Quantum phase gate in decoherence-free subspace with cavity QED system

We demonstrate how to perform quantum phase gate with cavity QED system in decoherence-free subspace by using only linear optics elements and photon detectors. The qubits are encoded in the singlet state of the atoms in cavities among spatially separated nodes, and the quantum interference of polarized photons decayed from the optical cavities is used to realized the desired quantum operation among distant nodes. In comparison with previous schemes, the distinct advantage is that the gate fidelity could not only resist collective noises, but also immune from atomic spontaneous emission, cavity decay, and imperfection of the photodetectors. We also discuss the experimental feasibility of our scheme.     

Schemes for preparing atomic qubit cluster states in cavity QED

Two schemes are proposed for generating atomic qubits cluster states in cavity quantum electrodynamics (QED). In the first scheme, only two-atom-cavity interactions are involved, and cluster states can be directly generated by using constructed two-qubit controlled phase gates. The second scheme needs the assistance of additional single-qubit rotations, but takes less time than the first one for two-atom operations in the cavity. In this scheme, two projective operators are constructed to prepare two-dimension or more complicated configurations of cluster states. Both schemes are insensitive to the cavity decay due to the fact that the cavity is only virtually excited during the interaction between atoms and the cavity. The idea can also be applied to the ion trap system.     

A phase sensitive optical rotation measurement in a scattered chiral medium using a Zeeman laser

A novel circular polarized optical heterodyne interferometer using a Zeeman laser to measure optical rotation both in nonscattered and scattered chiral medium is proposed. A pair of correlated orthogonal circular polarized light waves of different temporal frequency propagating in the chiral medium at different speed is studied. This results in phase retardation between circular polarized light waves of which the phase difference is proportional to the optical rotation angle of a linear polarized light in a chiral medium. In the mean time, two orthogonal circular polarized light waves can be treated as a circular polarized photon pair that is able to reduce the scattering effect in a scattered chiral medium. Then the optical rotation angle can be measured in the scattering medium. In addition, a common-path configuration with respect to circular polarized light waves immune the background noise. This further improves the sensitivity on optical rotation measurement based on phase difference detection.     

Entanglement-assisted quantum logic gates for two remote qubits

We propose a protocol to realize quantum logic gates for two remote qubits via entanglement swapping. According to the scheme of quantum repeater presented by H.-J. Briegel et al., we can complete long-distance communication and computation. Compared with previous schemes through noisy channels, our protocol can overcome the limitation that error probability scales exponentially with the length of the channel. We illustrate this protocol in cavity QED system, but the idea can also be realized in other physical systems.     

偏振光学系统中相位延迟机理及其应用

为了实现偏振编码的自由空间量子密钥分发实验,研制了偏振保持的光机系统,并对该系统所采用的相位延迟传输机理及应用进行了研究,建立了偏振误码率在允许范围内的量子链路.首先,采用矩阵光学理论对偏振光的方位角、相位延迟与消光比的关系进行了介绍.接着,通过矩阵光学理论及实验验证了偏振光学系统的相位延迟线性叠加原理.然后,在相位延迟线性叠加原理的基础上,设计了一套偏振保持光学系统,并通过理论分析及实验验证了此系统具备良好的偏振保持效果.最后,将偏振保持光学系统的设计机理应用于量子通信光机系统的设计之中,并取得了良好的设计效果.实验结果表明:相位之间的相互抵消可以有效地进行偏振保持设计,最终设计的量子通信光机系统的偏振消光比优于500∶1.满足了自由空间量子通信实验中对偏振误码率的要求.     

Robust schemes for the generation of multipartite entanglement for remote atoms with cross-Kerr nonlinearity

Based on the interference effect of indistinguishable polarized photons leaking out of separated cavities with each atom trapped in separate cavity, using quantum nondemolition detection, we propose the robust schemes for the generation of N-atom GHZ state, three-atom W state and four-atom cluster state with a certain success probability. In Lamb-Dicke limit, the schemes do not require the simultaneous click of the photon-detectors. These made the schemes more realizable in experiments. Meanwhile, the advantage of the scheme is that the fidelity of the entangled states is not affected by the atomic spontaneous, cavity decay, and imperfection of the photodetectors. The schemes would be useful steps towards long-distance quantum communication.     

