量子测量问题的研究及应用

文章结合最近完成的量子测量实验（如用冷原子Ｂｒａｇｇ散射实现的“ｗｈｉｃｈ－ｗａｙ”实验和用Ｃ６０分子完成的小系统量子干涉实验）,比较系统地介绍了量子测量问题及其相关的基本概念和基本思想,如量子相干、量子退相干和量子纠缠,从理论和实验结合的角度,讨论了外部环境和内部运动怎样诱导量子退相干和量子耗散,由此探讨了“薛定谔猫佯谬”和“宏观物体空间局域化描述”的物理解释,最后,文章简单地描述了量子计算和量     

基于量子存储的长距离测量设备无关量子密钥分配研究

针对量子中继器短时间内难以应用于长距离量子密钥分配系统的问题, 提出了基于量子存储的长距离测量设备无关量子密钥分配协议, 分析了其密钥生成率与存储效率、信道传输效率和安全传输距离等参数间的关系, 研究了该协议中量子存储单元的退相干效应对最终密钥生成率的影响, 比较了经典测量设备无关量子密钥分配协议和基于量子存储的测量设备无关量子密钥分配协议的密钥生成率与安全传输距离的关系. 仿真结果表明, 添加量子存储单元后, 协议的安全传输距离由无量子存储的216 km增加至500 km, 且量子存储退相干效应带来的误码对最终的密钥生成率影响较小. 实验中可以采取调节信号光强度的方式提高测量设备无关量子密钥分配系统的密钥生成率, 为实用量子密钥分配实验提供了重要的理论参数.     

量子理论若干基本问题研究的新进展

本结合最后的典型量子物理实验,如用冷原子Bragg散射实现的“which-way”实验,量子退相干过程的微腔QED检验和C60分子的量子干涉等,比较系统地介绍了量子理论基本问题若干研究的新进展,特别强调了处于其核心的量子测量问题及其相关的基本概念和基本思想,如EPR佯谬和Bell不等式,量子退相干和量子纠缠,从理论和实验结合的角度本阐述了被测系统和测量仪器的相互作用怎样导致量子测量的一般动力学过程,由此还讨论了外部环境和内部运动怎样诱导量子退相干和量子耗散,对“薛定谔猫佯谬”和“宏观物体空间局域化描述”给出了可能的物理解释,最后,通过具体例子,本简单地讨论了量子物理基本问题的研究结果对量子信息的应用。     

基于量子Fisher信息的量子计量进展

量子计量是超冷原子气体研究中的一个热点领域.超冷原子体系独特的量子性质(量子纠缠)和量子效应有助于大幅度提高待测物理量的测量精度,这已经成为量子精密测量中的共识.量子Fisher信息对该领域的发展起了非常重要的作用.本文首先介绍量子Fisher信息的基本概念和量子计量的主要内容;然后简要回顾这些理论在提高测量精度方面的应用,特别是多粒子量子纠缠态的产生及其判定;再介绍线性和非线性原子干涉仪的相关进展;最后论述量子测量过程中的统计方法的研究进展.     

量子计算机理论中的量子叠加和量子纠缠

讨论了量子计算、量子通讯与量子计算机中的核心问题 量子叠加和量子纠缠. 从量子态表示量子信息为出发点, 指出有关量子信息的所有问题都可采用量子力学理论来处理. 其中信息的演变遵从薛定谔方程, 信息的传输就是量子态在量子通道中的传送, 信息处理就是量子态的幺正变换, 信息提取则是对量子系统实行量子测量.     

