would push the atoms tothe center of the trap.

Despite the fact that we were in possession of a greatsource of cold atoms, the path to trapping was not yetclear to us for a number of reasons: (i) Optical trapsbased strictly on the scattering force seemed to be ruledout because of a no-trapping theorem referred to as the‘‘Optical Earnshaw theorem.’’ This theorem was pub-lished in response to earlier proposals to make atomtraps based on the scattering force (Ashkin and Gordon,1983).6 (ii) We believed a trap based on an opposing-beam geometry was not viable because of the severestimulated heating effects. (iii) Finally, we ruled out asingle focused laser beam because of the tiny trappingvolume. We were wrong on all counts.

Immediately after the molasses experiment, we triedto implement a large-volume ac light trap suggested byAshkin (1984). Our attempt failed and, after a fewmonths, we began to cast around for other alternatives.One possibility was another type of ac trap we proposedat a conference talk in December of 1984 (Chu,Bjorkholm, Ashkin, Hollberg, and Cable, 1985), but wewanted something simpler. Sometime in the winter of1986, during one of our brainstorming sessions on what

6A summary of this theorem and other ‘‘no-trapping’’ theo-rems can be found in Chu, 1992.


to do next, John Bjorkholm tried to resurrect the singlefocused-beam trap first proposed in Ashkin’s 1978 paper(Ashkin, 1978). I promptly rejected the idea because ofthe small trapping volume. A ;1-watt laser focused toproduce a ;5-mK-deep trap would have a trapping vol-ume of ;1027 cm3. Since the density of atoms in ouroptical molasses was 106 atoms/cm3, we would capturefewer than one atom in a trap surrounded by 106 atomsin molasses. A day or two after convincing the groupthat a trap based on a focused laser beam would notwork, I realized that many more atoms would be cap-tured by the trap than my original estimate. An atomclose to the trap might not be immediately captured, butit would have repeated opportunities to fall into the trapduring its random walk in optical molasses.

The trap worked. We could actually see the randomwalk loading with our own eyes. A tiny dot of light grewin brightness as more atoms fell into the trap. During thefirst days of trapping success, I ran up and down thehalls, pulling people into our lab to share in the excite-ment. My director, Chuck Shank, showed polite enthu-siasm, but I was not sure he actually picked out the sig-nal from the reflections in the vacuum can windows andthe surrounding fluorescence. Art Ashkin came downwith the flu shortly after our initial success. He con-fessed to me later that he began to have doubts: as helay in bed with a fever; he wasn’t sure whether the fevercaused him to imagine we had a working trap.


We tried to image the tiny speck of light onto an ap-ertured photomultiplier tube, but the slightest misalign-ment would include too much light from the surround-ing molasses. It was a frustrating experience not to beable to produce a repeatable signal on a photomultipliertube if we could actually see the atoms with our eyes.Then it dawned on me: if we could see the signal withour eyes, we could record it with a sensitive video cam-era and then analyze the video tape! A local RCA rep-resentative, tickled by the experiment, loaned us asilicon-intensified video camera. Our trapping paper in-cluded a still photo of our trapped atoms, the first colorpicture published in Physical Review Letters (Chu,Bjorkholm, Ashkin, and Cable, 1986).

As we began the atom trapping, Art decided to trapmicron-sized particles of glass in a single focused beamas a ‘‘proof of principle’’ for the atom trap. Instead of anatom in optical molasses, he substituted a silica (glass)sphere embedded in water. A micron-sized sphere is farmore polarizable than an atom and Ashkin felt that itcould be trapped at room temperature if the intensitygradient in the axial direction that would draw the glassbead into the focus of the light could overcome the scat-tering force pushing the particle out of the trap. Thismore macroscopic version of the optical tweezers trapwas demonstrated quickly and gave us more confidencethat the atom trap might work (Ashkin, Dziedzic,Bjorkholm, and Chu, 1986). At that time, none of usrealized how this simple ‘‘toy experiment’’ was going toflower.

Shortly after we demonstrated the optical trap, I hiredMara Prentiss as a new permanent staff member in mydepartment. She began to work on the super-molassesriddle with us when I got a phone call from Dave Prit-chard at MIT. He told me he and his student, Eric Raab,had been working on a scattering-force trap that wouldcircumvent the Optical Earnshaw theorem (Ashkin andGordon, 1983). This theorem states that a scattering-force trap is impossible provided the scattering forceFscatt is proportional to the laser intensity I. The proofwas straightforward: ¹•Fscatt50, since any region inempty space must have the net intensity flux inwardequal to the flux outward. Thus there cannot be a regionin space where all force lines Fscatt point inward to astable trapping point. Pritchard, Carl Wieman, and theircolleagues had noted that the assumption Fscatt}I neednot be true (Pritchard, Raab, et al., 1986). They went onto suggest possible combinations of external magnetic orelectric fields that could be used to create a stable opti-cal trap.

Raab had had difficulties in getting a scattering-forcetrap to work at MIT and, as a last attempt before givingup, they asked if we were interested in collaboratingwith them on this work. The basic idea is illustrated inFig. 5 for the case of an atom with F51 in the groundstate and F52 in the excited state, where F is the totalangular momentum quantum number. A weak sphericalquadrupole trap magnetic field would split the Zeemansublevels of a multilevel atom illuminated by counter-propagating circularly polarized laser beams. Due to the


slight Zeeman-shift, an atom to the right of the trap cen-ter optically pump predominant into the mF521 state.Once in this state, the large difference in the scatteringrates for s2 light and s1 light causes the atom to expe-rience a net scattering force towards the trap center. At-oms to the left of center would scatter more photonsfrom the s1 beam. Since the laser beams remain tunedbelow all of the Zeeman split resonance lines, opticalmolasses cooling would still be occurring. The generali-zation to three dimensions is straightforward.

All we needed to do to test this idea was insert a pairof modest magnetic-field coils into our apparatus. Iwound some refrigeration tubing for the magnetic-fieldcoils, but had to tear myself away to honor a previouscommitment to help set up a muonium spectroscopy ex-periment with Allan Mills, Ken Nagamine, andcollaborators.7 A few days later, the molasses was run-ning again, and I received a call at the muon facility inJapan from Alex, his voice trembling with excitement.The trap worked spectacularly well and the atom cloudwas blindingly bright compared to our dipole trap. In-stead of the measly 1000 atoms we had in our first trap,they were getting 107 to 108 atoms (Raab et al., 1987).

The basic idea for the trap was due to Jean Dalibard,a protege of Claude Cohen-Tannoudji. His idea wasstimulated by a talk given by Dave Pritchard on how theEarnshaw theorem could be circumvented. I called Jeanin Paris to convince him that his name should go on thepaper we were writing. Jean is both brilliant and modest,and felt it would be inappropriate to be a co-authorsince he did not do any of the work.8

The magneto-optic trap (commonly referred to as theMOT) immediately seized the attention of the growingcommunity of coolers and trappers. Carl Wieman’sgroup showed that atoms could be directly loaded froma tenuous vapor without the intermediate step of slow-ing an atomic beam (Monroe et al., 1990).9 By increasingthe size of the laser beams used in the trap, Kurt Gibbleand I showed that as many as ;431010 atoms could betrapped (Gibble, Kasapi, and Chu, 1992). Wolfgang Ket-terle, Pritchard, et al. (1993) showed that the density ofatoms in the MOT could be increased significantly bycausing atoms to scatter less light in the central portionof the trap by blocking the repumping beam in thatregion of space. Stimulated by their shadowing idea,my collaborators and I at Stanford showed that bysimply turning down the repumping light at the

7Allan Mills had persuaded me to participate in a muoniumspectroscopy experiment and had been working on that experi-ment in parallel with the laser cooling and trapping work since1985.

