Green Vertical‑Cavity Surface‑Emitting Lasers Based on InGaN Quantum Dots and Short Cavity

Room temperature low threshold lasing of green GaNbased vertical cavity surface emitting laser(VCSEL)was demonstrated under continuous wave(CW)operation.By using self-formed InGaN quantum dots(QDs)as the active region,the VCSEL emitting at 524.0 nm has a threshold current density of 51.97 A cm^(-2),the lowest ever reported.The QD epitaxial wafer featured with a high IQE of 69.94%and theδ-function-like density of states plays an important role in achieving low threshold current.Besides,a short cavity of the device(~4.0λ)is vital to enhance the spontaneous emission coupling factor to 0.094,increase the gain coefficient factor,and decrease the optical loss.To improve heat dissipation,AlN layer was used as the current confinement layer and electroplated copper plate was used to replace metal bonding.The results provide important guidance to achieving high performance GaN-based VCSELs.

Preparation of entangledW states based on the cavity QED system

We present a qubit-loss-free(QLF)fusion scheme for generating large-scale atom W states in cavity quantum electrodynamics(QED)system.Compared to the most current fusion schemes which are conditioned on the case where one particle can be extracted from each initial W state to the fusion process,our scheme will access one or two particles from each W state.Based on the atom–cavity-field detuned interaction,three jWin+m+t states can be generated from the jWin,jWim,and jWit states with the help of two auxiliary atoms,and three jWin+m+t+q states can be generated from jWin,jWim,jWit,and a jWiq state with the help of three auxiliary atoms.Comparing the numerical simulations of the resource cost of fusing three small-size W states based on the previous schemes,our fusion scheme seems to be more efficient.This QLF fusion scheme can be generalized to the case of fusing k different or identical particle W states.Furthermore,with no qubit loss,it greatly reduces the number of fusion steps and prepares W states with larger particle numbers.

A Single Mode980 nm In Ga As/Ga As/Al Ga As Large Optical Cavity Quantum Well Laser with Low Vertical Divergence Angle

To achieve high optical power as well as low vertical divergence angle,a new kind of optim ized large opti- cal cavity (L OC) structure is applied to a ridge waveguide 980 nm In Ga As/ Ga As/ Al Ga As m ulti- quantum well laser.The optical power density in the waveguide is successfully reduced.The maxim um output power is more than 40 0 m W with a slope efficiency of 0 .89W/ A and the far- field vertical divergence angle is lowered to 2 3°.

Quantum logic gates operation using SQUID qubits in bimodal cavity

We present a scheme to realize the basic two-qubit logic gates such as the quantum phase gate and SWAP gate using a detuned microwave cavity interacting with three-level superconducting-quantum-interference-device （SQUID） qubit（s）, by placing SQUID（s） in a two-mode microwave cavity and using adiabatic passage methods. In this scheme, the two logical states of the qubit are represented by the two lowest levels of the SQUID, and the cavity fields are treated as quantized. Compared with the previous method, the complex procedures of adjusting tile level spacing of the SQUID and applying the resonant microwave pulse to the SQUID to create transformation are not required. Based on superconducting device with relatively long decoherence time and simplified operation procedure, the gates operate at a high speed, which is important in view of decoherence.

Quantum dynamic behaviour in a coupled cavities system

The dynamic behaviour of the two-site coupled cavities model which is doped with ta wo-level system is investi-gated. The exact dynamic solutions in the general condition are obtained via Laplace transform. The simple analytical solutions are obtained in several particular cases, which demonstrate the clear and simple physical picture for the quan-tum state transition of the system. In the large detuning or hoppling case, the quantum states transferring between qubits follow a slow periodic oscillation induced by the very weak excitation of the cavity mode. In the large coupling case, the system can be interpreted as two Jaynes-Cummings model subsystems which interact through photon hop between the two cavities. In the case of λ≈△〉〉 g, the quantum states transition of qubits is accompanied by the excitation of the cavity, and the cavity modes have the same dynamic behaviours and the amplitude of probability is equM to 0.25 which does not change with the variation of parameter.

Scheme for implementing quantum dense coding with W-class state in cavity QED

An experimentally feasible protocol for realizing dense coding by using a class of W-state in cavity quantum electrodynamics （QED） is proposed in this paper. The prominent advantage of our scheme is that the successful probability of the dense coding with a W-class state can reach 1. In addition, the scheme can be implemented by the present cavity QED techniques.

