Decoherence of the field in quantum nondemolition photon number measurement using the Kerr nonlinearity

The role of decoherence of an electromagnetic field in the process of quantum nondemolition measurement of the number of photons using a nonlinear Mach-Zehnder interferometer is investigated. This decoherence is caused by distributed losses in a Kerr medium. A long interaction time between the field and the medium is required to achieve a high accuracy in the photon number measurement. The losses in the Kerr medium accompanying the resonance four-wave interaction between the measured and the probe fields lead to a measurement error close to unity at γτ ?1 (γ is the rate of losses in the medium, and τ is the time of interaction between the field and the medium); consequently, nondemolition quantum measurement turns out to be impossible in the scheme considered. Under these conditions, an increase in the intensity of the probe field does not result in achievement of the required measurement accuracy.     

Generation of spin squeezing via continuous quantum nondemolition measurement

Continuous quantum nondemolition monitoring of a collective atomic spin with an off-resonant laser beam has been performed. Squeezed atomic spin states have thereby been produced with spin noise reduction to 70% below the standard quantum limit expected for a coherent spin state.     

Nonunity gain quantum nondemolition measurements based on measurement and repreparation

We demonstrate experimentally a nonunity gain quantum nondemolition measurement based on a simple homodyne measurement and recreation strategy. Although the output state is an amplified version of the input state, the device meets standard criteria for QND measurements: the transfer coefficient was measured to 1.78, and the conditional variance was measured to 0.66.     

Quantum-nondemolition measurement of the number of photons in a microcavity

A new scheme for quantum-nondemolition measurement of the number and statistics of photons in a microwave cavity on the basis of interferometry of strongly-controlled V atoms which interact by a dispersion interaction with a cavity mode at a different transition is proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 58–63 (10 July 1997)     

Information analysis of quantum nondemolition measurement of a photon in a resonator

The concept of entangled quantum measurement was demonstrated by the example of the fundamental value of the experiment on quantum nondemolition measurement of a photon in a resonator with the use of a probe atom [1]. The quantum-information analysis of this experiment was performed. A mechanism of information transmission in the nondemolition measurement scheme in the case of the classical and quantum formalism was demonstrated. The results were shown to coincide in both cases. This is the result of copying the classical part of the initial quantum information attributed to a set of quantum object states.     

InAs 单量子点中级联辐射光子的关联测量

利用分子束外延生长InAs单量子点样品,温度为5 K时,测量了单量子点中单、双激子自发辐射的荧光(PL)光谱.研究了单、双激子发光强度随激发功率的变化及对应发光峰的偏振特性和精细结构劈裂.基于Hanbury-Brown Twiss(HBT) 实验,测量了单、双激子间发光光谱的关联函数,证实了其发光过程为级联发射过程. 关键词： InAs 单量子点 单、双激子 荧光光谱 级联辐射     

A critique of the quantum nondemolition concept

A formal quantum theory of new types of measurement of small forces was published recently. The purpose of this note is to show that this theory suffers from a serious inconsistency which makes its conclusions invalid.     

光量子纠缠态的制备和测量实验

以量子纠缠源为实验对象,以光量子纠缠态的制备、传输和纠缠性质的测量为内容设计实验,有利于学生学习量子通讯的基本原理及偏振纠缠、自发参量下转换产生双光子态量子理论.     

Squeezing and entangling atomic motion in cavity QED via quantum nondemolition measurement

We propose a scheme for preparing the squeezing of an atomic motion and an Einstein-Podolsky-Rosen state in position and momentum of a pair of distantly separated trapped atoms. The scheme utilizes the quantum nondemolition measurements with interaction between the cavity field and the motional state of the trapped atom in cavity QED. By illuminating the atoms with bichromatic light, the interaction Hamiltonian of the cross-Kerr effect between the cavity and atomic motion is generated to implement quantum nondemolition measurements.Received: 5 February 2003, Published online: 17 July 2003PACS: 03.67.Hk Quantum communication - 32.80.Lg Mechanical effects of light on atoms, molecules, and ions - 42.50.-p Quantum optics     

