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BOSE-EINSTEIN CONDENSATION –S.N. BOSE’S LEGACY LIVES ON

WOLFGANG KETTERLE*

ARTICLE

* Professor of Physics, Research Laboratory for Electronics, MIT-Harvard Center for Ultracold Atoms, and Department of Physics,Massachusetts Institute of Technology, Cambridge, MA 02139-4307, USA. Also a Nobel Prize Winner of the year 2001.e-mail: ketterle@mit.edu

Satyendra Nath Bose and Albert Einstein predictedBose-Einstein condensation in 1924 which is a newform of matter – a superfluid gas. This was realized

in atomic gases in 1995. As we look back at this thrivingfield, it’s clear that there is much to celebrate, and muchmore excitement to come.

Bose-Einstein condensation (BEC) has changed theface of atomic physics. It is for atoms or matter waveswhat the laser is for photons: a macroscopically occupiedquantum state. It was regarded as an elusive goal until itwas discovered in 1995. Although BEC was immediatelyviewed as a major accomplishment, its impact has farexceed expectations. Now, more than 20 years later, thereis no question that the field remains exciting. And maybethe best is yet to come.

With the advent of BEC, it became possible to obtainnanokelvin temperatures and create samples of atomsalmost free of entropy. Within a very short time, most ofthe many groups working on laser cooling transitioned tostudies of BECs. Evaporative cooling of atoms was just somuch more robust and applicable to higher densities thanany sub-recoil laser-cooling scheme. First demonstrated in1988 by Thomas Greytak, Harald Hess and Dan Kleppner,evaporative cooling was the enabling technology for thefirst atomic BECs, and is still the method of choice forcooling atoms to quantum degeneracy.

BEC was also immediately recognized as a newquantum liquid, extending the tradition from the superfluidquantum liquids helium-3 and helium-4 to gaseous systems

at a billion times lower density. And when it became areality, there was immediately a shopping list of scientificdirections quantum control and precision measurementcoming from laser cooling and atomic physics, and thestudy of phase transitions, sound and other collectiveexcitations, vortices and superfluidity drawing fromquantum liquids. Most of these goals were accomplishedin the years that followed, with the exception of precisionmeasurements, hampered by the high atomic densities inBECs that introduce interaction shifts of spectral signalsas a major systematic uncertainty. Instead, lower densitylaser cooled samples are often used.

But what most people couldn’t imagine in 1995 washow many other scientific directions could take advantageof BECs. I have often said that even in my boldest dreams,I would not have imagined how dramatically the fieldwould develop. To a large degree, this reflects the varietyof systems and methods that atomic physics brought to BECresearch.

First, many atomic species have now been condensed.This includes atoms that cannot be confined in magnetictraps, one of the key technologies used in earlier BECwork. They can, however, be confined in optical traps —a technique that has undergone major developments.Second, evaporative cooling to quantum degeneracy hasbeen extended to fermionic atoms and to molecules (moreabout this below).

Third, mixtures of different atomic species are beingstudied, including spinor condensates, which are mixturesof atoms in different internal states that interconvertdepending on external parameters, resulting in a rich phasediagram. Fourth, Feshbach resonances (scatteringresonances allowing a wide tunability of interactions viamagnetic fields) and optical lattices became powerful new

28 SCIENCE AND CULTURE, JANUARY-FEBRUARY, 2019

methods. Fifth, laser and opticaltechnology has rapidly advanced.Laser sources are now muchcheaper and more reliable, and itis possible to do experiments withmany different lasers of differentcolors. This is a big step forwardfrom my own labs in the mid andlate 1990s, which fully dependedon the dye lasers that are nowalmost universally thought of asgiant extinct species.

Another platform for Bose-Einstein condensation are excitonsand polaritons – bosonicexcitations in solid materials.Since these bosons are muchlighter than atoms, the BECtransition temperature is muchhigher, even room temperature ispossible.

Some major extensions of BEC were surprisinglystraight forward because they continued to use alkali atoms,for which almost all cooling and trapping methods werefirst developed. Fermionic isotopes of potassium andlithium, for example, could be cooled to degeneracy usingthe same basic methods as those used for bosonic gases— avoiding the freeze out of elastic collisions by using atwo-component gas. Further cooling induced the fermionsto form pairs and to become superfluid. Using a Feshbachresonance to adjust the atomic interactions, the nature ofthe pairing was changed from tight molecular pairing to alooser form of pairing analogous to Cooper pairs ofelectrons in superconductors and the BEC-BCS crossoverwas mapped out (BCS stands for the Bardeen-Cooper-Schrieffer theory for the simplest form of superconductors).Ultimately, these experiments weren’t much morecomplicated than the early BEC experiments. The differencewas that researchers had to understand Feshbach resonancesand cold collision physics in order to choose the rightconditions (in particular the magnetic field controlling theatomic interactions).