Phase‐Dependent Quantum Correlation in a Cavity‐Atom System

A scheme to manipulate quantum correlation between output lights of a cavity‐atom system by phase control is proposed. A driving‐field phase is introduced which has a similar value with that of building up quantum correlation in a Hanbury–Brown–Twiss setup. A closed‐loop phase is formed to improve quantum coherence by phase‐dependent electromagnetically induced transparency. The closed‐loop phase has been utilized to realize quantum correlation and even quantum entanglement in the atomic system of previous work. With these two phases, a steady and maximum quantum correlation has been obtained in the scheme here. Moreover, the maximum quantum correlation is free to decoherence of this cavity‐atom system. The study on field‐intensity correlation (quantum correlation) has potential applications on correlated imaging, image encryption transmission, and the improvement of noise resistance in a quantum network.     

Quantum computation and entangled state generation through a cavity output process

We propose a protocol to realize quantum phase gates and generate entangled states between two atoms trapped in one cavity. In Lamb-Dick limits, it is not necessary to require coincidence detections, which will relax the conditions for the experimental realization. The protocol can be generalized to generate N-atom entangled states.     

Resonant interaction scheme for GHZ state preparation and quantum phase gate with superconducting qubits in a cavity

A scheme is proposed to generate GHZ state and realize quantum phase gate for superconducting qubits placed in a microwave cavity. This scheme uses resonant interaction between the qubits and the cavity mode, so that the interaction time is short, which is important in view of decoherence. In particular, the phase gate can be realized simply with a single interaction between the qubits and the cavity mode. With cavity decay being considered, the fidelity and success probability are both very close to unity.     

基于极化旋转超表面的圆极化天线设计

本文通过设计出一种反射型极化旋转超表面,在8—12 GHz频域内实现高效的极化旋转,并将其加载于微带缝隙天线下方构成新型的极化旋转超表面天线,利用超表面的90°极化旋转效应,成功实现了天线的圆极化辐射调制.仿真与实验结果表明:圆极化天线的中心工作频率为GHz,阻抗带宽为8.3—10 GHz.当微带缝隙天线与极化旋转超表面的间距H=4.5mm时,天线在8.3—8.8 GHz频带内实现了圆极化辐射;当mm时,天线在8.8—9.3 GHz频带内实现了圆极化辐射;当=8mm时,天线在9.3—10 GHz频带内实现了圆极化辐射.实验结果与仿真结果相符,证明了此种设计方法的有效性,也为微带缝隙天线的圆极化设计提供了一种新的途径.     

Schemes of Implementing a Two-Atom Iswap Gate via Cavity QED

We propose two schemes for implementing a two-atom quantum iswap gate via cavity QED. In the first scheme, only one cavity and two classical fields are used to realizing iswap gate and thus reducing the operating time. The second scheme needs three cavities for atom-cavity interaction and single-qubit rotations without the assistant of the auxiliary level. Our schemes can effectively reduce the cavity decay owing to the reason that the cavity is only virtually excited during the interaction process between atoms and the cavity. Operator fidelity is applied to analyzing the influence when two atoms enter a cavity without accurate simultaneity.     

超导量子比特的耦合研究进展

超导量子比特以其在可控性、低损耗以及可扩展性等方面的优势被认为是最有希望实现量子计算机的固态方式之一.量子比特之间的相干可控耦合是实现大规模的量子计算的必要条件.本文介绍了超导量子比特耦合方式的研究进展,包括利用电容或电感实现量子比特的局域耦合,着重介绍一维传输线谐振腔作为量子总线实现多个量子比特的可控耦合的电路量子电动力学体系,并对最新的三维腔与超导量子比特的耦合结构的研究进展进行了论述.对各种耦合体系的哈密顿量进行了比较详细的分析,并按照局域性和可控性对不同耦合机制进行了分类.     

Optical control of spin-dependent thermal transport in a quantum ring

We report on calculation of spin-dependent thermal transport through a quantum ring with the Rashba spin-orbit interaction. The quantum ring is connected to two electron reservoirs with different temperatures. Tuning the Rashba coupling constant, degenerate energy states are formed leading to a suppression of the heat and thermoelectric currents. In addition, the quantum ring is coupled to a photon cavity with a single photon mode and linearly polarized photon field. In a resonance regime, when the photon energy is approximately equal to the energy spacing between two lowest degenerate states of the ring, the polarized photon field can significantly control the heat and thermoelectric currents in the system. The roles of the number of photon initially in the cavity, and electron–photon coupling strength on spin-dependent heat and thermoelectric currents are presented.     