8I gave a brief history of these events in Chu, 1992. See alsoPritchard and Ketterle, 1992.

9Raab and Pritchard had tried to get the scattering-force trapto work at MIT by trying to capture the atoms directly fromvapor, but the vapor pressure turned out to be too high forefficient capture.


FIG. 5. The magneto-optic trap for atoms: (a) an F52 ground state and an F53 excited state. The slight energy-level shifts of theZeeman sublevels cause symmetry to be broken and the atoms to optically pump predominantly into either the mF512 or themF522 state for B,0 or B.0. Once in the optically pumped states, the atoms are pushed towards the B50 region due to thelarge difference in relative strengths of the transition rates. The relative transition rates for s1 and s2 light for the mF522 andm521 states are shown; (b) A photograph of atoms confined in a magneto-optic trap. The line of fluorescence below the ball oftrapped atoms is due to the atomic beam used to load the trap.

end stage of molasses dramatically increases the densityof low-temperature atoms in the MOT (Lee, Adams,Kasevich, and Chu, 1996).

The invention and development of the MOTexemplifies how the field of laser cooling and trapping


grew out of the combined ideas and cooperation ofan international set of scientists. For this reason, I findit especially fitting that the magneto-optic trap is thestarting point of most experiments using laser-cooledatoms.


OPTICAL MOLASSES REVISITED

In the winter of 1987, I decided to leave the ‘‘ivorytower’’ of Bell Labs and accept an offer to become aprofessor at Stanford. When I left Bell Labs, we had justdemonstrated the magneto-optic trap and it was clearthe trap would provide an ideal starting point for a num-ber of experiments. I arrived at Stanford in the fall of1987, not knowing how long it would take to build a newresearch team.10 Bill Phillips and Claude Cohen-Tannoudji were assembling powerful teams of scientiststhat could not be duplicated at Stanford. Dave Pritchardhad been cultivating a powerful group at MIT. Other‘‘home run hitters’’ in atomic physics such as Carl Wie-man and Alain Aspect had just entered the field. Mean-while, I had to start over again, writing proposals andmeeting with prospective graduate students. If I hadthought carefully about starting a new lab in the face ofthis competition, I might not have moved.

As with many aspects of my career, I may not havemade the smart move, but I made a lucky move. From1988 to 1993, I entered into the most productive time inmy scientific career to date. My first three graduate stu-dents were Mark Kasevich, Dave Weiss, and Mike Fee. Ialso had two post-docs, Yaakov Shevy and Erling Riis,who joined my group my first year at Stanford. By Janu-ary of 1988, Dave and Yaakov had a magneto-optic trapgoing in the original chamber we used to demonstrateoptical molasses and the dipole trap. The plan was toimprove the optical trapping techniques and then usethe new laser cooling and trapping technology to ex-plore new physics that could be accessed with cold at-oms.

Another chamber was being assembled by Mark andErling with the intent of studying the ‘‘quantum reflec-tion’’ of atoms from cold surfaces. While I was at BellLabs, Allan Mills and Phil Platzman began to get meinterested in studying quantum reflection with ultracoldatoms. The problem can be simply posed as follows: con-sider an atom with a long de Broglie wavelength l inci-dent on an idealized, short-range, attractive potential. Ingeneral there is a transmitted wave and a reflected wave,but in the limit where l is much greater than the lengthscale of the potential, one gets the counterintuitive re-sult that the probability of reflection goes to unity. Areal surface potential has a power-law attraction of theform 1/zn and has no length scale. An atom near a sur-face will experience an attractive van der Waals forcewith a 1/z3 potential and, further away, the attractivepotential would become 1/z4 due to ‘‘retarded poten-tial’’ effects first discussed by Casimir. There are also

10I tried to persuade Alex Cable to come with me and be-come my first graduate student. By this time he was blossom-ing into a first-rate researcher and I knew he would do well asa graduate student at Stanford. He turned down my offer anda year later resigned from Bell Labs. While he was my techni-cian, he had started a company making optical mounts. Hiscompany, ‘‘Thor Labs,’’ is now a major supplier of opticalcomponents.


subtleties to the problem when inelastic scattering chan-nels are included. This problem had attracted the atten-tion of a considerable number of theorists and experi-mentalists.

My plan of research was soon tossed out the windowby a discovery that sent shock waves through the lasercooling community. By 1987, other groups began to pro-duce optical molasses in their labs and measured atomtemperatures near the expected limit (Sesko et al., 1988;Phillips, private communication), but in the spring of1988, Bill Phillips and co-workers reported that sodiumatoms in optical molasses could be cooled to tempera-tures far below the limit predicted by theory. The NISTgroup reported the temperature of sodium atoms cooledin optical molasses to be 43620 mK and that the tem-perature did not follow the frequency dependence pre-dicted by theory (Lett et al., 1988). The result was sosurprising, they performed three different time-of-flightmethods to confirm their result. Within a few months,three separate groups led by Wieman, Cohen-Tannoudji, and myself verified that sodium and cesiumatoms in optical molasses could be cooled well below theDoppler limit.

As with many big ‘‘surprises,’’ there were earlier hintsthat something was amiss. My group had been discussingthe ‘‘super-molasses’’ problem at conferences since1986. At a laser spectroscopy conference in Are, Swedenin 1987, the NIST group reported molasses lifetimeswith a very different frequency dependence from theone predicted by the simple formula ^x2&52Dt/a2 thatwe published in our first molasses paper (Gould et al.,1987). This group also found that the trap was morestable to beam imbalance than had been expected. Inour collective euphoria over cooling and trapping atoms,the research community had not performed the basictests to measure the properties of optical molasses, and Iwas the most guilty.

At the end of June, 1988, Claude and I attended aconference on spin-polarized quantum systems inTorino, Italy, and I gave a summary of the new surprisesin laser cooling known at that time (Shevy, Weiss, andChu, 1989). After our talks, Claude and I had lunch andcompared the findings in our labs. The theory that pre-dicted the minimum temperature for two-level atomswas beyond reproach. We felt the lower temperaturesmust be due to the fact that the atoms we were playingwith were real atoms with Zeeman sublevels and hyper-fine splittings. Our hunch was that the cooling mecha-nism probably had something to do with the Zeemansublevel structure and not the hyperfine structure, sincecesium (Dvhfs59.19 GHz) and sodium (Dvhfs51.77 GHz) were both cooled to temperatures corre-sponding to an rms velocity on the order 4 to 5 times therecoil velocity, vrecoil5\k/M and k 5 2p/l . By then wealso knew that the magnetic field had to be reduced tobelow 0.05 gauss to achieve the best cooling.

After the conference, Claude returned to Paris while Iwas scheduled to give several more talks in Europe. Mynext stop was Munich, where I told Ted Hansch that Ithought it had to be an optical pumping effect. My


knowledge of optical pumping was rudimentary, so Ispent half a day in the local physics library readingabout the subject. I was getting increasingly discouragedwhen I came across an article that referred to ‘‘Cohen-Tannoudji states.’’ It was beginning to dawn on me thatClaude and Jean were better positioned to figure outthis puzzle.