A scheme for conditional quantum phase gate via bimodal cavity and a∧-type three-level atom

We propose a scheme to implement a two-qubit conditional quantum phase gate for the intracavity field via a single three-level ∧-type atom driven by two modes in a high-Q cavity. The quantum inforraation is encoded on the Fock states of the bimodal cavity. The gate＇s averaged fidelity is expected to reach 99.8%.

A scheme for transferring an unknown atomic entangled state via cavity quantum electrodynamics

In this paper, we propose a scheme for transferring an unknown atomic entangled state via cavity quantum electrodynamics （QED）. This scheme, which has a successful probability of 100 percent, does not require Bell-state measurement and performing any operations to reconstruct an initial state. Meanwhile, the scheme only involves atomfield interaction with a large detuning and does not require the transfer of quantum information between the atoms and cavity. Thus the scheme is insensitive to the cavity field states and cavity decay. This scheme can also be extended to transfer ring an entangled state of n-atom.

Theoretical analysis of quantum game in cavity QED

Recent years, several ways of implementing quantum games in different physical systems have been presented. In this paper, we perform a theoretical analysis of an experimentally feasible way to implement a two player quantum game in cavity quantum electrodynamic（QED）. In the scheme, the atoms interact simultaneously with a highly detuned cavity mode with the assistance of a classical field. So the scheme is insensitive to the influence from the cavity decay and the thermal field, and it does not require the cavity to remain in the vacuum state throughout the procedure.

Quantum state transfer between atomic ensembles trapped in separate cavities via adiabatic passage

We propose a new approach for quantum state transfer（QST） between atomic ensembles separately trapped in two distant cavities connected by an optical fiber via adiabatic passage. The three-level Λ-type atoms in each ensemble dispersively interact with the nonresonant classical field and cavity mode. By choosing appropriate parameters of the system, the effective Hamiltonian describes two atomic ensembles interacting with ＂the same cavity mode＂ and has a dark state. Consequently, the QST between atomic ensembles can be implemented via adiabatic passage. Numerical calculations show that the scheme is robust against moderate fluctuations of the experimental parameters. In addition, the effect of decoherence can be suppressed effectively. The idea provides a scalable way to an atomic-ensemble-based quantum network, which may be reachable with currently available technology.

Scheme for splitting quantum information via W states in cavity QED systems

Assisted by multipartite entanglement, Quantum information may be split so that the original qubit can be reconstructed if and only if the recipients cooperate. This paper proposes an experimentally feasible scheme for splitting quantum information via W-type entangled states in cavity QED systems, where three-level Rydberg atoms interact with nonresonant cavities. Since W-type states are used as the quantum channel and the cavities are only virtually excited, the scheme is easy to implement and robust against decoherence, and the dependence on the quality factor of the cavities is greatly reduced.

Quantum discord dynamics of two qubits in single-mode cavities

The dynamics of quantum discord for two identical qubits in two independent single-mode cavities and a common single-mode cavity are discussed. For the initial Bell state with correlated spins, while the entanglement sudden death can occur, the quantum discord vanishes only at discrete moments in the independent cavities and never vanishes in the common cavity. Interestingly, quantum discord and entanglement show opposite behavior in the common cavity, unlike in the independent cavities. For the initial Bell state with anti-correlated spins, quantum discord and entanglement behave in the same way for both independent cavities and a common cavity. It is found that the detunings always stabilize the quantum discord.

Dynamic properties of atomic collective decay in cavity quantum electrodynamics

We theoretically study the collective decay of two atoms trapped in a single mode cavity and we describe the evolution of the population of Dicke states. We show that the collective decay property is strongly dependent on the phase of atomic radiation and the speeding up of collective decay can only be observed in a bad cavity regime. For in-or out-phase case,this occurs due to the quantum interference enhancement, no matter which atom is excited initially. For π/2 phase, the speeding up of collective decay takes place if the first atom is excited at the beginning. However, it disappears due to the quantum interference cancellation if the second atom is excited. Compared with the in-phase and out-phase cases,we also show that the speeding up of collective decay can be significantly enhanced in strong coupling regime for π/2 phase, although one atom is decoupled to the cavity in this condition. The study presented here is helpful to understand the physical mechanism of collective decay in cavity quantum electrodynamics and it provides a useful method to control the collective decay phenomenon via quantum interference effect.

