Session H10: Invited Session: Strongly Interacting Photons

Strong photon-photon interaction in a coupled quantum dot- photonic crystal nanocavity

Quantum dots (QDs) in photonic crystal nanocavities are interesting both as a testbed for fundamental cavity quantum electrodynamics (QED) experiments, as well as a platform for classical and quantum information processing. In addition to providing a scalable, on-chip, semiconductor platform, this system also enables very large dipole-field interaction strengths, as a result of the field localization inside of sub-cubic wavelength volumes (vacuum Rabi frequency is in the range of 10's of GHz). We have demonstrated controlled amplitude and phase modulation between two continuous wave (CW) optical beams at the single photon level (power less than a photon per cavity photon lifetime) interacting via a strongly coupled quantum dot - photonic crystal cavity system, and have subsequently extended this experiment to weak time-varying control field and a CW signal field. Recently, we have performed all-optical switching at the single photon level between two pulsed, resonant optical beams (with 40ps pulses and 80MHz repetition rate). In this experiment, we have measured transmission through the strongly coupled QD-cavity system as a function of delay between the two pulses, and have demonstrated a 22{\%} increase in the transmission at zero delay. The increase in the transmission is a result of the saturation of the strongly coupled QD-cavity system. We have also studied the effects of the photon blockade and photon induced tunneling which result from the anharmonicity of the ladder of dressed states in a strongly coupled QD-nanocavity system. These effects lead to dramatic changes in the transmitted photon statistics, which can be varied from sub-Poissonian to super-Poissonian, and can be employed to generate nonclassical states of light (such as Fock or NOON states) with high efficiency.   

Single-photon nonlinearity

Optical nonlinearity at the level of single photons will enable a variety of novel effects and applications, including the possibility of quantum gates between individual photons. We generate such a nonlinearity in an atomic ensemble by replacing the strong (classical) coupling field of electromagnetically induced transparency by the mode of an optical resonator. Then the resonant transmission of light through the atomic ensemble can be substantially altered even by the cavity vacuum. The vacuum induces a group delay of the optical pulse that corresponds to a group velocity of 1600 m/s. We also discuss possibilities for implementing a single-photon switch.   

Slow-light polaritons in Rydberg gases

Slow-light polaritons are quasi-particles generated in the interaction of photons with laser-driven atoms with a $\Lambda$- or ladder-type coupling scheme under conditions of electromagnetically induced transparency (EIT). They are a superposition of electromagnetic and collective spin excitations. If one of the states making up the atomic spin is a high lying Rydberg level, the polaritons are subject to a strong and non-local interaction mediated by a dipole-dipole or van-der Waals coupling between excited Rydberg atoms. I will present and discuss an effective many-body model for these Rydberg polaritons. Depending on the detuning of the control laser the interaction potential between the polaritons can be repulsive or attractive and can have a large imaginary component for distances less than the so-called blockade radius. The non-local effective interaction gives rize to interesting many-body phenomena such as the generation of photons with an avoided volume, visible in stronlgy suppressed two-particle correlations inside the blockade volume. Moreover the long-range, power-law scaling of the interaction can in the repulsive case give rize to the formation of quasi-crystalline structures of photons. In a one dimensional system the low-energy dynamics of the polaritons can be described in terms of a Luttinger liquid. Using DMRG simulations the Luttinger K parameter is calculated and conditions for the formation of a quasi-crystal are derived. When confined to a two-dimensional geometry, e.g. using a resonator with quasi-degenerate transversal mode spectrum, Rydberg polaritons are an interesting candidate to study the bosonic fractional quantum Hall effect. I will argue that the formation of photons with an avoided volume is essential for explaining recent experiments on stationary EIT in Rydberg gases [1,2].\\[4pt] [1] J.D. Pritchard et al., Phys. Rev. Lett. 105, 193603 (2010). \\[0pt] [2] D. Petrosyan, J. Otterbach, and M. Fleischhauer, arXiv:1106.1360   

Fast optical control of atom-light interactions using quantum dots coupled to photonic crystal cavities

Quantum dots (QDs) are stable, bright, semiconductor based light emitters that exhibit a quantized energy spectrum. For these reasons they are excellent candidates for development of lasers, optoelectronic components, and could serve as basic building blocks for future quantum information technology. By coupling these nanostructures to optical cavities the interaction strength between QDs and light can be significantly increased. Photonic crystals (materials with a periodic index of refraction) are particularly promising for enhancing these interactions due to their ability to guide and confine light on the size scale of an optical wavelength. Photonic crystal based optical cavities have already been shown to enable the strong coupling regime of cavity quantum electrodynamics (cQED). In this regime a significant modification of both the QD emission spectrum and cavity reflectivity can be observed due to quantum mechanical mixing of atom-photon states. Control of QD-photon interactions on fast timescales is an important capability that enables strong nonlinear optical effects, opening up the door for a new class of opto-electronic devices at ultra-low light levels. It could also provide a promising route towards quantum information processing using photons and QDs to store and transmit quantum coherence. Here we describe a method to achieve fast all-optical control of atom-light interactions using indium arsenide (InAs) QDs coupled to photonic crystal cavities. We show that a QD strongly coupled to a photonic crystal cavity can exhibit very large optical Stark shifts due to resonant cavity enhancement of the electromagnetic field. Stark shifts as large as 20 GHz are demonstrated with as few as 10 photons in the cavity. These shifts can be used to control the QD resonant frequency on fast time scales, and therefore modify its interactions with the optical cavity through resonant detuning. Using this approach we demonstrate the ability to perform all optical switching with control pulse energies as small as 400 photons and switching times as fast as 140 ps. The approach can be improved through better cavity coupling methods to approach nonlinear optics near the single photon level.