After Munich, I went to Pisa where I gave a talkabout our positronium and muonium spectroscopy work(Chu, 1989; Fee, Mills, et al., 1993; Fee, Chu, et al., 1993;Chu, Mills, Yodh, et al., 1988). There I finally realizedhow molasses was cooling the atoms. The idea wasstimulated by a intuitive remark made by one of thespeakers during his talk, ‘‘ . . . the atomic polarizationresponds in the direction of the driving light field . . . ’’.The comment reminded me of a ball-and-stick model ofan atom as an electron (cloud) tethered by a weaklydamped harmonic force to a heavy nucleus. I realizedthe cooling was due to a combination of optical pump-ing, light shifts, and the fact that the polarization in op-tical molasses changed at different points in space. Alinearly polarized laser field drives the atomic cloud upand down, while a circularly polarized field drives thecloud in a circle. In optical molasses, the x, y, and z-directed beams all have mutually perpendicular linearpolarizations and the polarization of the light field variesfrom place to place. As a simple example, consider aone-dimensional case where two opposing beams oflight have mutually perpendicular linear polarizations asshown in Fig. 6. The electron cloud wants to rotate withan elliptical helicity that is dependent on the atom’s po-sition in space.

Another effect to consider is the ac Stark (light) shift.In the presence of light, the energy levels of an atom areshifted, and the amount of the shift is proportional tothe coupling strength of the light. Suppose an atom withangular momentum F52 is in s1 light tuned belowresonance. It will optically pump into the mF512 stateand its internal energy in the field will be lowered asshown in Fig. 6(c). If it then moves into a region in spacewhere the light is s2, the transition probability is veryweak and consequently the ac Stark shift is small. Forsodium the mF512 energy is lowered 15 times more ins1 light than in s2 light. Hence, the atom gains internalenergy by moving into the s2 region. This increase ininternal energy must come at the expense of its kineticenergy. The final point is that the atom in the new re-gion in space will optically pump into the mF522 state[Fig. 6(d)]. Thus the atom will find itself again in a low-energy state due to the optical pumping process. Theensemble of atoms loses energy, since the spontaneouslyemitted photons are slightly blueshifted with respect tothe incident photons.

Cooling in polarization gradients is related to a cool-ing mechanism that occurs for two-level atoms in thepresence of two counter-propagating laser beams. In thelow-intensity limit, the force is described by our intuitivenotion of scattering from two independent beams oflight, as first discussed by Hansch and Schawlow (1975).However, at high intensities, the force reverses sign so


that one obtains a cooling force for positive detuning.This cooling force has been treated by Gordon and Ash-kin for all levels of intensity in the low-velocity limit(Gordon and Ashkin, 1980), and by Minogin and Seri-maa (1979; also see Minogin, 1981) in the high-intensitylimit for all velocities. A physical interpretation of thecooling force in the high-intensity limit based on thedressed-atom description was given Dalibard andCohen-Tannoudji (1985). In their treatment, the atomgains internal energy at the expense of kinetic energy asit moves in the standing-wave light field. The gain ininternal energy is dissipated by spontaneous emission,which is more likely to occur when the atom is at themaximum internal energy. When the atom makes a tran-sition, it will find itself most often at the bottom of thedressed-state potential hills. Following Albert Camus,Jean and Claude again revived Le Mythe de Sisyphe bynaming this form of cooling after the character in Greekmythology, Sisyphus, who was condemned to eternallyroll a boulder up a hill.

FIG. 6. Polarization-gradient cooling for an atom with an F51/2 ground state and an F53/2 excited state. (a) The inter-ference of two linearly polarized beams of light with orthogo-nal polarizations creates a field of varying elliptical polariza-tion as shown. Under weak excitation, the atom spends most ofits time in the ground states. (b) The energy of the mF561/2 ground states as a function of position in the laser field isshown. (c) An atom in a s1 field will optically pump mostlyinto the mF511/2 ground state, the lower internal energystate. (d) If the atom moves into a region of space where thelight is s2, its internal energy increases due to the decreasedlight shift (ac Stark shift) of the mF511/2 state in that field.The atom slows down as it goes up a potential hill created bythe energy-level shift. As the atom nears the top of the hill, itbegins to optically pump to the mF521/2 state, putting theatom into the lowest-energy state again. Laser cooling by re-peated climbing of potential hills has been dubbed ‘‘Sisyphuscooling’’ after the character in the Greek myth condemnedforever to roll a boulder up a hill.


The name ‘‘optical molasses’’ takes on a more pro-found meaning with this new form of cooling. Originally,I conceived of the name thinking of a viscous fluid asso-ciated with cold temperatures: ‘‘slow as molasses inJanuary.’’ With this new understanding, we now knowthat cooling in optical molasses has two parts: at highspeeds, the atom feels a viscous drag force, but at lowerspeeds where the Doppler shift becomes negligible, theoptical pumping effect takes over. An atom sees itselfwalking in a swamp of molasses, with each planted footsinking down into a lower energy state. The next steprequires energy to lift the other foot up and out of theswamp, and with each sinking step, energy is drainedfrom the atom.

The Pisa conference ended on Friday, and on Sunday,I went to Paris to attend the International Conferenceon Atomic Physics. That Sunday afternoon, Jean and Imet and compared notes. It was immediately obviousthat the cooling models that Jean and Claude and I con-cocted were the same. Jean, already scheduled to give atalk at the conference, gave a summary of their model(Dalibard, Solomon, et al., 1989). I was generously givena ‘‘post-post-deadline’’ slot in order to give my accountof the new cooling mechanism (Chu, Weiss, et al., 1989).

Detailed accounts of laser cooling in light fields withpolarization gradients followed a year later in a specialissue of JOSA B dedicated to cooling and trapping.Dalibard and Cohen-Tannoudji (1989) provided an el-egant quantum-mechanical treatment of simple modelsystems. They discussed two different types of cooling,depending on whether the counter-propagating lightbeams were comprised of mutually perpendicular linearpolarizations or opposing s1 –s2 beams. Their ap-proach allowed them to derive the cooling force anddiffusion of momentum (heating) as a function of ex-perimental parameters such as detuning, atomic line-width, optical pumping time, etc., that could be experi-mentally tested.

My graduate students and I presented our version ofthe Sisyphus cooling mechanism in the same issue (Un-gar et al., 1989). In order to obtain quantitative calcula-tions that we could compare to our experimental results,we chose to calculate the cooling forces using the opticalBloch equations generalized for the sodium F52ground state→F53 excited state transition. We derivedthe steady-state cooling forces as a function of atomicvelocity for the same two simple polarization configura-tions considered by Dalibard and Cohen-Tannoudji, butfor sodium atoms instead of a model system. However,we showed that steady-state forces cannot be used toestimate the velocity distribution and that the transientresponse of the atom in molasses with polarization gra-dients was significant.11 A weak point of our paper wasthat we made ad hoc assumptions in our treatment ofthe diffusion of momentum, and the predicted MonteCarlo calculations of the velocity distributions were sen-

11See Fig. 13 in Dalibard and Cohen-Tannoudji, 1989.


sitive to the details of these assumptions. Since thattime, more rigorous quantum Monte Carlo methodshave been developed.

In a companion experimental paper (Weiss, Riis,Shevy, et al., 1989) we measured nonthermal velocitydistributions of atoms cooled in the laser fields treated inour theory paper. We also measured the velocity distri-butions of atoms cooled in s1 –s1 light.12 Under theseconditions, sodium atoms will optically pump into an ef-fective two-level system consisting of the 3S1/2 , F52,mF512 and 3P3/2 , F53, mF513 states. This arrange-ment allowed us finally to verify the predicted frequencydependence of the temperature for a two-level atom,three years after the first demonstration of Dopplercooling. In the course of those experiments, Dave Weissdiscovered a magnetic-field-induced cooling (Weiss,Riis, Shevy, et al., 1989) that could be explained in termsof Sisyphus-like effects and optical pumping (Ungaret al., 1989). This cooling mechanism was explored infurther detail by Hal Metcalf and collaborators (Shanget al., 1990).

