Shaoyan Gao

Dissipative preparation of entangled states between two spatially separated nitrogen-vacancy centers

We present a novel scheme for the generation of entangled states of two spatially separated nitrogen-vacancy (NV) centers with two whispering-gallery-mode (WGM) microresonators, which are coupled either by an optical fiber-taper waveguide, or by the evanescent fields of the WGM. We show that, the steady state of the two NV centers can be steered into a singlet-like state through a dissipative quantum dynamical process, where the cavity decay plays a positive role and can help drive the system to the target state. The protocol may open up promising perspectives for quantum communications and computations with this solid-state cavity quantum electrodynamic system.

Hybrid Quantum Device Based on N V Centers in Diamond Nanomechanical Resonators Plus Superconducting Waveguide Cavities

We propose and analyze a hybrid device by integrating a microscale diamond beam with a single built-in nitrogen-vacancy (NV) center spin to a superconducting coplanar waveguide (CPW) cavity. We find that under an ac electric field the quantized motion of the diamond beam can strongly couple to the single cavity photons via dielectric interaction. Together with the strong spin-motion interaction via a large magnetic field gradient, it provides a hybrid quantum device where the dia- mond resonator can strongly couple both to the single microwave cavity photons and to the single NV center spin. This enables coherent information transfer and effective coupling between the NV spin and the CPW cavity via mechanically dark polaritons. This hybrid spin-electromechanical de- vice, with tunable couplings by external fields, offers a realistic platform for implementing quantum information with single NV spins, diamond mechanical resonators, and single microwave photons.

Robust continuous-variable entanglement of microwave photons with cavity electromechanics

We investigate the controllable generation of robust photon entanglement with a circuit cavity electromechanical system, consisting of two superconducting coplanar waveguide cavities (CPWCs) capacitively coupled by a nanoscale mechanical resonator (MR). We show that, with this electromechanical system, two-mode continuous-variable entanglement of cavity photons can be engineered deterministically either via coherent control on the dynamics of the system, or through a dissipative quantum dynamical process. The first scheme, operating in the strong coupling regime, explores the excitation of the cavity Bogoliubov modes, and is insensitive to the initial thermal noise. The second one is based on the reservoir-engineering approach, which exploits the mechanical dissipation as a useful resource to perform ground state cooling of two delocalized cavity Bogoliubov modes. The achieved amount of entanglement in both schemes is determined by the relative ratio of the effective electromechanical coupling strengths, which thus can be tuned and made much lager than that in previous studies.

Geometrical parameters controlled focusing and enhancing near field in infinite circular metal-dielectric multilayered cylinder

The plasmon resonance-induced near electric field focusing and enhancement of three-layered silver nano-cylinder has been studied by quasi-static electricity. A field enhancement factor of more than 102 times can be obtained in the middle dielectric wall between the inner silver wire and outer tube around the resonance wavelengths of 400–500 nm. Because of the anti-symmetric coupling between the bonding tube plasmon and the wire plasmon, the incident electric field could be fine focused between the two metallic surfaces by decreasing the middle wall thickness. As a result of the curvature-dependent surface charge concentration, thinner dielectric wall with small diameter provides stronger local field enhancement. It provides the potential applications of plamonic nano-structures for high-density and high-contrast optical data storage under the diffraction limit.

Engineering two-mode entangled states between two superconducting resonators by dissipation

We present an experimental feasible scheme to synthesize two-mode continuous-variable entangled states of two superconducting resonators that are interconnected by two gap-tunable superconducting qubits. We show that, with each artificial atom suitably driven by a bichromatic microwave field to induce sidebands in the qubit-resonator coupling, the stationary state of the photon fields in the two resonators can be cooled and steered into a two-mode squeezed vacuum state via a dissipative quantum dynamical process, while the superconducting qubits remain in their ground states. In this scheme the qubit decay plays a positive role and can help drive the system to the target state, which thus converts a detrimental source of noise into a resource.

