Nanophotonic Devices for Quantum Optics

Feb 13, 2013GCOE symposium

Takao Aoki

Waseda University

Atom-Light InteractionInteraction between a single two-level atom and single-mode near-resonant monochromatic light:

Strong optical nonlinearity at the single-photon level. Generation of non-classical light states. Quantum manipulation of atom/light states.

Atom-Light InteractionInteraction between a single two-level atom and single-mode near-resonant monochromatic light:

It had been extremely difficult to “isolate” individual atoms and single-mode light from the environment.

Interaction of Light and a Single Atom in Free Space

Resonant scattering cross section in the weak-driving limit

To control only the atom:

Just use strong enough light.

Interaction of Light and a Single Atom in Free Space

Resonant scattering cross section in the weak-driving limit:

To control both the atom and light:- Confine light in a small

volume- Tightly focus the light

beam down to .

Interaction of Light and a Single Atom in Free Space

Resonant scattering cross section in the weak-driving limit:

To control both the atom and light:- Confine light in a small

volume- Tightly focus the light

beam down to .

Tightly Focusing Laser Beam

Required NA of the lens ~ 0.65

Numerical Aperture (NA)

* NA ~ 0.93 for a clear aperture of 3x beam radius

Technical Difficulties

In both cases, just detecting a single emitter had been a challenging task.

Single (laser-cooled) atom in vacuum:hard to trap within a volume ~ l3

Single solid-state emitters (molecule, quantum dot, …):

suffer from dephasing due to interaction with phonons

Experimental Progress

“Collisional Blockade”

No Blockade (Poisson Law)

Collisional Blockade

Experimental ProgressNature 411, 1024 (2001)

Measurement of light-extinction by a single atom

Nature Physics 4, 924 (2008)

Light extinction (coupling between one atom and a single-mode light beam)

Single Photon SourceScience 309, 454 (2005)

Single-atom Rabi oscillation

Single Photon SourceNature 440, 779 (2006)

Imperfect interference due to mode mismatching

Remaining Problems

High collection efficiency of single photons into a single-mode fiber is demanded.

Collection efficiency into a single-mode fiber < 1%

Collection into lens aperture

Transmission through various optics

Coupling into single-mode fiber

~10% ~50% ~10%

Optical NanofiberPull in both direction

Commercial single-mode fiber Microtorch or heater

rmin < l

r0 = 62.5 mmr(z)

z

Field

Inte

nsity

F. Warken et al., Opt. Express 15, 11952 (2007)

Optical Nanofiber

Excitation

Collection Efficiency =

Atom-Nanofiber Interface

Achievements at Kyoto

rmin ~ 200 nm

r0 = 62.5 mmr(z)

z

Adiabatic condition:

(longer taper has lower coupling to higher-order modes, thus shows higher transmission)

With tapering length of ~4 cm, we have fabricated tapered fibers with transmission > 99%, which is the highest value ever achieved to date.

single-mode fiber(silica core, silica clad)

tapered region: multi-mode waveguide

single-mode waveguide(silica core, vacuum clad)

T. Aoki, JJAP 49, 118001 (2010)

Our Idea: “Lensed” Nanofiber

Nanofiber with a spherical tip = “Lensed” nanofiber

Preliminary Study at Kyoto (Numerical Simulations)

10l 5l 2l-10l -5l -2l

Preliminary Study at Kyoto (Fabrication)

Acknowledgement: I would like to thank Mr. M. Kawaguchi (currently at Dept. of Chem.) for his assistance in the early stage of this work.

1.4

1.3

1.2

1.1

1.0FWH

M /

Wav

elen

gth

1.20.80.40.0

Z / Wavelength

Interaction of Light and a Single Atom in Free Space

Resonant scattering cross section in the weak-driving limit:

To control both the atom and light:- Confine light in a small

volume- Tightly focus the light

beam down to .

Interaction of Light and a Single Atom in Free Space

Resonant scattering cross section in the weak-driving limit:

To control both the atom and light:- Confine light in a small

volume- Tightly focus the light

beam down to .

Enhancement of Spontaneous Emission

• Atom-Light Interaction

• Dissipation of Atom 　 g • Dissipation of Light 　 k

g2

G = gk2

Purcell effect

• cavity mode

Decay rates for• free space

Enhancement of spontaneous emission if G > g .

Silica microtoroidal cavities

Monolithically fabricated on a Si chip

High coupling efficiency to optical fibers （～ 99.9% ）

10 ～ 100 mm

High Q factor （ 107 ～ 1010 ）

D. K. Armani et al., Nature 421, 925-929 (2003).

Placing an atom in the evanescent field

Cesiumatom

S. M. Spillane et al., PRA 71, 013817 (2005).

Realization of strongly-coupled toroidal cQED system

Nature 443, 671 (2006)

Realization of strongly-coupled toroidal cQED system

Nature Physics 7, 159 (2011)

One-dimensional systemScience 319, 1062 (2008)

One-dimensional system

in

atom

photons out

out

“Routing of Single Photons”

PRL 102, 083601 (2009)

Achievements at Kyoto

Photolithography & etching CO2 laser irradiation

Si substrate SiO2 disk

We have achieved cavity Q factor as high as 3x108.

T. Aoki, JJAP 49, 118001 (2010)

Single Atom Trap in the Toroid’s Mode

Cesiumatom

S. M. Spillane et al., PRA 71, 013817 (2005).

Summary• We have proposed novel nanophotonic devices

for quantum optics.

• Numerical simulations show that a lensed nanofiber has focusing capability and ~30% collection efficiency, and a cleaved nanofiber has ~40% collection efficiency.

• We have successfully fabricated lensed nanofibers and cleaved nanofibers.

• We have fabricated ultra-high-Q microspherical resonators on a Si chip, which is more suitable for cQED experiments than microtoroidal resonators in terms of mode identification.