The NIST discovery of sub-Doppler temperaturesshowed that the limiting temperature based on the Dop-pler effect was not actually a limit. What is the funda-mental limit to laser cooling? One might think that thelimit would be the recoil limit kBT;(pr)

2/2M , since thelast photon spontaneously emitted from an atom resultsin a random velocity of this magnitude. However, eventhis barrier can be circumvented. For example, an iontightly held in a trap can use the mass of the trap toabsorb the recoil momentum. The so-called sidebandcooling scheme proposed by Dehmelt and Wineland(see, for example, Dehmelt, 1990) and demonstrated byWineland and collaborators (Diedrich, 1989) can inprinciple cool an ion so that the fractional occupancy oftwo states separated by an energy DE can have an ef-fective temperature Teff less than the recoil temperature,where Teff is defined by e2DE/kT.

For free atoms, it is still possible to cool an ensembleof atoms so that their velocity spread is less than thephoton recoil velocity by using velocity-selection tech-niques. The Ecole Normale group devised a clevervelocity-selection scheme based on a process theynamed ‘‘velocity-selective coherent population trapping(Aspect et al., 1988, 1989). In their first work, metastablehelium atoms were cooled along one dimension of anatomic beam to a transverse (one-dimensional) tem-perature of 2 mK, a factor of 2 below the single-photonrecoil temperature. The effective temperature of thevelocity-selected atoms decreases roughly as the squareroot of the time that the velocity-selection light is on, somuch colder temperatures may be achieved for longercooling times. In subsequent experiments, they used at-oms precooled in optical molasses and achieved muchcolder temperatures in two and three dimensions (La-wall et al., 1995). An important point to emphasize is

12We showed in Shevy, Weiss, Ungar, and Chu, 1988, that alinear force versus velocity dependence F(v)52av would re-sult in a Maxwell-Boltzmann distribution.


that this method has no strict cooling limit: the longerthe cooling time, the smaller the spread in velocity.However, there is a trade-off between the final tempera-ture and the number of atoms cooled to this tempera-ture since the atom finds it harder to randomly walk intoa progressively smaller section of velocity space. Even-tually, the velocity-selective cooling become velocity‘‘selection’’ in the sense that the number of atoms in thevelocity-selected state begins to decrease.

APPLICATIONS OF LASER COOLING AND TRAPPING

During the time we were studying polarization gradi-ent cooling, my group of three students and one post-doc at Stanford began to apply the newly developedcooling and trapping techniques, but even those planswere soon abandoned.

After the completion of the studies of polarizationgradient optical molasses cooling, Erling Riis, DaveWeiss, and Kam Moler constructed a two-dimensionalversion of the magneto-optic trap where sodium atomsfrom a slowed atomic beam were collected, cooled in allthree dimensions and compressed radially before beingallowed to exit the trap in the axial direction (Riis et al.,1990). This ‘‘optical funnel’’ increased the phase-spacedensity of an atomic beam by five orders of magnitude.Another five orders of magnitude are possible with acesium beam and proper launching of the atoms in afield with moving polarization gradients. Ertmer andcolleagues developed a two-dimensional compressionand cooling scheme with the magneto-optic trap (Nelle-son et al., 1990). These two experiments demonstratedthe ease with which laser cooling can be used to ‘‘focus’’an atomic beam without the limitations imposed by the‘‘brightness theorem’’ in optics.

In our other vacuum chamber, Mark Kasevich andErling Riis were given the task of producing an atomicfountain as a first step in the quantum reflection experi-ment that was to be Mark’s thesis. The idea was tolaunch the atoms upwards in an atomic fountain with aslight horizontal velocity. When the atoms reached theirzenith, they would strike a vertically oriented surface.As they were setting set up this experiment, I askedthem to do the first of a number of ‘‘quickie experi-ments.’’ ‘‘Quickies’’ were fast diversions that I promisedwould only take a few weeks, and the first detour was touse the atomic fountain to do some precision spectros-copy.

In the early 1950s, Zacharias attempted to make an‘‘atomic fountain’’ by directing a beam of atoms up-wards. Although most of the atoms would crash into thetop of the vacuum chamber, the very slowest atoms inthe Maxwell distribution were expected to follow a bal-listic trajectory and return to the launching position dueto gravity. The goal of Zacharias’ experiment was to ex-cite the atoms in the fountain with Ramsey’s separatedoscillatory field technique, the method used in the ce-sium atomic clock (see, for example, Ramsay, 1956). At-oms initially in state u1& would enter a microwave cavityon the way up and become excited into a superposition


of two quantum states u1& and u2&. While in that superpo-sition state, the relative phases of the two states wouldprecess with a frequency \v5E12E2 . When the atomspassed through the microwave cavity on the way down,they would again be irradiated by the microwave field. Ifthe microwave generator were tuned exactly to theatomic frequency v, the second pulse would excite theatoms completely into the state u2&. If the microwavesource were p radians (half a cycle) out of phase withthe atoms, they would be returned to state u1& by thesecond pulse. For a time Dt separating the two excita-tion pulses, the oscillation ‘‘linewidth’’ Dvrf of this tran-sition satisfies DvrfDt5p .

This behavior is a manifestation of the Heisenberguncertainty principle: the uncertainty DE in the mea-surement of an energy interval times the quantum mea-surement time Dt must be greater than Planck’s constantDEDt>\ . An atomic fountain would increase the mea-surement time by more than two orders of magnitude ascompared to conventional atomic clocks with horizon-tally moving thermal beams. Zacharias hoped to mea-sure the gravitational redshift predicted by Einstein:identical clocks placed at different heights in a gravita-tional field will be frequency shifted with respect to eachother. The atom fountain clock, during its trajectory,would record less time than the stationary microwavesource driving the microwave cavity.

Unfortunately, Zacharias’ experiment failed. Theslowest atoms in the Maxwell-Boltzmann distributionwere scattered by faster atoms overtaking them frombehind and never returned to the microwave cavity. Thefailure was notable in several respects. The graduate stu-dent and post-doc working on the project still got goodjobs, and the idea remained in the consciousness of thephysics community.13 With our source of laser-cooledatoms, it was a simple matter for us to construct anatomic fountain (Kasevich, Riis, et al., 1989). Atomswere first collected in a magneto-optic trap and thenlaunched upwards by pushing from below with anotherlaser beam. At the top of the ballistic trajectory, we ir-radiated the atoms with two microwave pulses separatedby 0.25 seconds, yielding a linewidth of 2 Hz. Ralph De-Voe at the IBM Almaden Research center joined ourexperiment and provided much needed assistance in mi-crowave technology. With our demonstration atomicfountain, we measured the sodium ground-state hyper-fine splitting to an accuracy of one part in 109.

After the theory of polarization-gradient cooling wasdeveloped, we realized that there was a much better wayto launch the atoms. By pushing with a single laser beamfrom below, we would heat up the atoms due to therandom recoil kicks from the scattered photons. How-ever, by changing the frequency of the molasses beams

13See, for example, De Marchi et al., 1984. I had heard of thisattempt while still a graduate student at Berkeley. We dis-cussed the advantages of a cesium fountain atomic clock inGibble and Chu, 1992.


so that the polarization gradients would be in a frame ofreference moving relative to the laboratory frame, theatoms would cool to polarization-gradient temperaturesin the moving frame. The atoms could be launched withprecise velocities and with no increase in temperature(Weiss, Riis, Kasevich, et al., 1991).

