Parameter accuracy analysis of weak-value amplification process in the presence of noise

We theoretically introduce the statistical uncertainty of photon number and phase error to discuss the precision of parameters to be measured based on weak measurements. When the photon counting scheme is used, we discuss the relative accuracy of the system in the presence of phase error by using the orthogonal and nonorthogonal pre-and postselected states, respectively. When using the measurement scheme of pointer shift, we discuss the measurement accuracy in the presence of phase error, pointer resolution, and statistical uncertainty. These results give a guide way to get the smallest relative precision and deepen our understanding about weak measurement.     

Improving the Precision of Multiparameter Estimation in the Teleportation of Qutrit under Amplitude Damping Noise

Since the initial discovery of quantum teleportation, it is devoted to transferring unknown quantum states from one party to another distant partner. However, in the scenarios of remote sensing, what people truly care about is the information carried by certain parameters. The problem of multiparameter estimation in the framework of qutrit teleportation under amplitude damping (AD) noise is studied. Particularly, two schemes are proposed to battle against AD noise and enhance the precision of multiparameter estimation by utilizing weak measurement (WM) and environment-assisted measurement (EAM). For two-phase parameters encoded in a qutrit state, the analytical formulas of the quantum Fisher information matrix can be obtained. The results prove that the scheme of EAM outperforms the WM one in the improvements of both independent and simultaneous estimation precision. Remarkably, the EAM scheme can completely ensure the estimation precision against the contamination by AD noise. The reason should be attributed to the fact that EAM is carried out after the AD noise. Thus, it extracts information from both the system and the environment. The findings show that the techniques of WM and EAM are helpful for remote quantum sensing and can be generalized to other qutrit-based quantum information tasks under AD decoherence.     

在有限温度下运用弱测量保护量子纠缠

根据单量子比特的映射, 提出一个在有限温度下运用弱测量保护两量子比特纠缠的方案. 在两个可通过局域幺正变化实现的等价的初始纠缠进入广义振幅阻尼信道前后, 对其分别进行弱测量, 并对四个弱测量的参数做全面的优化, 获取最大共生纠缠C_r 和弱测量参数m, n的解析表达式, 然后再进一步研究弱测量参数与信道参数的关系. 发现这种基于弱测量的纠缠保护方法在某些情况下可以有效地提高纠缠, 甚至可以避免纠缠的突然死亡. 当信道参数r一定时, 对不同的参数p,初始态|ψ>纠缠达到最大值时对应的弱测量参数m的取值一样, 共生纠缠关于p=0.5对称, 而初始态|φ>对应的参数m的取值不同; 在参数p一定、参数r不同、初始态|ψ>或|φ>的情况下, 当纠缠度取最大值时, 弱测量参数m的取值不变, 且随着r的增加, 纠缠度减少. 通过对信道参数的分析, 发现可以选择合适的信道参数和初始态来获得较大的纠缠.     

利用金刚石氮-空位色心精确测量弱磁场的探索

基于金刚石氮-空位色心对精确测量微弱静磁场进行了探索.金刚石氮-空位色心电子自旋的退相干时间高度依赖于外磁场,而不同的退相干特征时间对磁场的灵敏度不同.对金刚石氮-空位色心电子自旋在不同强度外磁场下的退相干过程进行模拟,得到不同退相干特征时间与磁场大小的高准确度关系,提出了基于响应度最高的退相干特征时间测量静态弱磁场大小和方向的方法,并分析了该方法测量静态弱磁场的灵敏度,证明该方法的测量灵敏度比一般磁场测量仪器更高.     

Increasing the efficiency of post-selection in direct measurement of the quantum wave function

Direct weak or strong measurement of quantum wave function based on post-selections has been widely explored; however, the efficiency of the measurement is heavily limited by the success probability of post-selection. Here we propose a modified scheme to directly measure photon's wave function by simply inserting a liquid crystal plate before the post-selection stage. Numerical simulations demonstrate that our modified method can significantly increase the efficiency of post selection. Our proposal would speed up the quantum wave function measurement with high resolution and high fidelity.     

利用弱测量保护两个三能级原子系统的束缚纠缠

高维量子系统的纠缠态因其诸多的优点而受到广泛的关注。目前,肖兴等人[Eur.Phys.J.D.(2013)67:204]研究了两个V-型三能级原子系统的演化动力学,得到了弱测量方案能保护系统自由纠缠的结论。本文研究了在振幅阻尼噪声中两个全同三能级原子系统束缚纠缠的演化动力学。在振幅阻尼通道中,前置弱测量和后置弱测量反转操作,实现了对系统束缚纠缠的保护。束缚纠缠保持的时间受量子态参数和弱测量强度的影响。相应的物理原因也得到了合理的解释。对于两个非全同三能级原子的情况,此结论依旧成立。另外,弱测量方案并不是对所有类型的束缚纠缠态都适用,有一定的局限性。     

