A Photon Turnstile Dynamically Regulated by One Atom

Construction of high-dimensional universal quantum logic gates using a Λ system coupled with a whispering-gallery-mode microresonator

High-dimensional quantum system provides a higher capacity of quantum channel, which exhibits potential applications in quantum information processing. However, high-dimensional universal quantum logic gates is difficult to achieve directly with only high-dimensional interaction between two quantum systems and requires a large number of two-dimensional gates to build even a small high-dimensional quantum circuits. In this paper, we propose a scheme to implement a general controlled-flip (CF) gate where the high-dimensional single photon serve as the target qudit and stationary qubits work as the control logic qudit, by employing a three-level Λ-type system coupled with a whispering-gallery-mode microresonator. In our scheme, the required number of interaction times between the photon and solid state system reduce greatly compared with the traditional method which decomposes the high-dimensional Hilbert space into 2-dimensional quantum space, and it is on a shorter temporal scale for the experimental realization. Moreover, we discuss the performance and feasibility of our hybrid CF gate, concluding that it can be easily extended to a 2n-dimensional case and it is feasible with current technology.

Quantum Computation Based on Photons with Three Degrees of Freedom

Quantum systems are important resources for quantum computer. Different from previous encoding forms using quantum systems with one degree of freedom (DoF) or two DoFs, we investigate the possibility of photon systems encoding with three DoFs consisting of the polarization DoF and two spatial DoFs. By exploring the optical circular birefringence induced by an NV center in a diamond embedded in the photonic crystal cavity, we propose several hybrid controlled-NOT (hybrid CNOT) gates operating on the two-photon or one-photon system. These hybrid CNOT gates show that three DoFs may be encoded as independent qubits without auxiliary DoFs. Our result provides a useful way to reduce quantum simulation resources by exploring complex quantum systems for quantum applications requiring large qubit systems.

Broadband photon-photon interactions mediated by cold atoms in a photonic crystal fiber

We demonstrate theoretically that photon-photon attraction can be engineered in the continuum of scattering states for pairs of photons propagating in a hollow-core photonic crystal fiber filled with cold atoms. The atoms are regularly spaced in an optical lattice configuration and the photons are resonantly tuned to an internal atomic transition. We show that the hard-core repulsion resulting from saturation of the atomic transitions induces bunching in the photonic component of the collective atom-photon modes (polaritons). Bunching is obtained in a frequency range as large as tens of GHz, and can be controlled by the inter-atomic separation. We provide a fully analytical explanation for this phenomenon by proving that correlations result from a mismatch of the quantization volumes for atomic excitations and photons in the continuum. Even stronger correlations can be observed for in-gap two-polariton bound states. Our theoretical results use parameters relevant for current experiments and suggest a simple and feasible way to induce interactions between photons.

Universal quantum gates for photon-atom hybrid systems assisted by bad cavities

We present two deterministic schemes for constructing a CNOT gate and a Toffoli gate on photon-atom and photon-atom-atom hybrid quantum systems assisted by bad cavities, respectively. They are achieved by cavity-assisted photon scattering and work in the intermediate coupling region with bad cavities, which relaxes the difficulty of their implementation in experiment. Also, bad cavities are feasible for fast quantum operations and reading out information. Compared with previous works, our schemes do not need any auxiliary qubits and measurements. Moreover, the schematic setups for these gates are simple, especially that for our Toffoli gate as only a quarter wave packet is used to interact the photon with each of the atoms every time. These atom-cavity systems can be used as the quantum nodes in long-distance quantum communication as their relatively long coherence time is suitable for multi-time operations between the photon and the system. Our calculations show that the average fidelities and efficiencies of our two universal hybrid quantum gates are high with current experimental technology.

Photon statistics on the extreme entanglement

The effects of photon bunching and antibunching correspond to the classical and quantum features of the electromagnetic field, respectively. No direct evidence suggests whether these effects can be potentially related to quantum entanglement. Here we design a cavity quantum electrodynamics model with two atoms trapped in to demonstrate the connections between the steady-state photon statistics and the two-atom entanglement. It is found that within the weak dissipations and to some good approximation, the local maximal two-atom entanglements perfectly correspond to not only the quantum feature of the electromagnetic field—the optimal photon antibunching, but also the classical feature—the optimal photon bunching. We also analyze the influence of strong dissipations and pure dephasing. An intuitive physical understanding is also given finally.