Andre Clairon and collaborators constructed the firstcesium atomic fountain (Clairon et al., 1991). KurtGibble and I analyzed the potential accuracy of anatomic fountain frequency standard and suggested thatthe phase shifts due to collisions might be a limiting fac-tor to the ultimate accuracy of such a clock (Gibble andChu, 1992). We then constructed an atomic fountain fre-quency source that surpassed the short-term stability ofthe primary Cs references maintained by standards labo-ratories (Gibble and Chu, 1993). In that work, we alsomeasured the frequency shift due to ultracold collisionsin the fountain, a systematic effect which may be thelimiting factor of a cesium clock. The group led byClairon has recently improved upon our short-term sta-bility. More important, they achieved an accuracy esti-mated to be Dv/v<2310215, limited by the stability oftheir hydrogen maser reference (Ghezali et al., 1996).Such a clock started at the birth of the universe wouldbe off by less than four minutes today, ;15 billion yearslater.

The next ‘‘quickie’’ to follow the atomic fountain wasthe demonstration of normal incidence reflection of at-oms from an evanescent wave. Balykin et al. (1998) de-flected an atomic beam by a small angle with an evanes-cent sheet of light extending out from a glass prism. Ifthe light is tuned above the atomic resonance, the in-duced dipole p will be out of phase with the driving field.The atom with energy 2p•E is then repelled from thelight by the dipole force. The demonstration of normalincidence reflection with laser-cooled atoms was a nec-essary first step towards the search for quantum reflec-tion. With our slow atoms, we wanted to demonstrate an‘‘atomic trampoline’’ trap by bouncing atoms from acurved surface of light created by internally reflecting alaser beam from a plano-concave lens. Unfortunately,the lens we used produced a considerable amount ofscattered light, and the haze of light ‘‘levitated’’ the at-oms and prevented us from seeing bouncing atoms.Mark ordered a good quality lens and we settled forbouncing atoms from the dove-prism surface (Kasevich,Weiss, and Chu, 1990), with the intent of completing thework when the lens arrived. We never used the lens heordered because of another exciting detour in our re-search. A few years later, a trampoline trap was demon-strated by Cohen-Tannoudji’s group (Aminoff et al.,1993). The evolution of gravito-optic atom traps is sum-marized in Fig. 7.

While waiting for the delivery of our lens, we began tothink about the next stage of the quantum reflection ex-periment. The velocity spread of the atoms in the hori-zontal direction would be determined by a collimatingslit, but I was unhappy with this plan. The quantum re-flection experiment would require exquisitely cold at-oms with a velocity spread corresponding to an effectivetemperature of a small fraction of a micro-kelvin. Given


the finite size of our atoms confined in the MOT, verynarrow collimating slits would reduce the flux of atomsto distressingly low levels. Ultimately, collimating slitswould cause the atoms to diffract.

While flying home from a talk, the solution to thevelocity-selection problem came to me. Instead of usingcollimating slits, we could perform the velocity selectionwith the Doppler effect. Usually, the Doppler sensitivityis limited by the linewidth of the optical transition. How-ever, if we induced a two-photon transition between twoground states with lasers beams at frequencies v1 andv2 , there is no linewidth associated with an excited state.If the frequency of v2 is generated by an electro-opticmodulator so that n25n11nrf , the frequency jitter ofthe excitation laser would not enter since the transitionwould depend on the frequency difference n22n15nrf .The linewidth Dn would be limited by the transition timeDt it took to induce the two-photon transition, and ouratomic fountain would give us lots of time. Despite thefact that the resonance would depend on the frequencydifference, the Doppler sensitivity would depend on the

FIG. 7. Some of the highlights in the development of the re-flection of atoms from optical sheets of light tuned to the blueside of the atomic resonance. Since the atoms spend a consid-erable amount of time in free fall, blue detuned optical trapsallow fairly efficient cooling with stimulated Raman pulses.Over 33106 sodium atoms have been Raman cooled to therecoil temperature at densities of 231011 atoms/cm3 in an in-verted pyramid trap.


frequency sum if the two laser beams were counter-propagating. This idea would allow us to achieve Dop-pler sensitivity equivalent to an ultraviolet transition,but with the frequency control of the microwave do-main, and it would be easy. In a proof of principle ex-periment, we created an ensemble of atoms with a ve-locity spread of 270 mm/sec corresponding to aneffective one-dimensional ‘‘temperature’’ of 24 pi-cokelvin and a de Broglie wavelength of 51 mm(Kasevich, Weiss, Riis, et al., 1991) We also used theDoppler sensitivity to measure velocity distributionswith sub-nanokelvin resolution.

By 1990, we were aware of several groups trying toconstruct atom interferometers based on the diffractionof atoms by mechanical slits or diffraction gratings, andtheir efforts stimulated us to think about different ap-proaches to atom interferometry. We knew there is aone-to-one correspondence between the Doppler sensi-tivity and the recoil an atom experiences when it makesan optical transition. With a two-photon Raman transi-tion with counterpropagating beams of light, the recoil isDp5\keff , where keff5(k11k2), and it is this recoil ef-fect that allowed us to design a new type of atom inter-ferometer.

If an atom with momentum p and in state u1&, de-scribed by the combined quantum state u1,p&, is excitedby a so-called ‘‘p/2’’ pulse of coherent light, the atom isdriven into an equal superposition of two states u1,p&and u2,p1\keff&. After a time Dt , the two wave packetswill have separated by a distance (\keff /M)Dt. Excita-tion by a p pulse induces the part of the atom in stateu1,p& to make the transition u1,p&→u2,p1\keff& and thepart of the atom in u2,p1\keff& to make the transitionu2,p1\keff&→u1,p& . After another interval Dt , the twoparts of the atom come back together and a second p/2pulse with the appropriate phase shift with respect to theatomic phase can put the atom into either of the statesu1,p& or u2,p1\keff&. This type of atom interferometer isthe atomic analog of an optical Mach-Zender interfer-ometer and is closely related to an atom interferometerfirst discussed by Borde (1989). In collaboration with thePTB group led by Helmcke, Borde used this atom inter-ferometer to detect rotations (Riehle et al., 1991).

By January of 1991, shortly after we began seeing in-terference fringes, we heard that the Konstanz group ledby Jurgen Mlynek (Carhal and Mlynek, 1991) had dem-onstrated a Young’s double-slit version atom interfer-ometer and that the MIT group, led by Dave Pritchard(Keith et al., 1991) had succeeded in making a gratinginterferometer. Instead of using atoms in a thermalbeam, we based our interferometer on an atomic foun-tain of laser-cooled atoms. We knew we had a poten-tially exquisite measuring device because of the longmeasurement time and wanted to use our atom interfer-ometer to measure something before we submitted a pa-per.

As we began to think of what we could easily measurewith our interferometer, Mark made a fortuitous discov-ery: the atom interferometer showed a phase-shift thatscaled as Dt2, the delay time between the p/2 and p


pulses, and correctly identified that this phase shift wasdue to the acceleration of the atoms due to gravity. Anatom accelerating will experience a Doppler shift withrespect to the lasers in a laboratory frame of referencepropagating in the direction of g. Even though the laserbeams were propagating in the nominally horizontal di-rection, the few-milliradian ‘‘misalignment’’ createdenough of phase shift to be easily observable.