量子理论若干基本问题研究的新进展

本文结合最近的典型量子物理实验 ,如用冷原子Bragg散射实现的“which way”实验 ,量子退相干过程的微腔QED检验和C60 分子的量子干涉等 ,比较系统地介绍了量子理论基本问题若干研究的新进展 ,特别强调了处于其核心的量子测量问题及其相关的基本概念和基本思想 ,如EPR佯谬和Bell不等式 ,量子退相干和量子纠缠。从理论和实验结合的角度 ,本文阐述了被测系统和测量仪器的相互作用怎样导致量子测量的一般动力学过程。由此还讨论了外部环境和内部运动怎样诱导量子退相干和量子耗散 ,对“薛定谔猫佯谬”和“宏观物体空间局域化描述”给出了可能的物理解释。最后 ,通过具体例子 ,本文简单地讨论了量子物理基本问题的研究结果对量子信息的应用     

石墨烯纳米带量子点中的量子混沌现象

在20 mK的极低温下测量了石墨烯纳米带量子点的电子输运性质,观测到清晰的库仑阻塞菱形块和对应量子点激发态的电导峰.对库仑阻塞近邻电导峰间距和峰值进行了统计分析,发现其统计分布分别满足无规矩阵理论描述的Wigner-Dyson分布和Porter-Thomas分布,说明石墨烯纳米带量子点在低温下出现了量子混沌现象.还讨论了这种长方形量子点中量子混沌的可能成因. 关键词： 石墨烯纳米带 量子点 库仑阻塞 量子混沌     

量子信道干涉

单个信道的性质可以用量子测量和量子层析技术等方法描述，但是仅仅单个信道的性质还不足以描述两个信道同时作用时的情况。J．Aberg告诉我们仅仅知道单个信道的信息并不足以帮助我们说明两个映射（一个信道的作用可以用一个映射来描述）同时作用时的情况。但是两个信道之间的干涉可以提供给我们单个信道描述中所没有的附加信息，即两个信道间的相干性质。因此，干涉测量可以用来提取量子层析技术所不能获得的信息。     

双原子Jaynes-Cummings模型下的量子失协和超量子失协

在量子信息的研究中,量子关联起着至关重要的作用,而弱测量对揭示量子关联有重要作用.在双原子Jaynes-Cummings(JC)模型下,分析了量子失协和超量子失协两种量子关联随时间演化过程中的差异性,以及测量强度参数、平均光子数在决定量子关联和影响其动态演化的一般作用.结果表明,弱测量可以更好地揭示量子关联,不仅可以更好地揭示量子关联的复活,还可以获得更大的关联最大值;平均光子数会有效影响两种关联的消失与复活,平均光子数越大,量子关联的消失与复活越快;平均光子数较大时,使用旋波近似得到的弱测量下的超量子失协比强测量下的量子失协与实际误差更小.     

量子密钥分发与量子安全直接通信

量子通信具有高安全性等优点,是当前的国际研究前沿,量子安全直接通信和量子密钥分发是两种重要的量子信息方式.量子密钥分发通过量子信道产生随机的密钥,而量子安全直接通信直接在量子信道中传输秘密信息.本文力图利用浅显易懂的语言介绍量子安全直接通信和量子密钥分发的基本原理;重点描述几个典型的量子安全直接通信方案,介绍目前的发展状态并展望未来.     

Quantum state measurement in double quantum dots with a radio-frequency quantum point contact

We study the dynamics of two electron spins in coupled quantum dots （CQDs） monitored by a quantum point contact （QPC） detector. Their quantum state can be measured by embedding the QPC in an LC circuit. We derive the Bloch-type rate equations of the reduced density matrix for CQDs. Special attention is paid to the numerical results for the weak measurement condintion under a strong Coulomb interaction. It is shown that the evolution of QPC current always follows that of electron occupation in the right dot. In addition, we find that the output voltage of the circuit can reflect the evolution of QPC current when the circuit and QPC are approximately equal in frequency. In particular, the wave shape of the output voltage can be improved by adjusting the circuit resonance frequency and bandwidth.     

Using a quantum dot as a high-frequency shot noise detector

We present the experimental realization of a quantum dot (QD) operating as a high-frequency noise detector. Current fluctuations produced in a nearby quantum point contact (QPC) ionize the QD and induce transport through excited states. The resulting transient current through the QD represents our detector signal. We investigate its dependence on the QPC transmission and voltage bias. We observe and explain a quantum threshold feature and a saturation in the detector signal. This experimental and theoretical study is relevant in understanding the backaction of a QPC used as a charge detector.     

