非线性双光束共振时的量子非破坏测量

运用全量子方法建立双光束Λ型三能级原子相互作用系统的理论模型，讨论了该系统的动力学行为和光学量子非破坏测量特性.研究表明在非线性双光束共振状态下，可以实现光学量子非破坏测量. 关键词：     

Editorial

The principles of quantum-mechanical measurements are a key issue in the foundation of modern physics. A beautiful consequence of these is the concept of Quantum Nondemolition (QND) measurements. Such measurements permit, in principle, to measure particular observables of a quantum system with unlimited precision, leaving their temporal evolution undisturbed. The application of QND measurements may play an important role in future high-sensivity experiments.Experimental QND measurements require appropriate measurements schemes and apparats, whose development took place in the past few years, thanks to strong progress in ultra-low noise mechanical, electronic and optical detection techniques, coherent light sources, and nonlinear optical systems. At present, interest is focused on measurements of mechanical observables, such as displacement of macroscopic masses and of intensity or phases of electomagnetic field modes in the optical spectral range.This special issue contains original papers on theory and experiments of QND measurements. Recent experimental developments are presented, and proposals and analyses of novel QND schemes are made.The issue focuses on contributions presented at the Workshop on QND Measurements, held March 27–29, 1996, on the Island of Reichenau, Lake Constance (Germany). The workshop was made possible through a generous grant from the Directorats General XII for Sciences, Research and Development of the European Commission.We thank Dr. W. Petrich for his help in preparing this special issue.     

QND measurements using dual ported cavities

 Optical QND measurements may be made using devices which act as nonlinear beam splitters, transmitting noise in a phase-sensitive way. One class of such devices are resonant optical cavities containing a nonlinear medium. By appropriate choice of nonlinearity and operating parameters, various QND observables may be measured. We consider, in particular, the extent to which the QND performance of these devices is degraded by intracavity losses. Received: 8 March 1996     

Quantum limits and symphotonic states in free-mass gravitational-wave antennae

Quantum mechanics sets severe limits on the sensitivity and required circulating energy in traditional free-mass gravitational-wave antennae. One possible way to avoid these restrictions is the use of intracavity QND measurements. We analyze a new QND observable, which possesses a number of features that make it a promising candidate for such measurements, and propose a practical scheme for the realization of this measurement. In combination with an advanced coordinate meter, this scheme makes it possible to lower substantially the requirements on the circulating power.     

Continuous QND measurements: no quantum noise

Monitoring an arbitrary observable is analyzed in the framework of Restricted-Path-Integral (RPI) theory of continuous quantum measurements. While in an usual (quantum-demolition) continuous measurement the measurement noise contains both classical and quantum parts, only the classical noise is shown to be present in a quantum nondemolition (QND) continuous measurement. As a result, no absolute restrictions exist on measurability of a QND observable and the measurement output satisfies the classical equation of motion. Monitoring the energy gives an example of a discrete-spectrum observable. Received: 7 April 1996 / Revised version: 7 August 1996     

Approximate Real–Time Visualization of a Quantum Transition by Means of Continuous Fuzzy Measurement

For the detection of gravitational waves the quantum mechanical properties of the detector have to be taken into account. Not all gravitational wave detectors allow a quantum nondemolition (QND) measurement. Continuous weak or fuzzy measurements are an alternative to study the evolution of a quantum mechanical system under the influence of an external field. In the present paper we investigate this alternative by applying it to a simplified system. We numerically simulate continuous fuzzy measurements of the oscillations of a two-level atom subjected to a resonant external light field. We thereby address the question whether it is possible to measure characteristic features of the evolution of a single quantum system in real time without relying on a QND scheme. We compare two schemes of continuous measurement: continuous measurement with constant fuzziness and with fuzziness changing in the course of the measurement. Because the sensitivity of the two-level atom to the influence of the measurement depends on the state of the atom, it is possible to optimize the continuous fuzzy measurement by varying its fuzziness.     

Splitting Single-Photon Polarization State with Weak Cross—Kerr Nonlinearity

We propose a three-party scheme for quantum information splitting (QIS) of an arbitrary single-photon polarization state based on weak cross-Kerr nonlinearity combined with linear optics elements such as polarization beam splitters (PBSs) and half wave plates (HWPs). The scheme is generalized to the arbitrary-party case. With the help of quantum nondemolition (QND) measurements, our schemes can be accomplished in an almost deterministic way. The two schemes are feasible with the current technology.     

Splitting Single-Photon Polarization State with Weak Cross–Kerr Nonlinearity

We propose a three-party scheme for quantum information splitting(QIS) of an arbitrary single-photon polarization state based on weak cross-Kerr nonlinearity combined with linear optics elements such as polarization beam splitters(PBSs) and half wave plates(HWPs). The scheme is generalized to the arbitrary-party case. With the help of quantum nondemolition(QND) measurements, our schemes can be accomplished in an almost deterministic way. The two schemes are feasible with the current technology.     

光学简并三波混频中的量子非破坏测量

运用半经典量子理论方法建立了光学简并三波混频在两输入场均不为零的情况下的理论模型 ,讨论了在两输入场均不为零时系统的动力学行为以及相应的量子非破坏测量 (QND)特性。研究结果表明 ,该系统的最佳量子非破坏测量工作点与以往的情况不同 ,不是出现在非线性共振点处而是在双稳转变点     

Quantum non-demolition measurement of nonlocal variables and its application in quantum authentication

Quantum non-demolition (QND) variables are generalized to the nonlocal ones by proposing QND measurement networks of Bell states and multi-partite GHZ states, which means that we can generate and measure them without any destruction. One of its prospective applications in the quantum authentication (QA) system of the quantum security automatic Teller machine (QSATM) which is much more reliable than the classical ones is also presented.     