基于部分测量增强量子隐形传态过程的量子Fisher信息

量子Fisher信息(QFI)是量子度量学中的一个重要物理量,可给出预估参数精度的最优值.本文研究如何引入弱测量和测量反转操作,来提高有限温环境下以Greenberger-Horne-Zeilinger态作为量子通道的隐形传态过程中的QFI.依据隐形传态过程中量子比特的传输情形,考虑了三种不同方案相应的QFI.首先,通过构造每种量子隐形传态方案的量子线路图,分析了QFI与推广振幅衰减噪声参数的变化关系.随后对各种方案中的受噪声粒子施加弱测量和测量反转操作,并对相应的部分测量参数进行优化,着重探讨了施加最优部分测量操作后QFI的改进量.结果表明,经过优化后的部分测量操作能有效提高有限温环境下量子隐形传态过程输出态的QFI;而且量子系统所处的环境温度越低,QFI的提高效果可越显著.     

针对天空背景亮度测量的弱信号检测

天空亮度分布在大气光学领域有着重要的应用.随着天空亮度测量仪器的迅速发展,对测量的精度和工作稳定性的要求也越来越高.为了提高测量仪器的动态测量范围,需要实现对探测器输出的弱信号的高精度测量.讨论了用高精度运算放大器对弱信号进行放大滤波处理,并采用一种低噪声高精度△-∑型模数转换器(ADC)来解决探测器中弱信号高精度测量...     

Pre-and post-selected measurements with coupling-strength-dependent modulation

Pre-and post-selected(PPS) measurement, especially the weak PPS measurement, has been proved to be a useful tool for measuring extremely tiny physical parameters. However, it is difficult to retain both the attainable highest measurement sensitivity and precision with the increase of the parameter to be measured. Here, a modulated PPS measurement scheme based on coupling-strength-dependent modulation is presented with the highest sensitivity and precision retained for an arbitrary coupling strength. This idea is demonstrated by comparing the modulated PPS measurement scheme with the standard PPS measurement scheme in the case of unbalanced input meter. By using the Fisher information metric, we derive the optimal pre-and post-selected states, as well as the optimal coupling-strength-dependent modulation without any restriction on the coupling strength. We also give the specific strategy of performing the modulated PPS measurement scheme, which may promote practical application of this scheme in precision metrology.     

独立环境相互作用下的二聚体的相干性传输操控

研究了二聚体中两个分别与独立环境作用的色素(供体色素与受体色素)间相干性的传输. 通过对供体色素进行作用前的弱测量以及对受体色素进行作用后的反转测量或弱测量,提出了一个能最优化地 把相干性从供体传输到受体的方案. 建立了两次测量强度和演化时间的确切关系,在该条件下受体的相干性可以达到最大值0.5,但代价是成果概率的减小.     

Beating Standard Quantum Limit with Weak Measurement

Weak measurements have been under intensive investigation in both experiment and theory. Numerous experiments have indicated that the amplified meter shift is produced by the post-selection, yielding an improved precision compared to conventional methods. However, this amplification effect comes at the cost of a reduced rate of acquiring data, which leads to an increasing uncertainty to determine the level of meter shift. From this point of view, a number of theoretical works have suggested that weak measurements cannot improve the precision, or even damage the metrology information due to the post-selection. In this review, we give a comprehensive analysis of the weak measurements to justify their positive effect on prompting measurement precision. As a further step, we introduce two modified weak measurement protocols to boost the precision beyond the standard quantum limit. Compared to previous works beating the standard quantum limit, these protocols are free of using entangled or squeezed states. The achieved precision outperforms that of the conventional method by two orders of magnitude and attains a practical Heisenberg scaling up to n=106 photons.     

基于弱非线性实现非破坏性测量两光子Bell态及三光子Greenberger-Horne-Zeilinger态

基于弱非线性及线性光学元件提出非破坏性测量两光子Bell态及三光子 Greenberger-Horne-Zeilinger (GHZ)态方案. 方案中, 首先应用光束分束器及交叉克尔非线性介质对两光子Bell态进行对称性分析, 进而结合控制非门提出三光子分析方案实现对八个三光子GHZ态完全且非破坏性区分. 关键词： Bell态测量 Greenberger-Horne-Zeilinger态测量 弱非线性 量子非破坏性测量     

Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures the magnetic susceptibility of an atomic spin ensemble along the $x$ and $z$ directions and produces parameter estimation with precision beating the standard quantum limit. The atomic ensemble is initialized via one-axis spin squeezing with optimized squeezing time and parameter $\phi$ (to be estimated) assumed as uniformly distributed between 0 and $2\pi$ while fixed in each estimation. One estimation of $\phi$ can be produced with every two magnetic susceptibility data measured along the two axes respectively, which has an imprecision scaling $({1.43\pm0.02})/N^{0.687\pm0.003}$ with respect to the number $N$ of the atomic spins. The measurement scheme is easy to implement and is robust against the measurement fluctuation caused by environment noise and measurement defects.     