Quantum nondestructive determination of qubit states in low-Q cavities via single-photon input-output process

We present an efficient proposal to realize quantum nondestructive determination (QNDD) of unknown single-qubit states in two types of low-Q cavities, i.e., single-sided and double-sided cavity QED systems. In the dispersive regime, we demonstrate that the QNDD of single-qubit states can be realized by detecting qubit-state-dependent phase shifts (QSDPSs) of the reflected or transmitted photons from the cavity in the single-photon input-output process. Our proposal could be straightforwardly extended to the case of multiple-qubit states. Furthermore, the experimental feasibility of our proposal is also analyzed in experimentally-demonstrated circuit QED systems. The distinct feature of our proposal is that our proposal works in the dispersive regime of low-Q cavities and it is robust to both cavity decay and atomic spontaneous emission.

Exact optimal control of photon blockade with weakly nonlinear coupled cavities

We propose a scheme of photon blockade in a system comprising of coupled cavities embedded in Kerr nonlinear material, where two cavities are driven and dissipated. We analytically derive the exact optimal conditions for strong photon antibunching, which are in good agreement with those obtained by numerical simulations. We find that conventional and unconventional photon blockades have controllable flexibilities by tuning the strength ratio and relative phase between two complex driving fields. Such unconventional photon-blockade effects are ascribed to the quantum interference effect to avoid two-photon excitation of the coupled cavities. We also discuss the statistical properties of the photons under given optimal conditions. Our results provide a promising platform for the coherent manipulation of photon blockade, which has potential applications for quantum information processing and quantum optical devices.

Universal remote quantum computation assisted by the cavity input–output process

Quantum computing may provide potential superiority to solve some difficult problems. We propose a scheme for scalable remote quantum computation based on an interface between the photon and the spin of an electron confined in a quantum dot embedded in a microcavity. By successively interacting auxiliary photon pulses with spins charged in optical cavities, a prototypical quantum controlled–controlled flip gate (Toffoli gate) is achieved on a remote three-spin system using only one Einstein–Podolsky–Rosen entanglement, and local operations and classical communication. Our proposed model is shown to be robust to practical noise and experimental imperfections in current cavity–quantum electrodynamics techniques.

Strong Coupling between a Trapped Single Atom and an All-Fiber Cavity

We demonstrate an all-fiber cavity quantum electrodynamics system with a trapped single atom in the strong coupling regime. We use a nanofiber Fabry-Perot cavity, that is, an optical nanofiber sandwiched by two fiber-Bragg-grating mirrors. Measurements of the cavity transmission spectrum with a single atom in a state-insensitive nanofiber trap clearly reveal the vacuum Rabi splitting.

Electrically controllable single-photon switch based on graphene

We propose an electrically controllable single-photon switch that consists of a graphene nanoribbon side-coupled to a dynamically modulated graphene nanodisk. With a harmonic modulation of graphene conductivity induced by the gate voltage, interband photonic transition between two eigenstates of the nanodisk is introduced, which leads to extraordinary single-photon transport properties such as electromagnetically induced transparency-like phenomena. This effect is utilized to realize a highly efficient single-photon switch. With properly designed parameters, numerical results show that an extinction ratio of up to 20.8 dB can be achieved. The influence of the coupling strength between nanoribbon and nanodisk, the coupling strength between two eigenstates of the nanodisk, and dissipations in the nanodisk are also investigated in this paper.