Dynamics of dispersive photon-number QND measurements in a micromaser

A numerical analysis of dispersive quantum nondemolition measurement of the photon number of a microwave cavity field is presented. Simulations show that a key property of the dispersive atom-field interaction used in Ramsey interferometry is the extremely high sensitivity of the dynamics of atomic and field states to basic parameters of the system. When a monokinetic atomic beam is sent through a microwave cavity, a qualitative change in the field state can be caused by an uncontrollably small deviation of parameters (such as atom path length through the cavity, atom velocity, cavity mode frequency detuning, or atom-field coupling constants). The resulting cavity field can be either in a Fock state or in a super-Poissonian state (characterized by a large photon-number variance). When the atoms have a random velocity spread, the field is squeezed to a Fock state for arbitrary values of the system’s parameters. However, this makes detection of Ramsey fringes impossible, because the probability of detecting an atom in the upper or lower electronic state becomes a random quantity almost uniformly distributed over the interval between zero and unity, irrespective of the cavity photon number.     

基于光子轨道角动量的密码通信方案研究

设计了一个基于两个正交的光子轨道角动量态的量子密码通信方案.在该方案中,Alice使用具有独特设计的激光器,随机发送有确定轨道角动量的光子;Bob采用由两个达夫棱镜组成的光束旋转器,对光子的轨道角动量态进行测量.对系统安全性的讨论表明,Eve采用截获重发、攻击单臂等攻击手段,其窃听行为都会被发现.理论证明,该方案不需要通信双方实时监测和调整参考系,同时避免了BB84,B92协议因发送基和测量基不一致而丢弃一半信息的问题,从而提高了密钥生成效率. 关键词： 量子保密通信 轨道角动量     

Field theory of spin-singlet quantum Hall states

We formulate a field theory for the Haldane-Rezayi quantum Hall state and its variants. A new essential ingredient is a class of super Chern-Simons field.     

Direct measurement for general quantum states using parametric quantum circuits

In the field of quantum information,the acquisition of information for unknown quantum states is very important.When we only need to obtain specific elements of a state density matrix,the traditional quantum state tomography will become very complicated,because it requires a global quantum state reconstruction.Direct measurement of the quantum state allows us to obtain arbitrary specific matrix elements of the quantum state without state reconstruction,so direct measurement schemes have obtained...     

Polarization quantum states of light in nonlinear distributed feedback systems; quantum nondemolition measurements of the Stokes parameters of light and atomic angular momentum

Received: 28 March 1996/In final form: 31 March 1997     

Procedure for direct measurement of general quantum states using weak measurement

Recent work by Lundeen et al. [Nature (London) 474, 188 (2011)] directly measured the wave function by weakly measuring a variable followed by a normal (i.e., "strong") measurement of the complementary variable. We generalize this method to mixed states by considering the weak measurement of various products of these observables, thereby providing the density matrix an operational definition in terms of a procedure for its direct measurement. The method only requires measurements in two bases and can be performed in situ, determining the quantum state without destroying it.     

Effect of the spatial inhomogeneity of a field in a micromaser experiment with the selective measurement of atomic states

It is shown that the theoretical analysis of a micromaser experiment with the selective measurement of atomic states is impossible without consideration of the spatial structure of the field and of the statistical spread of the atomic velocities in the cavity. An approximate theory, ignoring the dynamic effects associated with the mentioned factors, yields not only quantitatively, but also qualitatively different theoretical predictions. The dynamics of the field and the mean number of photons in the trapping states realized in this case differ significantly from the predictions of the idealized standard theory used at present. Along with the process of establishment of the stationary Fock state of the field, under certain conditions, it appears possible to form the quasistationary quantum state of the field with large fluctuations of the number of photons, accompanied by the exit of the field from the cavity (“quasi-dissipation”), and the following stepwise transition into the state of the electromagnetic vacuum. It is shown that, under the conditions of the micromaser experiment, the information about the states of atoms, passed through the cavity, in the case considered, does not allow one to draw an unambiguous conclusion about the state of the field in the cavity.     

Continuous nondemolition measurement of the Cs clock transition pseudospin

We demonstrate a weak continuous measurement of the pseudospin associated with the clock transition in a sample of Cs atoms. Our scheme uses an optical probe tuned near the D1 transition to measure the sample birefringence, which depends on the component of the collective pseudospin. At certain probe frequencies the differential light shift of the clock states vanishes, and the measurement is nonperturbing. In dense samples the measurement can be used to squeeze the collective clock pseudospin and has the potential to improve the performance of atomic clocks and interferometers.