Two-photon scattering by a cavity-coupled two-level emitter in a one-dimensional waveguide

Abstract We show that two-photon transport can be modulated by a two-level emitter coupled to a cavity in a one-dimensional waveguide. In the ordinary case, the transmitted light has a wider frequency spectrum than the situation without the cavity because it is reflected and scattered many times. But when the two photons are resonant with the cavity resonance reflection frequency, the frequency spectrum of the transmitted light becomes narrower than that without the cavity. This means that properly tuning the cavity resonance frequency can improve the photon–photon interaction. In addition, we show that the two-photon intensity correlation functions are nearly opposite to each other at the two sides of the emitter transition frequency rather than the same, which is exactly the Fano resonance line shape for two photons. Such an effect is important for lowering the power threshold in optical bistable devices and for sensing applications. When the emitter transition frequency equals to the cavity resonance frequency for a high-Q cavity, our results agree with the recent experiments and theories.

Coherent frequency down-conversions and entanglement generation in a Sagnac interferometer

A waveguide loop coupled to two external line waveguides by a 50/50 beam splitter forms a Sagnac interferometer. We consider the situation where two Λ-type three-level emitters are symmetrically coupled to the loop of a Sagnac interferometer and a single photon is input through one end of the line waveguides. Since the incoming photon is always in a superposition of the clockwise and counterclockwise modes of the loop and the two emitters are positioned symmetrically with respect to the input port of photon, the processes of photon scattering at the two emitters are symmetric and coherent. When the separation of the emitters and the coupling strengths of the emitters with the waveguide loop take some special values, due to quantum interference, a frequency down-conversion can certainly happen at one of the two emitters during the photon scattering but one cannot know at which emitter the frequency down-conversion takes place. This indistinguishability of the coherent frequency down-conversion processes can result in the generation of the symmetric or antisymmetric two-qubit maximally entangled states of the emitters. In the present scheme, a single photon comes in and goes out of the waveguide loop, and no photon localization modes exists. The entangled states result from the coherent frequency down-conversion processes of the emitters. Thus, the resulting entangled states are stable if the two lower-lying states of the emitters have no decay. We also investigate the influence of the dissipation of the emitters and the finite bandwidth of an input photon wavepacket on the success probability of entanglement generation, and find that the present scheme is robust to these effects and feasible with current available technologies.

Two-mode squeezing generation in hybrid chains of superconducting resonators and nitrogen-vacancy-center ensembles

We consider a hybrid quantum system consisting of two independent chains of nearest-neighbor linearly interacting superconducting transmission-line resonators, each of which a nitrogen-vacancy-center ensemble (NVE) is magnetically coupled to. In addition, the first-beginning pair of the resonators is locally driven by a two-mode microwave squeezed bath. We show that in the steady state a series of NVE pairs in the up and down chains is in a two-mode spin squeezed state. In the low excitation limit, collective excitation modes of the up- and down- NVE pairs are in the two-mode squeezed state of the driven field, i.e., realizing a perfect squeezed state replication.

Quantum information transfer with hybrid NV center-photon qubit encoding

We propose a scheme to perform quantum information transfer based on hybrid spin-photon qubit encoding in superconducting quantum circuits. The hybrid qubit consists of a nitrogen-vacancy center spin ensemble coherently coupled to a microwave photon confined in a frequency tunable superconducting co-planar waveguide cavity. The spin ensemble-cavity hybrid system forms quantum nodes capable of sending, receiving, storing, and releasing photonic quantum information by the hybrid encoding. Quantum information transfer between distinct nodes is achieved by the coherent exchange of a single photon. We show the faithful transfer of an arbitrary superposition hybrid quantum state between two separate identical nodes via coherent control on the system’s dynamics. The protocol may have interesting applications in quantum networking with this hybrid spin-photon structure.

Coherent control of one-photon and two-photon optical fluorescence channels in three-level ladder system

We consider a three-level atomic system interacting with two coherent fields in the ladder configuration. Namely, the pumping field couples the ground and first excited state while the driving field interacts with the two excited states. The intensity spectrum of resonant optical fluorescence at the two-photon transition is studied. It is shown that coherent driving in this scheme allows for efficient control of fluorescence distribution between the one-photon (elastic with respect to pumping field) and two-photon (anti-Stoke Raman with respect to the driving field) channels.