Our analysis of this phase shift based on Feynman’spath-integral approach to quantum mechanics was out-lined in the first demonstration of our atom interferom-eter (Kasevich and Chu, 1991) and expanded in our sub-sequent publications (Kasevich and Chu, 1992a; Moleret al., 1991; Young et al., 1997; Peters et al., 1997). Storeyand Cohen-Tannoudji have published an excellent tuto-rial paper on this approach as well (Storey and Cohen-Tannoudji, 1994). Consider a laser beam propagatingparallel to the direction of gravity, as shown in Fig. 8.The phase shift of the atom has two parts: (i) a free-evolution term e2iSCll\, where SCl5*GLdt is the actionevaluated along the classical trajectory, and (ii) a phaseterm due to the atom interacting with the light. Theevaluation of the integrals for both paths shows that thefree-evolution part contributes no net phase shift be-tween the two arms of the interferometer. The part ofthe phase shift due to the light/atom interaction is calcu-lated by using the fact that an atom that makes a tran-sition u1,p&→u2,p1\k& acquires a phase factore2i(kLz2vt), where z is the vertical position of the atomand kL5k11k2 is the effective k vector of the light. Atransition u2,p1\k&→u1,p& adds a phase factore1i(kLz2vt). If the atom does not make a transition, the

FIG. 8. An atom interferometer based on optical pulses oflight. The phase of the optical field is read into the atom duringa transition from one state to another. In the absence of grav-ity (solid lines), the part of the atom moving along the upperpath from the first p/2 pulse to the p pulse will experiencethree cycles of phase less than the part of the atom that movesalong the lower path. The lower path experiences an identicalloss of phase during the time between the p pulse and thesecond p/2 pulse. If the excitation frequency is exactly on reso-nance, the atom will be returned back to the initial state. Withgravity (dotted lines), the phase loss of the upper path is morethan the phase loss of the lower path since zC –zB8 is greaterthan zB –zA . Thus, by measuring displacements during a timeDt in terms of a phase difference, we can measure velocitychanges due to g or photon-recoil effects (Fig. 9) in the timedomain.


phase factor due to the light is unity. If kL is parallel tog, the part of the atom in the upper path will have a totalphase fupper5kL•(zA2zB) read into the atom by thelight. The part of the atom in the lower path will have agiven phase angle f lower5kL•(zB82zC). In the absenceof gravity, zA2zB5zB82zC , and there is no net phaseshift between the two paths. However, with gravity, zB2zA5 1

2 gDt2, while zB82zC53gDt2/2. Thus the netphase shift is Df5kLgDt2. Notice that the accelerationis measured in the time domain: we record the change inphase Df5kLDz that occurs after a time Dt .

The phase shift of the interferometer is measured byconsidering the relative phase between the atomic phaseof the two parts of the atom and the phase of the light atposition C. If the light is in phase with the atom, thesecond p/2 pulse will cause the atom to return to stateu1,p&. If it is p out of phase, the atom will be put intostate u2,p1\k&. Thus the phase shift is measured interms of the relative populations of these two states.

For the long interferometer times Dt that are obtain-able in an atomic fountain, the phase shift can be enor-mous. For Dt50.2 seconds, over 43106 cycles of phasedifference accumulate between the two paths of the in-terferometer. In our first atom interferometer paper, wedemonstrated a resolution in g of Dg/g51026, and, withimproved vibration isolation, achieved a resolution ofDg/g,331028 (Kasevich and Chu, 1992a). With a num-ber of refinements, including the use of an actively sta-bilized vibration-isolation system (Hensley, Peters, andChu, 1998) we have been able maintain the full fringecontrast for times up to Dt50.2 seconds and have im-proved the resolution to one part in 10210 (Peters et al.,1997).

Soon after the completion of our first atom interfer-ometer measurements, Mark Kasevich thought of a wayto cool atoms with stimulated Raman transitions(Kasevich and Chu, 1992b). Since the linewidth of a Ra-man transition is governed by the time to make the tran-sition, we had a method of addressing a very narrowvelocity slice of atoms within an ensemble alreadycooled by polarization-gradient cooling. Atoms are ini-tially optically pumped into a particular hyperfine stateu1&. A Raman transition u1&→u2& is used to push a smallsubset of them towards v50. By changing the frequencydifference n12n2 for each successive pulse, differentgroups of atoms in the velocity distribution can bepushed towards v50, analogous to the ‘‘frequencychirp’’ cooling methods used to slow atomic beams. Acritical difference is that Raman pulses permit muchhigher-resolution Doppler selectivity. After each Ramanpulse, a pulse is used to optically pump the atom backinto u1&. During this process, the atom will spontaneouslyemit one or more photons and can remain near v50.The tuning of the optical pulses are adjusted so that anatom that scatters into a velocity state near v50 willhave a low probability of further excitation. This methodof cooling is analogous to coherent population trappingexcept that the walk in velocity space is directed towardsv50. In our first demonstration of this cooling process,sodium atoms were cooled to less than 0.1 Trecoil in one


dimension, with an eightfold increase in the number ofatoms near v50. In later work, we extended this coolingtechnique to two and three dimensions (Davidson, Lee,Kasevich, and Chu, 1994).

This cooling technique has also been shown to work inan optical dipole trap. We were stimulated to return todipole traps by Phillips’ group (Rolston et al., 1992) andby Dan Heinzen’s group (Miller et al., 1993), who dem-onstrated dipole traps tuned very far from resonance. Inthis type of trap, the heating due to the scattering oftrapping light is greatly reduced. A nondissipative dipoletrap turns out to be useful in a number of applications.In traps formed by sheets of blue detuned light (a suc-cessor to the trampoline traps), we showed that atomiccoherences can be preserved for 4-sec despite hundredsof bounces (Davidson, Lee, Adams, et al., 1995). Wealso demonstrated evaporative cooling in a red detuneddipole trap made from two crossed beams of light (Ad-ams, Lee, Davidson, et al., 1995).14 Atoms can be Ra-man cooled in both red and blue detuned dipole traps(Kasevich, Lee, et al., 1996; Lee; Adams, Kasevich, andChu, 1996). In our most recent work, over 106 atomshave been Raman cooled in a blue-detuned dipole trapto less than Trecoil. This is a factor of ;300 below what isneeded for Bose condensation, but a factor of 400 im-provement over the ‘‘dark-spot’’ magneto-optic trapphase-space densities. Unfortunately, a heating processprevented us from evaporatively cooling to achieve Bosecondensation with an optical trap. Recently, WolfgangKetterle and collaborators have loaded an optical dipoletrap with a Bose condensate created in a magnetic trap(Stamper-Kurn et al., 1998). With this trap they havebeen able to find the Feshbach resonances (Feshbach,1962) calculated for sodium (Moerdijk et al., 1995), inwhich in the s-wave scattering length changes sign. Sincethis resonance is only one gauss wide, a nonmagnetictrap makes its detection much easier. Optical traps couldalso be used to hold Bose condensates in magnetic-field-insensitive states for precision atom interferometry.

Our ability to measure small velocity changes withstimulated Raman transitions suggested another applica-tion of atom interferometry. If an atom absorbs a pho-ton of momentum pg5hn/c , it will receive an impulseDp5MDv . Thus h/M5cDv/n , and since Dv can bemeasured as a frequency shift, the possibility of makinga precision measurement of h/M dropped into our lap.After realizing this opportunity, I called Barry Taylor atNIST and asked if this measurement with an indepen-dent measurement of Planck’s constant could put theworld on an atomic mass standard. He replied that thefirst application of a precise h/M measurement would bea better determination of the fine-structure constant a,since a can be expressed as

a25~2R` /c !~mp /me!~Matom /mp!~h/Matom!.