Quantum interference induced photon blockade in a coupled single quantum dot-cavity system

We propose an experimental scheme to implement a strong photon blockade with a single quantum dot coupled to a nanocavity. The photon blockade effect can be tremendously enhanced by driving the cavity and the quantum dot simultaneously with two classical laser fields. This enhancement of photon blockade is ascribed to the quantum interference effect to avoid two-photon excitation of the cavity field. Comparing with Jaynes-Cummings model, the second-order correlation function at zero time delay g⁽²⁾(0) in our scheme can be reduced by two orders of magnitude and the system sustains a large intracavity photon number. A red (blue) cavity-light detuning asymmetry for photon quantum statistics with bunching or antibunching characteristics is also observed. The photon blockade effect has a controllable flexibility by tuning the relative phase between the two pumping laser fields and the Rabi coupling strength between the quantum dot and the pumping field. Moreover, the photon blockade scheme based on quantum interference mechanism does not require a strong coupling strength between the cavity and the quantum dot, even with the pure dephasing of the system. This simple proposal provides an effective way for potential applications in solid state quantum computation and quantum information processing.

Optical-coupling of distant spins via collective enhancement in multi-mode whispering gallery resonators

The quantum coupling of spatially distant spins via optical photons using cavity quantum electrodynamic (cQED) methods has proved experimentally challenging due to the large spin-photon coupling strengths required. To achieve such coupling strengths using traditional cQED methods requires either individual spins and ultra-small cavities or an ensemble of identical spins coupled to larger cavities. In this work we describe a method to couple distant spins via the collective enhanced coupling to a large ensemble ∼ N, of degenerate optical Whispering Gallery Modes (WGM) in a spherical resonator where the spins are spatially located at the antipodes. The setup can be scaled-up to build 1D, 2D and 3D cQED lattices to enable quantum simulation or computing.

Preparation of three-dimensional entanglement for distant atoms in coupled cavities via atomic spontaneous emission and cavity decay

We propose a dissipative scheme to prepare a three-dimensional entangled state for two atoms trapped in separate coupled cavities. Our work shows that both atomic spontaneous emission and cavity decay, which are two typical obstacles in unitary-dynamics-based schemes, are no longer detrimental, but necessary for three-dimensional entangled state preparation without specifying initial state and controlling the evolution time precisely. Final numerical simulation with one group of experimental parameters indicates that the performance of our scheme could be better than the unitary-dynamics-based scheme.

Coherent optical non-reciprocity in axisymmetric resonators

We describe an approach to optical non-reciprocity that exploits the local helicity of evanescent electric fields in axisymmetric resonators. By interfacing an optical cavity to helicity-sensitive transitions, such as Zeeman levels in a quantum dot, light transmission through a waveguide becomes direction-dependent when the state degeneracy is lifted. Using a linearized quantum master equation, we analyze the configurations that exhibit non-reciprocity, and we show that reasonable parameters from existing cavity QED experiments are sufficient to demonstrate a coherent non-reciprocal optical isolator operating at the level of a single photon.

Single-photon quantum router with multiple output ports

The routing capability is a requisite in quantum network. Although the quantum routing of signals has been investigated in various systems both in theory and experiment, the general form of quantum routing with many output terminals still needs to be explored. Here we propose a scheme to achieve the multi-channel quantum routing of the single photons in a waveguide-emitter system. The channels are composed by the waveguides and are connected by intermediate two-level emitters. By adjusting the intermediate emitters, the output channels of the input single photons can be controlled. This is demonstrated in the cases of one output channel, two output channels and the generic N output channels. The results show that the multi-channel quantum routing of single photons can be well achieved in the proposed system. This offers a scheme for the experimental realization of general quantum routing of single photons.

Concentration of entangled nitrogen-vacancy centers in decoherence free subspace

Exploiting the input-output process of low-Q cavities confining nitrogen-vacancy centers, we present an efficient entanglement concentration protocol on electron spin state in decoherence free subspace. Less entangled state can be concentrated to maximally entangled state with the assistance of single photon detection. With its robustness and scalability, the present protocol is immune to dephasing and can be further applied to quantum repeaters and distributed quantum computation.

Analytic properties of two-photon scattering matrix in integrated quantum systems determined by the cluster decomposition principle

We consider a general class of integrated quantum systems where photon-photon interaction occurs in a quantum device that is localized in space. Using techniques that are closely related to cluster decomposition principles in quantum field theory, we provide a general constraint on the analytic properties of a two-photon S matrix in this class of systems. We also show that the photon-photon interaction in these systems inevitably leads to frequency mixing and entanglement and that frequencies of the single photons cannot be preserved in these systems.

