Observation of strong coupling between one atom and a monolithic microresonator.

Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics with diverse physical systems, including single atoms in Fabry-Perot resonators, quantum dots coupled to micropillars and photonic bandgap cavities and Cooper pairs interacting with superconducting resonators. Experiments with single, localized atoms have been at the forefront of these advances with the use of optical resonators in high-finesse Fabry-Perot configurations. As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems. Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks, scalable quantum logic with photons, and quantum information processing on atom chips.

Early time diffusion for the quantum kicked rotor with narrow initial momentum distributions.

We investigate analytically and numerically early-time momentum diffusion rates for the delta-kicked rotor across the quantum to classical transition, i.e., as increased total system action produces more macroscopic dynamics. For sufficiently narrow initial momentum distributions we find a rich structure of resonances in these diffusion rates as a function of the effective Planck's constant. Our study is set in the physical context of the atom optics kicked rotor, and numerical simulations confirm that the resonances persist with kicks of finite duration and with other typical experimental imperfections, such as spontaneous emission noise. Our results should be testable in experiments where narrow initial momentum distributions are prepared using, for example, velocity selective Raman transitions or Bose-Einstein condensates.

Diffusion resonances in action space for an atom optics kicked rotor with decoherence.

We numerically investigate momentum diffusion rates for the pulse kicked rotor across the quantum to classical transition as the dynamics are made more macroscopic by increasing the total system action. For initial and late time rates we observe an enhanced diffusion peak which shifts and scales with changing kick strength, and we also observe distinctive peaks around quantum resonances. Our investigations take place in the context of a system of ultracold atoms which is coupled to its environment via spontaneous emission decoherence, and the effects should be realizable in ongoing experiments.

Efficient routing of single photons by one atom and a microtoroidal cavity.

Single photons from a coherent input are efficiently redirected to a separate output by way of a fiber-coupled microtoroidal cavity interacting with individual cesium atoms. By operating in an overcoupled regime for the input-output to a tapered fiber, our system functions as a quantum router with high efficiency for photon sorting. Single photons are reflected and excess photons transmitted, as confirmed by observations of photon antibunching (bunching) for the reflected (transmitted) light. Our photon router is robust against large variations of atomic position and input power, with the observed photon antibunching persisting for intracavity photon number 0.03 < or approximately similar n < or approximately similar 0.7.

Dynamical quantum phase transitions in the dissipative Lipkin-Meshkov-Glick model with proposed realization in optical cavity QED.

We present an optical cavity QED configuration that is described by a dissipative version of the Lipkin-Meshkov-Glick model of an infinitely coordinated spin system. This open quantum system exhibits both first- and second-order nonequilibrium quantum phase transitions as a single, effective field parameter is varied. Light emitted from the cavity offers measurable signatures of the critical behavior, including that of the spin-spin entanglement.

A photon turnstile dynamically regulated by one atom.

Beyond traditional nonlinear optics with large numbers of atoms and photons, qualitatively new phenomena arise in a quantum regime of strong interactions between single atoms and photons. By using a microscopic optical resonator, we achieved such interactions and demonstrated a robust, efficient mechanism for the regulated transport of photons one by one. With critical coupling of the input light, a single atom within the resonator dynamically controls the cavity output conditioned on the photon number at the input, thereby functioning as a photon turnstile. We verified the transformation from a Poissonian to a sub-Poissonian photon stream by photon counting measurements of the input and output fields. The results have applications in quantum information science, including for controlled interactions of single light quanta and for scalable quantum processing on atom chips.

Unconditional two-mode squeezing of separated atomic ensembles.

We propose schemes for the unconditional preparation of a two-mode squeezed state of effective bosonic modes realized in a pair of atomic ensembles interacting collectively with optical cavity and laser fields. The scheme uses Raman transitions between stable atomic ground states and under ideal conditions produces pure entangled states in the steady state. The scheme works both for ensembles confined within a single cavity and for ensembles confined in separate, cascaded cavities.

Entanglement and entropy engineering of atomic two-qubit States.

We propose a scheme employing quantum-reservoir engineering to controllably entangle the internal states of two atoms trapped in a high-finesse optical cavity. Using laser and cavity fields to drive two separate Raman transitions between stable atomic ground states, a system is realized corresponding to a pair of two-state atoms coupled collectively to a squeezed reservoir. Phase-sensitive reservoir correlations lead to entanglement between the atoms, and, via local unitary transformations and adjustment of the degree and purity of squeezing, one can prepare entangled mixed states with any allowed combination of linear entropy and entanglement of formation.