Feb 13, 2013
GCOE symposium
Nanophotonic Devices for Quantum Optics
Takao Aoki
Waseda University
Atom-Light Interaction
Interaction 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 Interaction
Interaction 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:
- Tightly focus the light beam
down to .
- Confine light in a small volume
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:
- Tightly focus the light beam
down to .
- Confine light in a small volume
Tightly Focusing Laser Beam
Numerical Aperture (NA)
Required NA of the lens ~ 0.65
* NA ~ 0.93 for a clear aperture of 3x beam radius
Technical Difficulties
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
In both cases, just detecting a single emitter had been a challenging task.
Experimental Progress
“Collisional Blockade”
Collisional Blockade
No Blockade (Poisson Law)
Experimental Progress
Nature 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 Source
Science 309, 454 (2005)
Single-atom Rabi oscillation
Single Photon Source
Nature 440, 779 (2006)
Imperfect interference due to
mode mismatching
Remaining Problems
Collection efficiency into a single-mode fiber < 1%
Collection into
lens aperture
Transmission through
various optics
Coupling into single-mode
fiber
~10%
~50%
~10%
High collection efficiency of single photons into a single-mode fiber is demanded.
Optical Nanofiber
Pull in both direction
Microtorch or heater
Commercial single-mode fiber
r0 = 62.5 mm
rmin < l
r(z)
Field Intensity
z
F. Warken et al., Opt. Express 15, 11952 (2007)
Optical Nanofiber
Excitation
Collection Efficiency =
Atom-Nanofiber Interface
Achievements at Kyoto
T. Aoki, JJAP 49, 118001 (2010)
r0 = 62.5 mm
r(z)
rmin ~ 200 nm
z
single-mode fiber
tapered region:
(silica core, silica clad) multi-mode waveguide
single-mode waveguide
(silica core, vacuum clad)
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.
Our Idea: “Lensed” Nanofiber
Nanofiber with a spherical tip = “Lensed” nanofiber
Preliminary Study at Kyoto (Numerical Simulations)
-10l
10l
-5l
5l
-2l
2l
FWHM / Wavelength
Preliminary Study at Kyoto (Fabrication)
1.4
1.3
1.2
1.1
1.0
0.0
0.4
0.8
1.2
Z / Wavelength
Acknowledgement: I would like to thank Mr. M. Kawaguchi (currently at Dept. of Chem.)
for his assistance in the early stage of this work.
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:
- Tightly focus the light beam
down to .
- Confine light in a small volume
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:
- Tightly focus the light beam
down to .
- Confine light in a small volume
Enhancement of Spontaneous Emission
• Atom-Light Interaction
• Dissipation of Atom
• Dissipation of Light
Purcell effect
Decay rates for
• free space
g
2
g
2
• cavity mode G =
k
Enhancement of spontaneous emission if G > g .
g
k
Silica microtoroidal cavities
10 ～ 100 mm
Monolithically fabricated on a Si chip
High Q factor（107～1010）
High coupling efficiency to optical fibers（～99.9%）
D. K. Armani et al., Nature 421, 925-929 (2003).
Placing an atom in the evanescent field
Cesium
atom
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 system
Science 319, 1062 (2008)
One-dimensional system
PRL 102, 083601
(2009)
“Routing of Single Photons”
in
out
photons
out
atom
Achievements at Kyoto
T. Aoki, JJAP 49, 118001 (2010)
Si substrate
SiO2 disk
Photolithography & etching
CO2 laser irradiation
We have achieved cavity Q factor as high as 3x108.
Single Atom Trap in the Toroid’s Mode
Cesium
atom
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.