Nanophotonic Devices for Quantum Optics Feb 13, 2013 GCOE symposium 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: -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 ~ 3 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 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 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 Nanofiber Pull in both direction Commercial single-mode fiber Microtorch or heater r min < r 0 = 62.5  m r(z)r(z) z Field Intensity F. Warken et al., Opt. Express 15, (2007)

Optical Nanofiber Excitation Collection Efficiency =

Atom-Nanofiber Interface

Achievements at Kyoto r min ~ 200 nm r 0 = 62.5  m r(z)r(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, (2010)

Our Idea: “Lensed” Nanofiber Nanofiber with a spherical tip = “Lensed” nanofiber

Preliminary Study at Kyoto (Numerical Simulations)

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.

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  Dissipation of Light   2  = g  2 Purcell effect cavity mode Decay rates for free space Enhancement of spontaneous emission if 

Silica microtoroidal cavities Monolithically fabricated on a Si chip High coupling efficiency to optical fibers （～ 99.9% ） 10 ～ 100  m High Q factor （ 10 7 ～ ） D. K. Armani et al., Nature 421, (2003).

Placing an atom in the evanescent field Cesium atom S. M. Spillane et al., PRA 71, (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 in atom photons out “Routing of Single Photons” PRL 102, (2009)

Achievements at Kyoto Photolithography & etching CO 2 laser irradiation Si substrate SiO 2 disk We have achieved cavity Q factor as high as 3x10 8. T. Aoki, JJAP 49, (2010)

Single Atom Trap in the Toroid’s Mode Cesium atom S. M. Spillane et al., PRA 71, (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.