Fiber-optical switch controlled by a single atom

We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection. Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.

Quantum-criticality-induced strong Kerr nonlinearities in optomechanical systems

We investigate a hybrid electro-optomechanical system that allows us to realize controllable strong Kerr nonlinearities even in the weak-coupling regime. We show that when the controllable electromechanical subsystem is close to its quantum critical point, strong photon-photon interactions can be generated by adjusting the intensity (or frequency) of the microwave driving field. Nonlinear optical phenomena, such as the appearance of the photon blockade and the generation of nonclassical states (e.g., Schrödinger cat states), are demonstrated in the weak-coupling regime, making the observation of strong Kerr nonlinearities feasible with currently available optomechanical technology.

Silica microtoroid resonator sensor with monolithically integrated waveguides

Due to their wide operating range, silica toroidal whispering gallery mode microresonators have enabled numerous applications from fundamental physics to lasing and sensing. However, the integration of a waveguide with these microresonators has not been achieved which limits their integration with additional on-chip components. Here, we demonstrate a novel approach for monolithically integrating a silica microtoroid with an on-chip waveguide to form a fully integrated microtoroid-waveguide system with quality factors in excess of 4 million. Similar to the conventional toroidal cavities, power-independent operation is demonstrated. UV and temperature sensing experiments are also performed using the monolithically integrated microtoroid-waveguide system.

Universal quantum controlled phase gate on photonic qubits based on nitrogen vacancy centers and microcavity resonators

Here we investigate a physical implementation of the universal quantum controlled phase (CPHASE) gate operation on photonic qubits by using nitrogen vacancy (N-V) centers and microcavity resonators. The quantum CPHASE gate can be achieved by sending the photons through the microcavity and interacting with the N-V center. The proposed scheme can be further used for scalable quantum computation. We show that this technique provides us a deterministic source of cluster state generation on photonic qubits. In this scheme, only single photons and single N-V center are required and the proposed schemes are feasible with the current experimental technology.

All-optical switch and transistor gated by one stored photon

The realization of an all-optical transistor, in which one "gate" photon controls a "source" light beam, is a long-standing goal in optics. By stopping a light pulse in an atomic ensemble contained inside an optical resonator, we realized a device in which one stored gate photon controls the resonator transmission of subsequently applied source photons. A weak gate pulse induces bimodal transmission distribution, corresponding to zero and one gate photons. One stored gate photon produces fivefold source attenuation and can be retrieved from the atomic ensemble after switching more than one source photon. Without retrieval, one stored gate photon can switch several hundred source photons. With improved storage and retrieval efficiency, our work may enable various new applications, including photonic quantum gates and deterministic multiphoton entanglement.

Strong coupling between single atoms and nontransversal photons

Light is often described as a fully transverse-polarized wave, i.e., with an electric field vector that is orthogonal to the direction of propagation. However, light confined in dielectric structures such as optical waveguides or whispering-gallery-mode microresonators can have a strong longitudinal polarization component. Here, using single (85)Rb atoms strongly coupled to a whispering-gallery-mode microresonator, we experimentally and theoretically demonstrate that the presence of this longitudinal polarization fundamentally alters the interaction between light and matter.

Coupling a Single Trapped Atom to a Nanoscale Optical Cavity

Hybrid quantum devices, in which dissimilar quantum systems are combined in order to attain qualities not available with either system alone, may enable far-reaching control in quantum measurement, sensing, and information processing. A paradigmatic example is trapped ultracold atoms, which offer excellent quantum coherent properties, coupled to nanoscale solid-state systems, which allow for strong interactions. We demonstrate a deterministic interface between a single trapped rubidium atom and a nanoscale photonic crystal cavity. Precise control over the atom's position allows us to probe the cavity near-field with a resolution below the diffraction limit and to observe large atom-photon coupling. This approach may enable the realization of integrated, strongly coupled quantum nano-optical circuits.