14Improved results are reported in Lee, Adams, Davidson,et al., 1995.


All of the quantities15 in the above relation can be mea-sured precisely in terms of frequencies or frequencyshifts.

Dave Weiss’ thesis project was changed to measureh/M with our newly acquired Doppler sensitivity. Theinterferometer geometry he chose was previously dem-onstrated as an extension of the Ramsey technique intothe optical domain (Borde, 1989). If two sets of p/2pulses are used, two interferometers are created withdisplaced endpoints as shown in Fig. 9. The displace-ment is measured in terms of a phase difference in therelative populations of the two interferometers, analo-gous to the way we measured the acceleration of gravity.This displacement was increased by sandwiching a num-ber of p pulses in between the two pairs of p/2 pulses.With Brent Young, Dave Weiss obtained a resolution ofroughly a part in 107 in h/MCs (Weiss, Young, and Chu,1993, 1994). In that work, systematic effects were ob-served at the 106 level, but rather than spending signifi-cant time to understand those effects, we decided to de-velop a new atom interferometer method with a verticalgeometry to measure h/M .

Instead of using impulses of momentum arising fromoff-resonant Raman pulses, I wanted to use an adiabatictransfer method demonstrated by Klaus Bergmann andcollaborators (Gaubatz et al., 1988). The beauty of anadiabatic transfer method is that it is insensitive to thesmall changes in experimental parameters such as inten-sity and frequency that adversely affect off-resonant ppulses. In addition, we showed that the ac Stark shift, apotentially troublesome systematic effect when usingoff-resonant Raman transitions, is absent when usingadiabatic transfer in a strictly three-level system (Weitz,Young, and Chu, 1994a).

Consider an atom with two ground states and one ex-cited state as shown in Fig. 10(a). Bergmann et al.showed that the rediagonalized atom/light system willalways have a ‘‘dark’’ eigenstate, not connected to theexcited state. Suppose, for simplicity, that the ampli-tudes A15^euHEMug1& and A25^euHEMug2& are equal,where HEM is the Hamiltonian describing the light/atominteraction. An atom initially in state ug1& is in the darkstate provided only v2 light is on. If we then slowly in-crease the intensity of v1 light until the beams haveequal intensities, the dark state will adiabatically evolveinto 1/&[ug1&2ug2&]. If we then turn down the intensityof v2 while leaving v1 on, the atom will evolve into stateug2& . Thus we can move the atom from state ug1& to stateug2& without ever going through the excited state.

The work of Bergmann prompted Marte, Zoller, andHall (1991) to suggest that this transfer process couldalso be used to induce a momentum change on an atom.Groups led by Mara Prentiss (Lawall and Prentiss, 1994)and Bill Phillips (Goldner et al., 1994) soon demon-strated the mechanical effect of this transition. In theseexperiments, the time-delayed light pulses were gener-

15The speed of light c is now a defined quantity. ‘‘2’’ is alsoknown well.


ated by having the atoms intersect spatially displacedlaser beams. With atoms moving slowly in an atomicfountain, we could independently vary the intensity ofeach beam with acousto-optic modulators. This freedomallowed us to construct an atom interferometer using theadiabatic transfer method (Weitz, Young, and Chu,1994b). At different light/atom interaction points, differ-ently shaped pulses are required, as shown in Fig. 10.For example, the first ‘‘adiabatic beamsplitters’’ requirethat v2 turn on first, but both v1 and v2 turn off inunison, while the second interaction point has both v1and v2 turn on together and then v1 turn off first. With

FIG. 9. (a) The basic method for measuring the recoil velocityrequires two counterpropagating pulses of light. Because ofenergy and momentum conservation, the excitation light mustsatisfy \v2\v125k•v6(\k)2/2m , where v is the velocity ofthe atom relative to the k vector of the light and the signdepends on whether the initial state is higher or lower in en-ergy than the final state. An atom at rest in ground state u1& isexcited by a p pulse at frequency v5v121\k2/2m . The atom,recoiling with velocity v5\k/m in state u2& is returned back tou1& with a counterpropagating photon v85v122\kk8/m1\k2/2m . The two resonances are shifted relative to eachother by Dv5v82v5\(k1k8)2/2m . (b) In order to increasethe resolution without sacrificing counting rate, two sets ofcounterpropagating p/2 pulses are used instead of two ppulses. Thus we are naturally led to use two atom interferom-eters whose end points are separated in space due to thephoton-recoil effect. Since the measurement is based on therelative separation of two similar atom interferometers, thereare a number of ‘‘common mode’’ subtractions that add to theinherent accuracy of the experiment. (c) To further increasethe resolution of the measurement, we sandwich many ppulses, each pulse coming from alternate directions. Only 2 ppulses are shown in the figure, but up to 60 p pulses are used inthe actual experiment, where each p pulse separates the twointerferometers by a velocity of 4\k/m .


an adiabatic transfer interferometer, we are able toseparate the two atom interferometers by up to ;250\kunits of momentum without significant degradation ofsignal.

Currently, our atom interferometric method of mea-suring h/MCs has a resolution of ;2 parts in 1029 (1 ppbin a), corresponding to a velocity resolution less than1/30 of an atom diameter/second. In terms of Dopplerspectroscopy, this precision corresponds to a resolutionin a Doppler shift of less than 100 mHz out of ;1015 Hz.We have been looking for systematic effects for the pastfive months and there are a few remaining tests to per-form before publishing a value for h/M . The other mea-surements needed to determine a, such as the mass ra-tios me /mp and mp /MCs , and the frequency of the CsD1 line will be measured to a precision of better thanone part in 1029 in the near future. Curiously, some ofthe most accurate methods of determining a are directapplications of three Nobel Prizes: the Josephson effect(56 ppb), the quantized Hall effect (24 ppb) and theequating of the ion-trap measurement of the electron

FIG. 10. (a) An atomic system consisting of ground states ug1&and ug2& and an excited state ue& connected by amplitudes A1and A2 . (b) In this space-time diagram of our adiabatic inter-ferometer, the first interaction transfers the atom from ug1& tothe superposition state (1/&)(ug1&1ug2&). In the second re-gion, both frequencies are turned on simultaneously, project-ing the atomic state uC(T1t)& onto the dark superpositionstate uC(T1t)&D evaluated at the start of the pulse. This stateis then adiabatically evolved into ug1& by the light profilesshown in the second interaction region. To complete the inter-ferometer, the sequence is repeated for adiabatic transferswith keff in the opposite direction as in Fig. 9. Atom paths thatdo not contribute to the interference signal are not shown.


magnetic moment with the QED calculation (4.2 ppb).16

OTHER APPLICATIONS IN ATOMIC PHYSICS

The topics I have discussed above are a small, person-ally skewed sampling of the many applications of thenew technology of laser cooling and atom trapping.These techniques have already been used in nonlinearoptics and quantum optics experiments. Laser-cooled at-oms have spawned a cottage industry in the study ofultracold collisions. Atom traps offer the hope that ra-dioactive species can be used for more precise studies ofparity nonconservation effects due to the weak interac-tions and more sensitive searches for a breakdown oftime-reversal invariance.