Ultracold molecules were assembled from ultracoldalkali atoms using Feshbach resonances. Currently, thisapproaches to ultracold molecules features lowesttemperatures and highest phase space densities. A newsubfield, ultracold chemistry, is opening up. Tri-atomicmolecules (so called Effimov trimers) provided new insightinto universal features of three-body systems. I continueto be surprised at how big a market share of research the

Fig. 1. One of the first observations of Bose-Einstein condensation (MIT, 1995). Shown are absorption(or shadow) pictures of the atomic cloud after ballistic expansion. The Bose-Einstein condensate ischaracterized by its slow expansion observed after 6 msec time of flight. The left picture shows anexpanding cloud cooled to just above the transition point; middle: just after the condensate appeared;right: after further evaporative cooling has left an almost pure condensate. The width of the images is1.0 mm. The total number of atoms at the phase transition is about 7x105, the temperature at thetransition point is 2 microkelvin.

alkali atoms still command.

The systems and methods mentioned above are nowoften combined in a platform called a quantum simulator.This is a toolbox in which atoms, multiple laser beamsand radio-frequency or microwave radiation are used to‘quantum engineer’ interesting, often paradigmatic,Hamiltonians to study their properties. Prime examples arethe BEC–BCS crossover, fermions with infinitely stronginteractions, population-imbalanced fermion systems, Bose–Hubbard and Fermi–Hubbard models, and Andersonlocalization.

All of the recent developments are featured at theBiannual International Conferences on Bose–Einsteincondensation. These conferences started in 1993 whenprogress towards achieving BEC intensified. Despitekeeping the name, the meeting now covers all frontiers inquantum gases. The 2015 conference opened with acelebratory session ‘BEC 20 years’, but most speakers, aftersome historic remarks, focused on new results. Bill Phillips,1997 Nobel Laureate for laser cooling, captured the spiritof the meeting in a talk entitled “40 years of laser cooling,20 years of BEC: still surprises”.

It seemed apt that the 2015 senior BEC prize wasgiven to Greytak, Hess and Kleppner for their earlydemonstration of evaporative cooling of atoms. The awardtalk highlighted the major obstacles during the early daysof research towards Bose–Einstein condensates, and thesolutions that many younger researchers now take forgranted.

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thermalization, the entanglement of fewatoms, and polarons in BECs and Fermigases.Although the community ofresearchers in this field has grown rapidly,it still feels like a big family marked by afriendly atmosphere with a collaborativespirit.

One major goal for the future is toobtain a deeper understanding ofentanglement, strong interactions andcorrelations in few and many-bodysystems.This can be realized by usingultracold atoms and molecules to assembleinteresting quantum systems. Materialswith topological properties, including thefractional quantum Hall effect, topologicalinsulators and Majorana fermions, newforms of superfluidity (including p-waveand d-wave pairing, FFLO states, modelsfor high-temperature superconductors) andfrustrated spin systems all rank high onthis list. A challenge is to use fermions asparticles and bosons as fields to simulatedynamic gauge fields, such as toy modelsfor quantum electrodynamics and quantumchromodynamics.

Atomic physics can go beyond therealizations available to an electron systemby using bosonic and fermionic atoms invarious spin states — finding the bosonicversion of fractional quantum Hall states,for example, or superfluidity in a three-component Fermi system.But almostcertainly, there will be surprises andunexpected breakthroughs. In the longterm, the hope is that insight into new

quantum phases of matter will pave the way towardsfundamentally new materials and new devices.

S.N. Bose was born on January 1, 1894, 125 yearsago. I am sure he would have been very pleased to see inhow many ways his theoretical concepts are nowapplied.

References1. Wolfgang Ketterle, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139, USA e-mail: ketterle@mit.edu2. This article is based on an earlier Nature Physics paper by the

same author (Nature Physics 11, 982 (2015))

Some of the new frontiers include new systems suchas ultracold atomic gases with Rydberg excitations to obtainstrong interactions and correlations, highly magnetic atomsthat show strong dipolar interactions, and quantum fluidsof photons. New techniques are being developed, such assingle atom microscopy, and shaping quantum gases intotwo reservoirs connected by a thin channel for transportmeasurements. And new scientific avenues have emerged,including spin–orbit coupling and artificial gauge fields,the creation of topological defects (Kibble–Zurek physics)during quenches across the BEC phase transition, disorderand many-body localization, ergodicity and pre-

Fig. 2. Vortices in quantum gases: Shown is a Vortex pattern in bosonic Sodium atoms (greencartoon) in a magnetic trap, Vortices in tightly bound Lithium molecules (red-blue cartoon)and a vortex lattice in loosely bound Fermion pairs created on the “BCS-side” of a Feshbachresonance. The background shows a classical vortex (Hurricane Isabel in summer 2003, NASAimage ISS007E14887).