Entangled Fields in Multiple Cavities as a Testing Ground for Quantum Mechanics

Entangled states provide the necessary tools for conceptual tests of quantum mechanics and other alternative theories. These tests include local hidden variables theories, pre- and postselective quantum mechanics, QND measurements, complementarity, and tests of quantum mechanics itself against, e.g., the so-called causal communication constraint. We show how to produce various nonlocal entangled states of multiple cavity fields that are useful for these tests, using cavity QED techniques. First, we discuss the generation of the Bell basis states in two entangled cavities, when there is at most one photon in either of the cavities, and then a straightforward generalization to similar N-cavity states. We then show how to produce a nonlocal entangled state when there is precisely one photon hiding in three cavities. These states can be produced by sending appropriately prepared atoms through the cavities. As applications we briefly review two proposals: one to test quantum mechanics against the causal communication constraint using a two-cavity entangled state and the other to test pre- and postselective quantum mechanics using a three-cavity entangled state. The outcome of the latter experiment can be discussed from the viewpoint of the consistent histories interpretation of quantum mechanics and therefore provides an opportunity to subject quantum cosmological ideas to laboratory tests. Finally, we point out the relation between these schemes and the schemes suggested for quantum computing, teleportation, and quantum copying.     

Quantum nondemolition measurements of a qubit

The concept of quantum nondemolition (QND) measurement is extended to coherent oscillations in an individual two-state system. Such a measurement enables direct observation of an intrinsic spectrum of these oscillations avoiding the detector-induced dephasing that affects the standard (non-QND) measurements. The suggested scheme can be realized in Josephson-junction qubits which combine flux and charge dynamics.     

Real-Time Quantum Feedback Control: ManipulatingQuantum State in Non-Markovian System

In this paper, we propose a closed-loop real-time feedback design for manipulating a quantum state to a target eigenstate via sequential measurements. To this end, based on the Lyapunov stability theorem, considering the controllability and convergence of the system, we select one measured observable and two control channels, which feedback part of the output signal to the input end, forming a closed-loop control. By dynamical programming, we find the optimal parameters to achieve state transfer with a high probability by real-time feedback control. Numerical simulation experiments show that, in a stochastic quantum system with non-Markovian noise, the real-time control strategy moves the system from initial state to the target eigenstate with fast convergence.

     

Predictions of quantum mechanics and stochastic electrodynamics in travelling wave second harmonic generation

We show that stochastic electrodynamics and quantum mechanics give quantitatively different predictions for the quantum nondemolition (QND) correlations in travelling wave second harmonic generation. Using phase space methods and stochastic integration, we calculate correlations in both the positive-P and truncated Wigner representations, the latter being equivalent to the semi-classical theory of stochastic electrodynamics. We show that the semi-classical results are different in the regions where the system performs best in relation to the QND criteria, and that they significantly overestimate the performance in these regions.     

Cascaded χ(2) nonlinearity in QND measurement

 QND measurement schemes often use Kerr nonlinearity to couple the intensity fluctuations of the signal beam to the phase fluctuations of the probe beam by means of the cross-phase modulation effect. Such schemes use materials with intrinsic χ⁽³⁾nonlinearity. However, it has already been shown that Kerr-effect-like correlation between two light waves may be achieved through cascaded χ⁽²⁾: χ⁽²⁾ processes. The value of the nonlinearity induced by cascading χ⁽²⁾ can, in many cases, be much higher than fibre χ⁽³⁾. The authors point out the possibility of using the cascaded χ⁽²⁾ nonlinearity instead of intrinsic χ⁽³⁾ nonlinearity in the scheme which resembles the well-known photon number QND measurement scheme. The non-degenerate second-harmonic generation (SHG) with a large phase mismatch is considered. The harmonic wave influence on the measurements is shown. High QND measurement characteristics can be achieved with the use of an appropriate set of parameters. Received: 6 April 1996     

Some Remarks on Unsharp Quantum Measurements,Quantum Non-Demolition,and All That

The theory of unsharp quantum measurements is reviewed as a generalization of the von Neumann-Lüders measurement theory and applied to measurements of continuous quantities. A generalized notion of repeatability is proposed which is applicable even to intrinsically unsharp continuous observables. As an illustration a precise formulation of a quantum non-demolition (QND) measurement performed on a harmonic oscillator is given.     

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 in Two-Photon Optical Bistability

We evaluate the effectiveness of the quantum nondemolition (QND) measurement scheme in an effective two-level atomic configuration by using the input-output formalism and the quantum regression theorem. The QND capability can be promoted greatly as the cavity detuning is swept towards the bistability threshold.     

Numerical study on formation of electronic quantum states due to self-coherency in a non-periodic system

In order to investigate formation process of electronic quantum states in a confined system, we simulate motion of a wavepacket state and show how an eigenstate is formed due to coherence of electronic wave from the viewpoint that an eigenstate arises as a result of self-interference of a moving electron. Numerical results for a Hénon–Heiles potential in which chaotic motion can occur in the classical mechanics indicate that electronic eigenstates can arise even when motion of an electron is non-periodic. The results show that, in the quantum mechanics, periodicity is unnecessary for the formation of eigenstates.     

耗散介观电容耦合电路的量子涨落

给出耗散介观电容耦合电路的量子化,在此基础上研究电荷和电流在能量本征态下的量子涨落,并对其进行讨论 关键词： 耗散电容耦合电路 量子涨落