A particularly spectacular use of cold-atom technol-ogy has been the demonstration of Bose condensation ina dilute gas by Eric Cornell, Carl Weiman, and collabo-rators (Anderson et al., 1995), and later by teams led byWolfgang Ketterle (Davis et al., 1995) and Randy Hulet(Bradley et al., 1997). The production of this new stateof matter opens exciting opportunities to study collec-tive effects in a quantum gas with powerful diagnosticmethods in laser spectroscopy. The increase in phase-space density of Bose condensed atoms will also gener-ate new applications, just as the phase-space density in-crease due to laser cooling and trapping started anumber of new areas of research.

APPLICATIONS IN BIOLOGY AND POLYMER SCIENCE

In 1986, the world was excited about atom trapping.During this time, Art Ashkin began to use optical twee-zers to trap micron-sized particles. While experimentingwith colloidal tobacco mosaic viruses (Ashkin and Dz-iedzic, 1987), he noticed tiny, translucent objects in hissample. Rushing into my lab, he excitedly proclaimedthat he had ‘‘discovered Life.’’ I went into his lab, halfthinking that the excitement of the last few years hadfinally gotten the better of him. In his lab was a micro-scope objective focusing an argon laser beam into a petridish of water. Off to the side was an old Edmund Scien-tific microscope. Squinting into the microscope, I sawmy eye lashes. Squinting harder, I occasionally saw sometranslucent objects. Many of these objects were ‘‘float-ers,’’ debris in my vitreous humor that could be movedby blinking my eyes. Art assured me that there wereother objects there that would not move when I blinkedmy eyes. Sure enough, there were objects in the waterthat could be trapped and would swim away if the lightwere turned off. Art had discovered bugs in his appara-tus, but these were real bugs, bacteria that had eventu-ally grown in his sample beads and water.

His discovery was quickly followed by the demonstra-tion that infrared light focused to megawatts/cm2 couldbe used to trap live e-coli bacteria and yeast for hourswithout damage (Ashkin, Dziedzic, and Yamane, 1987).

16For a review of the current status of a, see Kinoshita, 1966.


FIG. 11. (a) A series of video images showing the relaxation of a single-molecule ‘‘rubber band’’ of DNA initially stretched byflowing fluid past the molecule. The DNA is stained with approximately one dye molecule for every five base-pairs and visualizedin an optical microscope (Perkins, Smith, and Chu, 1994). (b) (Color) The relaxation of a stained DNA molecule in an entangledsolution of unstained DNA. The molecule, initially pulled through the polymer solution with an optical tweezers, is seen to relaxalong a path defined by its contour. This work graphically shows that polymers in an entangled solution exhibit ‘‘tubelike’’ motion(Perkins, Smith, and Chu, 1994). This result and a separate measurement of the diffusion of the DNA in a similar polymer solution(Smith, Perkins, and Chu, 1995) verifies de Gennes’ reptation theory used to explain a general scaling feature of viscoelasticmaterials.



Other work included the internal cell manipulation ofplant cells, protozoa, and stretching of viscoelastic cyto-plasm (Ashkin and Dziedzik, 1989). Steve Block andHoward Berg soon adapted the optical tweezers tech-nique to study the mechanical properties of the flagellamotor (Block, Blair, and Berg, 1989), and MichaelBurns and collaborators used the tweezers to manipu-late live sperm (Tadir et al., 1989). Objects on the mo-lecular scale could also be manipulated with opticaltweezers. Block et al. sprinkled a low-density coverageof kinesin motor molecules onto a sphere and placed thesphere on a microtubule. When the kinesin was acti-vated with ATP, the force and displacement generatedby a single kinesin molecule could be measured (Block,Goldstein, and Schnapp, 1990; also see Svoboda andBlock, 1994, and references therein). Related experi-ments on the molecular motor actin/myosin associatedwith skeletal muscles have also been performed by JeffFiner, Bob Simmons, and Jim Spudich (1994) using anactive-feedback optical tweezers developed in my lab(Warnick et al., 1993; Simmons et al., 1996). Steve Kronand I developed a method to hold and simultaneouslyview a single molecule of DNA by attaching polystyrenehandles to the ends of the molecule (Chu and Kron,1990; Kasevich, Moler et al., 1990; Chu, 1991). Theseearly experiments introduced an important tool for bi-ologists, at both the cellular and the molecular level. Theapplications of this tool in biology have exploded andmay eventually overtake the activity in atomic physics.17

My original goal in developing methods to manipulateDNA was to study, in real time, the motion of enzymesmoving on the molecule. However, once we began toplay with the molecules, we noticed that a stretchedmolecule of DNA would spring back like a rubber bandwhen the extensional force was turned off, as shown inFig. 11. The ‘‘springiness’’ of the molecule is due to en-tropy considerations: the configurations of a flexiblepolymer are enumerated by counting the possible waysof taking a random walk of a large, but finite number ofsteps. A stretched molecule is in an unlikely configura-tion, and the system will move towards the much morelikely equilibrium configuration of a random coil. Ouraccidental observation of a single-molecule rubber bandcreated yet another detour: DNA from a lambda-phagevirus is large enough to visualize and manipulate, andyet small enough so that the basic equations of motiondescribing a polymer are still valid. We stumbled onto anew way of addressing long-standing questions in poly-mer dynamics and began a program in polymer physicsthat is continuing today18 (Perkins, Smith, and Chu,1994; Perkins, Quake, et al., 1994; Perkins, Smith, Lar-son, and Chu, 1994; Smith, Perkins, and Chu, 1995;Smith, Perkins, and Chu, 1996; Quake and Chu, 1997;Perkins, Smith, and Chu 1997).

17Much of the activity has been reviewed by Ashkin (1997).

18Preliminary results were reported by Chu (1991).


CLOSING REMARKS

The techniques I have discussed, to borrow an adver-tising slogan of AT&T, have enabled us to ‘‘reach outand touch’’ atoms and other neutral particles in power-ful new ways. Laser beams can now reach into a vacuumchamber, capture and cool atoms to micro-kelvin tem-peratures, and toss them upwards in an atomic fountain.With this technique, a new generation of atomic clocksis being developed. Atoms quantum mechanically splitapart and brought back together in an interferometerhave given us inertial sensors of exquisite precision andwill allow us to measure fundamental constants with un-precedented accuracy. Atom trapping and cooling meth-ods allow us to Bose-condense a gas of atoms. With thiscondensate, we have begun to examine many-body ef-fects in a totally new regime. The condensates are begin-ning to provide a still brighter source of atoms which wecan exploit. Laser traps allow us to hold onto living cellsand organelles within cells without puncturing the cellmembrane. Single molecules of DNA are being used tostudy fundamental questions in polymer dynamics. Theforce and displacement generated by a dynesin moleculeas it burns one ATP molecule can now be measured.This proliferation of applications into physics, chemistry,biology, and medicine has occurred in less than a decadeand is continuing, and we will no doubt see further ap-plications of this newfound control over matter.

In 1985, when my colleagues and I first demonstratedoptical molasses, I never foresaw the wealth of applica-tions that would follow in just a few years. Instead ofworking with a clear vision of the future, I followed mynose, head close to the ground where the scent wasstrongest.

All of my contributions cited in this lecture were theresult of working with the numerous gifted collaboratorsmentioned in this lecture. Without them, I would havedone far less. On a larger scale the field of cooling andtrapping was built out of the interwoven contributions ofmany researchers. Just as my associates and I were in-spired by the work of others, our worldwide colleagueshave already added immensely to our contributions. Iconsider this Nobel Prize to be the recognition of ourcollective endeavors. As scientists, we hope that otherstake note of what we have done and use our work to goin directions we never imagined. In this way, we con-tinue to add to the collective scientific legacy.

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