Atom lasers, coherent states, and coherence. I. Physically realizable ensembles of pure states
Atom lasers, coherent states, and coherence. I. Physically realizable ensembles of pure states
复制标题
原子激光器、相干态和相干性。
DOI：
10.1103/physreva.65.043605
复制
发表时间：
1999
影响因子：
2.9
通讯作者：
J. Vaccaro
中科院分区：
文献类型：
--
作者：
H. Wiseman;J. Vaccaro
激光器，无论是光学激光器还是原子激光器，都是一个开放的量子系统，它产生玻色子（分别是光子或原子）的相干光束。远高于阈值，激光模式的稳态rho（ss）是具有随机相位的相干场状态的混合，或者等效地，是数态的泊松混合。本文回答了这样一个问题：像这样的描述，ρ（ss）作为一个固定的纯态系综，可以在物理上实现？在这里，物理实现是我们以前定义的[H。M. Wiseman和J.A. Vaccaro，Phys. Lett. A 250，241（1998）]：如果在不改变系统动力学的情况下，实验者可以（原则上）在任何时候知道系统处于系综的一个纯态成员中，那么特定系统的纯态系综可以在物理上实现。这样的知识可以通过监测系统耦合的浴来获得，只要耦合可以通过马尔可夫主方程来描述。使用一个家庭的主方程（原子）激光器，我们解决了物理上可实现的（PR）合奏。我们发现，对于任何有限的自能chi的玻色子在激光模式，相干态系综是不是PR;最接近的一个可以来它是一个系综的压缩状态。这对于原子激光器尤其重要，因为弹性碰撞产生的自能预计会很大。相比之下，数态系综总是PR。随着自能chi的增加，PR系综中最接近相干态系综的态变得越来越压缩。尽管如此，仍然存在chi的值，对于这些值，具有明确定义的相干振幅的状态是PR，即使原子激光器不是相干的（在具有玻色简并输出的意义上）。我们讨论的物理意义，这种异常的条件相干性（因此条件玻色简并）。
A laser, be it an optical laser or an atom laser, is an open quantum system that produces a coherent beam of bosons (photons or atoms, respectively). Far above threshold, the stationary state rho(ss) of the laser mode is a mixture of coherent-field states with random phase, or, equivalently, a Poissonian mixture of number states. This paper answers the question: can descriptions such as these, of rho(ss) as a stationary ensemble of pure states, be physically realized? Here physical realization is as defined previously by us [H. M. Wiseman and J. A. Vaccaro, Phys. Lett. A 250, 241 (1998)]: an ensemble of pure states for a particular system can be physically realized if, without changing the dynamics of the system, an experimenter can (in principle) know at any time that the system is in one of the pure-state members of the ensemble. Such knowledge can be obtained by monitoring the baths to which the system is coupled, provided that coupling is describable by a Markovian master equation. Using a family of master equations for the (atom) laser, we solve for the physically realizable (PR) ensembles. We find that for any finite self-energy chi of the bosons in the laser mode, the coherent-state ensemble is not PR; the closest one can come to it is an ensemble of squeezed states. This is particularly relevant for atom lasers, where the self-energy arising from elastic collisions is expected to be large. By contrast, the number-state ensemble is always PR. As the self-energy chi increases, the states in the PR ensemble closest to the coherent-state ensemble become increasingly squeezed. Nevertheless, there are values of chi for which states with well-defined coherent amplitudes are PR, even though the atom laser is not coherent (in the sense of having a Bose-degenerate output). We discuss the physical significance of this anomaly in terms of conditional coherence (and hence conditional Bose degeneracy).