Quantum state engineering with nitrogen-vacancy centers coupled to low-Q microresonator

We demonstrate efficient schemes of deterministic entanglement generation and quantum state transfer (QST) with the nitrogen-vacancy (NV) centers in diamond confined in separated microtoroidal resonators via single-photon input-output process. Assisted by the polarization of input photon pulse and the electron spin state of NV center, high fidelity NV center entangled states and photonic entangled states can be generated, respectively. The analyses of experimental feasibility show that our schemes work well with low quality resonators and weak coupling between qubits, which may be beneficial for exploring large-scale quantum information processing with diamond-based solid-state devices.

Atomic entanglement purification and concentration using coherent state input-output process in low-Q cavity QED regime

We investigate an atomic entanglement purification protocol based on the coherent state input-output process by working in low-Q cavity in the atom-cavity intermediate coupling region. The information of entangled states are encoded in three-level configured single atoms confined in separated one-side optical micro-cavities. Using the coherent state input-output process, we design a two-qubit parity check module (PCM), which allows the quantum nondemolition measurement for the atomic qubits, and show its use for remote parities to distill a high-fidelity atomic entangled ensemble from an initial mixed state ensemble nonlocally. The proposed scheme can further be used for unknown atomic states entanglement concentration. Also by exploiting the PCM, we describe a modified scheme for atomic entanglement concentration by introducing ancillary single atoms. As the coherent state input-output process is robust and scalable in realistic applications, and the detection in the PCM is based on the intensity of outgoing coherent state, the present protocols may be widely used in large-scaled and solid-based quantum repeater and quantum information processing.

Achieving maximum entanglement between two nitrogen-vacancy centers coupling to a whispering-gallery-mode microresonator

We investigate the entanglement generation between two nitrogen-vacancy (NV) centers in diamond nanocrystal coupled to a high-Q counterpropagating twin whispering-gallery modes (WGMs) of a microtoroidal resonator. For looking into the degree and dynamics of the entanglement, we calculate the concurrence using the microscopic master equation approach. The influences of the coupling strength between the WGMs (or the size of the two spherical NV centers), the distance between two NV centers, the frequency detuning between the NV center and microresonator, and the initial state of the system on the dynamics of concurrence are discussed in detail. It is found that the maximum entanglement between the two NV centers can be created by properly adjusting these controllable system parameters. Our results may provide further insight into future solid-state cavity quantum electrodynamics (CQED) system for quantum information engineering.

Cascaded two-photon nonlinearity in a one-dimensional waveguide with multiple two-level emitters

We propose and theoretically investigate a model to realize cascaded optical nonlinearity with few atoms and photons in one-dimension (1D). The optical nonlinearity in our system is mediated by resonant interactions of photons with two-level emitters, such as atoms or quantum dots in a 1D photonic waveguide. Multi-photon transmission in the waveguide is nonreciprocal when the emitters have different transition energies. Our theory provides a clear physical understanding of the origin of nonreciprocity in the presence of cascaded nonlinearity. We show how various two-photon nonlinear effects including spatial attraction and repulsion between photons, background fluorescence can be tuned by changing the number of emitters and the coupling between emitters (controlled by the separation).

Photon blockade in the ultrastrong coupling regime

We explore photon coincidence counting statistics in the ultrastrong coupling regime, where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. In this regime, usual normal order correlation functions fail to describe the output photon statistics. By expressing the electric-field operator in the cavity-emitter dressed basis, we are able to propose correlation functions that are valid for arbitrary degrees of light-matter interaction. Our results show that the standard photon blockade scenario is significantly modified for ultrastrong coupling. We observe parametric processes even for two-level emitters and temporal oscillations of intensity correlation functions at a frequency given by the ultrastrong photon emitter coupling. These effects can be traced back to the presence of two-photon cascade decays induced by counterrotating interaction terms.

Optical nonlinearity for few-photon pulses on a quantum dot-pillar cavity device

Giant optical nonlinearity is observed under both continuous wave and pulsed excitation in a deterministically coupled quantum dot-micropillar system, in a pronounced strong-coupling regime. Using absolute reflectivity measurements we determine the critical intracavity photon number as well as the input and output coupling efficiencies of the device. Thanks to a near-unity input-coupling efficiency, we demonstrate a record nonlinearity threshold of only 8 incident photons per pulse. The output-coupling efficiency is found to strongly influence this nonlinearity threshold. We show how the fundamental limit of single-photon nonlinearity can be attained in realistic devices, which would provide an effective interaction between two coincident single-photons.