Enhancing the precision of phase estimation by weak measurement and quantum measurement reversal

The enhancement of the precision of phase estimation in quantum metrology is investigated by employing weak measurement （WM） and quantum measurement reversal （QMR）. We derive the exact expressions of the optimal quantum Fisher information （QFI） and success probability of phase estimation for an exactly solving model consisting of a qubit interacting with a structured reservoir. We show that the QFI can be obviously enhanced by means of the WM and QMR in different regimes. In addition, we also show that the magnitude of the decoherence involved in the WM and QMR can be a general complex number, which extends the applicable scope of the WM and QMR approach.     

Bidirectional multi-qubit quantum teleportation in noisy channel aided with weak measurement

Recently,bidirectional quantum teleportation has attracted a great deal of research attention.However,existing bidirectional teleportation schemes are normally discussed on the basis of perfect quantum environments.In this paper,we first put forward a bidirectional teleportation scheme to transport three-qubit Greenberger-Horne-Zeilinger(GHZ) states based on controled-not(CNOT) operation and single-qubit measurement.Then,we generalize it to the teleportation of multi-qubit GHZ states.Further,we discuss the influence of quantum noise on our scheme by the example of an amplitude damping channel,then we obtain the fidelity of the teleportation.Finally,we utilize the weak measurement and the corresponding reversing measurement to protect the quantum entanglement,which shows an effective enhancement of the teleportation fidelity.     

自旋对弱耦合库仑束缚磁极化子基态能量的影响

采用改进的线性组合算符和LLP正则变换方法,在计及反冲效应的情况下讨论了自旋对弱耦合库仑束缚磁极化子基态能量的影响。以GaP晶体为例进行计算,结果表明自旋对极化子基态能量的影响并不能总是忽略不计。自旋能是否可以忽略取决于能带电子的有效质量与自由电子的质量之比、晶体本身的性质、外磁场强度等因素。     

A method for precise neutron beam polarisation analysis using an opaque spin filter

Free neutron beta decay provides sensitive tests of the Standard Model of electroweak interaction. A particular class of experiments has now reached a limit set by the imperfect knowledge of the neutron polarisation and of the efficiency of a spin flipper. Standard techniques of polarisation analysis reach a precision of ≥0.25%. In a recent experiment it was shown that an accuracy on the level of a few times 10⁻⁴ can be reached within a classical scheme of polarisation analysis, using an opaque spin filter as analyser. In this letter a new scheme is given which takes advantage of a natural degree of freedom of spin filters, resulting in a more sensitive and simpler measurement of neutron polarisation and flipper efficiency.     

Enhancement of the Quantum Coherence in a Hierarchical Environment System by Weak Measurement and Weak Measurement Reversal

The quantum coherence of a two-qubit system in which each qubit is coupled to its own hierarchical environment is studied. The effect of the hierarchical environment is explored in order to improve the coherence effectively. It is discovered that the dynamics of coherence can be manipulated by the number N of the second-layer cavities ($m_n$), the coupling strengths Ω₀, Ω, κ, respectively for the qubit-cavity (m₀), nearest-neighbor, and m₀-$m_n$ cavities. It depends also on the decay rate Γ of second-layer cavities. Furthermore, an effective scheme for enhancing coherence is proposed by using weak measurement and weak measurement reversal. The explicit conditions of the optimal measurement strengths to improve the coherence comparatively are derived. It is seen that the final stable value of the coherence is independent of the number N of second-layer cavities but is related to the weak measurement strength m when the coupling strength between two nearest-neighbor cavities is taken into account.     

High Fidelity Symmetric Telecloning and Entanglement Distribution of Spin Quantum States by Weak Measurement and Reversal

We propose a physical realization of robust symmetric telecloning scheme for spin quantum states by employing the weak measurement and reversal (WMR) operation. Using proper WMR, the ultrahigh telecloning fidelity and long distance of quantum state transfer with certain success probability can be achieved. More interestingly, the lowest average telecloning fidelity can attain 80 %, which is almost independent of the spin chain length. We also study the properties of entanglement distribution via the spin chain for arbitrary two-qubit entangled pure states as inputs and find that the WMR operation indeed helps for protecting distributed entanglement.     

Precision Mass Measurement with Optomechanically Induced Transparency in an Optomechanical System

We present a scheme for all-optical precision mass sensing with squeezed field in an optomechanical system in terms of optomechanically induced transparency (OMIT). We demonstrate that the splitting of the two peaks of the OMIT, which is almost inverse proportion to square root of the accreted mass landing on nanomechanical resonator (NAMR). We also show that the mass measurement scheme for the squeezed fields can be robust against temperature and cavity decay in somehow. Specifically, the precision measurement is from the noise spectrum, for these reasons, our scheme may provide a new paradigm for precision measurement based on the noise in the optomechanical system.