Effect of Quantum Point Contact Measurement on Electron Spin State in Quantum Dots

We study the time evolution of two electron spin states in a double quantum-dot system, which includes a nearby quantum point contact （QPC） as a measurement device. We find that the QPC measurement induced decoherence is in the microsecond timescale. We also find that the enhanced QPC measurement will trap the system in its initial spin states, which is consistent with the quantum Zeno effect.     

Hybrid double-dot qubit measurement with a quantum point contact

We present the measurement of a hybrid double-dot qubit using a quantum point contact （QPC）. To study the dy- namics, we derive the rate equations of the entire system. Numerical results show that QPC current can directly reflect the evolution of the qubit. By adjusting Coulomb interaction, energy mismatch, and QPC tunneling rate, the efficiency and dephasing time can be improved. In addition, the initial state with a hybrid triplet state is superior to that with the purely triplet states on the efficiency. Moreover, the decoherence time is estimated on the magnitude of a microsecond, long enough to implement quantum operations.     

固态量子比特的量子测量

文章介绍了作者用介观输运器件[如量子点接触（QPC）或单电子晶体管（SET）]测量固态量子比特的原理和特性，特别着重地介绍了作者最近在处理被测量子比特和介观测量仪器之间的关联方面的新进展。     

Counterflow of electrons in two isolated quantum point contacts

We study the interaction between two adjacent but electrically isolated quantum point contacts (QPCs). At high enough source-drain bias on one QPC, the drive QPC, we detect a finite electric current in the second, unbiased, detector QPC. The current generated at the detector QPC always flows in the opposite direction than the current of the drive QPC. The generated current is maximal, if the detector QPC is tuned to a transition region between its quantized conductance plateaus and the drive QPC is almost pinched-off. We interpret this counterflow phenomenon in terms of an asymmetric phonon-induced excitation of electrons in the leads of the detector QPC.     

Counting statistics and detector properties of quantum point contacts

Quantum detector properties of the quantum point contact (QPC) are analyzed for an arbitrary electron transparency and coupling strength to the measured system and are shown to be determined by the electron counting statistics. Conditions of the quantum-limited operation of the QPC detector, which prevent information loss through the scattering time and scattering phases, are found for arbitrary coupling. We show that the phase information can be restored and used for the quantum-limited detection by inclusion of the QPC detector in the electronic Mach-Zehnder interferometer.     

Measurement efficiency and n-shot readout of spin qubits

We consider electron spin qubits in quantum dots and define a measurement efficiency e to characterize reliable measurements via n-shot readouts. We propose various implementations based on a double dot and a quantum point contact (QPC) and show that the associated efficiencies e vary between 50% and 100%, allowing single-shot readout in the latter case. We model the readout microscopically and derive its time dynamics in terms of a generalized master equation, calculate the QPC current, and show that it allows spin readout under realistic conditions.     

Measuring the Luttinger liquid parameter with shot noise

We explore the low-frequency noise of interacting electrons in a one-dimensional structure (quantum wire or interaction-coupled edge states) with counterpropagating modes, assuming a single channel in each direction. The system is driven out of equilibrium by a quantum point contact (QPC) with an applied voltage, which induces a double-step energy distribution of incoming electrons on one side of the device. A second QPC serves to explore the statistics of outgoing electrons. We show that measurement of a low-frequency noise in such a setup allows one to extract the Luttinger liquid constant K which is the key parameter characterizing an interacting 1D system. We evaluate the dependence of the zero-frequency noise on K and on parameters of both QPCs (transparencies and voltages).     

Fault-Tolerate Quantum Private Comparison Based on GHZ States and ECC

There are some quantum private comparison (QPC) schemes proposed previously. In this paper we study these QPC protocols in non-ideal scenario and find that they are not secure there. For resolving the problem, we propose a QPC scheme which could be performed in practical scenario. By the use of Greenberger-Horne-Zeilinger (GHZ) states and error-correcting code (ECC), the scheme has the capability of fault-tolerate.