Dynamical Autler-Townes control of a phase qubit

Routers, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmission.

Loss-enabled sub-poissonian light generation in a bimodal nanocavity

We propose an implementation of a source of strongly sub-poissonian light in a system consisting of a quantum dot coupled to both modes of a lossy bimodal optical cavity. When one mode of the cavity is resonantly driven with coherent light, the system will act as an efficient single photon filter, and the transmitted light will have a strongly sub-poissonian character. In addition to numerical simulations demonstrating this effect, we present a physical explanation of the underlying mechanism. In particular, we show that the effect results from an interference between the coherent light transmitted through the resonant cavity and the super-poissonian light generated by photon-induced tunneling. Peculiarly, this effect vanishes in the absence of the cavity loss.

Mechanical tuning of whispering gallery modes over a 0.5 THz tuning range with MHz resolution in a silica microsphere at cryogenic temperatures

We experimentally demonstrate the mechanical tuning of whispering gallery modes in a 40 μm diameter silica microsphere at 10K, over a tuning range of 450 GHz and with a resolution less than 10 MHz. This is achieved by mechanically stretching the stems of a double-stemmed silica microsphere with a commercially available piezo-driven nano-positioner. The large tuning range is made possible by the millimeter long slip-stick motion of the nano-positioner. The ultrafine tuning resolution, corresponding to sub-picometer changes in the sphere diameter, is enabled by the use of relatively long and thin fiber stems, which reduces the effective Poisson ratio of the combined sphere-stem system to approximately 0.0005. The mechanical tuning demonstrated here removes a major obstacle for the use of ultrahigh Q-factor silica microspheres in cavity QED studies of solid state systems and, in particular, cavity QED studies of nitrogen vacancy centers in diamond.

Whispering gallery modes formed by partial barriers in ultrasmall deformed microdisks

Unexpected formation of regular high-Q whispering gallery modes in a deformed microdisk where the radius is of the order of the vacuum wavelength is explained in terms of partial barriers in phase space. Using a semiclassical approach to determine the action flux of the partial barriers, we successfully predict spectral ranges in which the high-Q modes can exist. Our analysis enables optimization of emission directionality and the Q factor of deformed ultrasmall microcavities.

Invited review article: Single-photon sources and detectors

We review the current status of single-photon-source and single-photon-detector technologies operating at wavelengths from the ultraviolet to the infrared. We discuss applications of these technologies to quantum communication, a field currently driving much of the development of single-photon sources and detectors.

Two-photon scattering by a driven three-level emitter in a one-dimensional waveguide and electromagnetically induced transparency

We study correlated two-photon transport in a (quasi-)one-dimensional photonic waveguide coupled to a three-level Λ-type emitter driven by a classical light field. Two-photon correlation is much stronger in the waveguide for a driven three-level emitter (3LE) than a two-level emitter. The driven 3LE waveguide shows electromagnetically induced transparency (EIT), and we investigate the scaling of EIT for one and two photons. We show that the two transmitted photons are bunched together at any distance separation when energy of the incident photons meets "two-photon resonance" criterion for EIT.

A composite microcavity of diamond nanopillar and deformed silica microsphere with enhanced evanescent decay length

We report the experimental realization of a composite microcavity system, in which negatively-charged nitrogen vacancy (NV) centers in diamond nanopillars couple evanescently to whispering-gallery modes (WGMs) in a deformed, non-axisymmetric silica microsphere. We show that the deformed microsphere can feature an evanescent decay length four times larger than that of a regular silica microsphere. With the enhanced evanescent coupling, WGMs can in principle couple to NV centers that are 100 to 200 nm beneath the diamond pillar surface, providing a promising avenue for exploring evanescently-coupled cavity QED systems of NV centers in ultrahigh purity diamond.