Scheme for concentration of the unknown GHZ entangled states

We propose two schemes for concentrating unknown nonmaximally tripartite GHZ entangled states via cavity quantum electrodynamics (QED) techniques. The finial pure states obtained from the two schemes are shared by two or three parties. Our schemes only require large-detuned interaction between two driven atoms and the quantized cavity mode, which is insensitive to both the cavity decay and thermal field, thus the schemes are well within current experimental technology.     

非阿贝尔腔量子电动力学模型下偏振光场的影响

通过使用场正交算符,而不是传统的玻色算符,研究了非阿贝尔腔量子电动力学(QED)模型中原子和偏振光场的相互作用。讨论了初始双模偏振光场对于原子布居数反转以及偏振光场的压缩特性的影响。结果表明,原子布居数反转的演化不仅与偏振椭圆的相位角有关,也与偏振椭圆的椭率角有关;只有当偏振椭圆是右旋圆偏振光时,原子布居数反转随时间的演化基本不变,趋近于初始值0,而当偏振椭圆是左旋圆偏振光时,原子布居数反转随时间的演化呈现周期性的崩塌复苏变化。另外,当初始光场是左旋圆偏振光时,光场可以出现周期性的压缩;而当初始光场是右旋圆偏振光时,光场的压缩不会持续出现。     

基于低Q腔光子Faraday旋转的远程态制备

基于低Q腔中单光子的输入与输出关系,提出了利用偏振光Faraday旋转分别遥远制备单原子态和两原子纠缠态的可行方案.研究结果表明,当初始原子态的系数为实数时,通过选择合适的偏振光、腔场与原子相互作用系统的参数,单原子态与两原子纠缠态的远程制备均可确定性地得以实现.与以前的原子态远程制备方案相比,本文方案采用光子作为飞行比特来传递量子信息,故原则上可实现原子态的真正长距离制备.由于原子态的信息编码在耗散单边腔囚禁的Λ型三能级原子的两个基态能级,且原子仅虚激发,因此本文方案对腔衰减和原子自发辐射不敏感.此外,本文所提出的两种方案不需要两体或多体正交测量,仅涉及单体直积态测量,而且两种方案都工作在低Q腔,不需要原子与光腔的强耦合,从而有效降低了实验难度.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity. We also find that the stationary quantum discord can be increased by applying a classical driving field. Furthermore, we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence. Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Controlling quantum discord dynamics in cavity QED systems by applying a classical driving field with phase decoherence

We investigate a two-level atom interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence and find that a stationary quantum discord can arise in the interaction of the atom and cavity field as the time turns to infinity.We also find that the stationary quantum discord can be increased by applying a classical driving field.Furthermore,we explore the quantum discord dynamics of two identical non-interacting two-level atoms independently interacting with a quantized cavity field and a classical driving field in the presence of phase decoherence.Results show that the quantum discord between two atoms is more robust than entanglement under phase decoherence and the classical driving field can help to improve the amount of quantum discord of the two atoms.     

Superintense laser fields in circular array: effects of phase and pulse jitters

Extremely intense laser field that makes nonlinear quantum vacuum can be generated by coherent superposition of multiple lasers in circular configuration that incorporates optical fibers synchronization scheme and piecewise mirrors in circular array operating below typical damage threshold. Coherent amplification and large laser beams can produce intensity reaching nonlinear quantum vacuum regime. The effects of phase jitter and envelope timing of the pulses due to imperfect synchronization are simulated and analyzed for both linear and circularly polarized pulses. We obtain simple analytical expressions that well describe the envelope jitter and phase jitter. Several practical aspects are discussed, including implications of scaling the laser dimension and pulse duration, with possibility for giant laser facility.     

On the phase-space analysis of photon mediated quantum state transfer in nanophotonic waveguidance

A cavity quantum electrodynamics (QED) based approach for transferring quantum state between quantum nodes has been proposed, wherein a rubidium (⁸⁷Rb) atom trapped inside a two-mode optical cavity forms the quantum node and photons serve as the information carrier between two such nodes. Information is encoded into polarized photon states generated through the application of a system of lasers. The focus is made on the phase-space analysis of the approach, wherein two subspaces of the hyperfine energy levels with magnetic sub-levels of rubidium (⁸⁷Rb) atom represent the logic states ‘0’ and ‘1’. The system of lasers initiates a cavity assisted Raman process which, in turn, generates a right- or left-circularly polarized photon depending on the logic state of the transmit node. Once the photon is received (at the receive node), the logic state of the transmit node is restored into the receive node through a cavity QED process.