Broadband tunable external cavity laser using a bent-waveguide quantum-dot superluminescent diode as gain device

A broadband tunable grating-coupled external cavity laser is realized by employing a self-assembled InAs/GaAs quantum-dot （QD） superluminescent diode （SLD） as the gain device. The SLD device is processed with a bent-waveguide structure and facet antireflection （AR） coating. Tuning bandwidths of 106 nm and 117 nm are achieved under a-A and 3.5-A injection currents, respectively. The large tuning range originates essentially from the broad gain spectrum of self-assembled QDs. The bent waveguide structure combined with the facet AR coating plays a role in suppressing the inner-cavity lasing under a large injection current.

Efficient scheme for realizing quantum dense coding with GHZ state in separated low-Q cavities

We propose an efficient scheme for realizing quantum dense coding with three-particle GHZ state in separated low-Q cavities. In this paper, the GHZ state is first prepared with three atoms trapped, respectively, in three spatial separated cavities. Meanwhile, with the assistance of a coherent optical pulse and X-quadrature homodyne measurement, we can im- plement quantum dense coding with three-particle GHZ state with a higher probability. Our scheme can also be generalized to realize N-particle quantum dense coding.

Micropillar Cavity Design for 1.55-μm Quantum-Dot Single-Photon Sources

The 1.55-μm quantum-dot (QD) micropillar cavities are strongly required as single photon sources (SPSs) for silica-fiber-based quantum information processing. Theoretical analysis shows that the adiabatic distributed Bragg reflector (DBR) structure may greatly improve the quality of a micropillar cavity. An InGaAsP/InP micropillar cavity is originally difficult, but it becomes more likely usable with inserted tapered (thickness decreased towards the center) distributed DBRs. Simulation turns out that, incorporating adiabatically tapered DBRs, a Si/SiO2- InP hybrid micropillar cavity, which enables weakly coupling InAs/InP quantum dots (QDs), can even well satisfy strong coupling at a smaller diameter. Certainly, not only the tapered structure, other adiabatic designs, e.g., both DBR layers getting thicker and one thicker one thinner, also improve the quality, reduce the diameter, and degrade the fabrication difficulty of Si/SiO2-InP hybrid micropillar cavities. Furthermore, the problem of the thin epitaxial semiconductor layer can also be greatly resolved by inserting adiabatic InGaAsP/InP DBRs. With tapered DBRs, the InGaAsP/InP-air-aperture micro-pillar cavity serves as an efficient, coherent, and monolithically producible 1.55-μm single-photon source (SPS). The adiabatic design is thus an effective way to obtain prospective candidates for 1.55-μm QD SPSs.

Scheme for implementing quantum secret sharing via cavity QED

An experimentally feasible scheme for implementing quantum secret sharing via cavity quantum electrodynamics （QED） is proposed. The scheme requires the large detuning of the cavity field from the atomic transition, the cavity is only virtually excited, thus the requirement on the quality factor of the cavity is greatly loosened.

Three-qubit Fredkin gate based on cavity quantum electrodynamics

This paper presents a scheme for implementing a Fredkin gate on three modes of a cavity. The scheme is based on the dispersive atom-cavity interaction. By modulating the cavity frequency and the atomic transition frequency appropriately, it obtains the effective form of nonlinear interaction between photons in the three-mode cavity. This availability is testified via numerical analysis. It also considers both the situations with and without dissipation.

An Alternative Scheme for Transferring Quantum States and Preparing a Quantum Network in Cavity QED

An alternative scheme is proposed to transfer quantum states and prepare a quantum network in cavity QED. It is based on the interaction of a two-mode cavity field with a three-level V-type atom. In the scheme, the atom-cavity field interaction is resonant, thus the time required to complete the quantum state transfer process is greatly shortened, which is very important in view of decoherence. Moreover, the present scheme does not require one mode of the cavities to be initially prepared in one-photon state, thus it is more experimentally feasible than the previous ones.

Scheme for Implementation of Quantum Game in Cavity QED

We propose an experimentally feasible scheme to implement two-player quantum game in cavity quantum electrodynamics （QED）. During the process, the cavity is only virtually excited, thus our scheme is insensitive to the cavity field states and cavity decay. The scheme can be realized in the range of current cavity QED techniques.