Coherent transport of nanowire surface plasmons coupled to quantum dots

The coherent transport of surface plasmons with nonlinear dispersion relations on a metal nanowire coupled to two-level emitters is investigated theoretically. Real-space Hamiltonians are used to obtain the transmission and reflection spectra of the surface plasmons. For the single-dot case, we find that the scattering spectra can show completely different features due to the non-linear quadratic dispersion relation. For the double-dot case, we obtain the interference behavior in transmission and reflection spectra, similar to that in resonant tunneling through a double-barrier potential. Moreover, Fano-like line shape of the transmission spectrum is obtained due to the quadratic dispersion relation. All these peculiar behaviors indicate that the dot-nanowire system provides a onedimensional platform to demonstrate the bandgap feature widely observed in photonic crystals.

Rigorous criterion for characterizing correlated multiphoton emissions

Strong correlation of photons, particularly in the single-photon regime, has recently been exploited for various applications in quantum information processing. Existing correlation measurements, however, do not fully characterize multi-photon correlation in a relevant context and may pose limitations in practical situations. We propose a conceptually rigorous, but easy-to-implement, criterion for detecting correlated multi-photon emission out of a quantum optical system, drawn from the context of wavefunction collapse. We illustrate the robustness of our approach against experimental limitations by considering an anharmonic optical system.

Spontaneous dressed-state polarization in the strong driving regime of cavity QED

We utilize high-bandwidth phase-quadrature homodyne measurement of the light transmitted through a Fabry-Perot cavity, driven strongly and on resonance, to detect excess phase noise induced by a single intracavity atom. We analyze the correlation properties and driving-strength dependence of the atom-induced phase noise to establish that it corresponds to the long-predicted phenomenon of spontaneous dressed-state polarization. Our experiment thus provides a demonstration of cavity quantum electrodynamics in the strong-driving regime in which one atom interacts strongly with a many-photon cavity field to produce novel quantum stochastic behavior.

Giant Kerr nonlinearities in circuit quantum electrodynamics

The very small size of optical nonlinearities places strict restrictions on the types of novel physics one can explore. This work describes how a single artificial multilevel Cooper pair box molecule, interacting with a superconducting microwave coplanar resonator, when suitably driven, can generate extremely large optical nonlinearities at microwave frequencies, with no associated absorption. We describe how the giant self-Kerr effect can be detected by measuring the second-order correlation function and quadrature squeezing spectrum.

Van der Waals enhancement of optical atom potentials via resonant coupling to surface polaritons

Contemporary experiments in cavity quantum electrodynamics (cavity QED) with gas-phase neutral atoms rely increasingly on laser cooling and optical, magneto-optical or magnetostatic trapping methods to provide atomic localization with sub-micron uncertainty. Difficult to achieve in free space, this goal is further frustrated by atom-surface interactions if the desired atomic placement approaches within several hundred nanometers of a solid surface, as can be the case in setups incorporating monolithic dielectric optical resonators such as microspheres, microtoroids, microdisks or photonic crystal defect cavities. Typically in such scenarios, the smallest atom-surface separation at which the van der Waals interaction can be neglected is taken to be the optimal localization point for associated trapping schemes, but this sort of conservative strategy generally compromises the achievable cavity QED coupling strength. Here we suggest a new approach to the design of optical dipole traps for atom confinement near surfaces that exploits strong surface interactions, rather than avoiding them, and present the results of a numerical study based on (39)K atoms and indium tin oxide (ITO). Our theoretical framework points to the possibility of utilizing nanopatterning methods to engineer novel modifications of atom-surface interactions.

Ultralow-power four-wave mixing with Rb in a hollow-core photonic band-gap fiber

We demonstrate extremely efficient four-wave mixing with gains greater than 100 at microwatt pump powers and signal-to-idler conversion of 50% in Rb vapor confined to a hollow-core photonic band-gap fiber. We present a theoretical model that demonstrates such efficiency is consistent with the dimensions of the fiber and the optical depths attained. This is, to our knowledge, the largest four-wave mixing gain observed at such low total pump powers and the first demonstrated example of four-wave mixing in an alkali-metal vapor system with a large (approximately 30 MHz) ground state decoherence rate.