Evolution of a quantum system during measurement and decoherence

Xiang‐Guo Meng, Ji‐Suo Wang, Bao‐Long Liang

Patrick D. Kumavor, Eric Donkor, Bing Wang

A theoretical model is presented for the interaction of a quantum system with an orthogonal polarized entangled photon-pair measurement probe. The theoretical framework is based on solving the generalized Jaynes-Cummings and Shroeder's equations to determine the phase evolution of the interacting system. The measurement-induced decoherence is expressed in terms of the temporal evolution of the relative phases of the superposition states induced by the measurement probe. The method is applied to determine the rate of decoherence of a two-qubit rubidium quantum system. Quantitative results are given to contrast measurement-induced of (i) single photon probe and (ii) orthogonal polarized and entangled probe.

Михаил Борисович Менский

E. D. Vol

We study the influence of a continuous measurement in a composite quantum system C on the evolution of the states of its parts. It is shown that the character of the evolution (decoherence or recoherence) depends on the type of measured quantity and on the initial state of the system. A number of conditions under which the states of the subsystems of C decohere during the measuring process are established. We propose a model of the composite system and specify the observable the measurement of which may result in the recoherence of the state of one of the subsystems of C. In the framework of this model we find the optimal regime for the exchange of information between the parts of C during the measurement. The main characteristics of such a process are computed. We propose a scheme of detection of the recoherence under the measurement in a concrete physical experiment.

Gershon Kurizki, Abraham G. Kofman

Abstract The purpose of this article, which is our contribution to Wolfgang Schleich’s Festschrift, is to present an unconventional perspective of two fundamental, interrelated problems: the emergence of classicality and the observer’s role in quantum mechanics. In our perspective, the decoherence of quantum states of a system or its measuring device (meter) by the environment cannot replace the role of the observer, nor can it provide the ultimate explanation of the transition of the system from quantumness to classicality. The reason is that decoherence, despite appearances, typically does not irretrievably obliterate the information on the system’s state. Even in cases where the environment has the potential of erasing this information, the observer can negate decoherence effects in the meter or the system by dynamical control. We contend that the choice of measuring bases and observables is largely up to the controller-observer (CO) that need not possess unlimited resources in order to steer or freeze ( at will ) the system-state evolution and its quantumness, notwithstanding the environment effects. An analogy is drawn between the outlined approach and Lamarckism, the theory of evolution that preceded Darwinism. Quantum Lamarckism expresses our view of evolution as functional adaptation of the system to the interplay between environmental and CO effects . In our view, both conceptually and practically, decoherence may be deemed non-essential for determining the evolution of a quantum system embedded in its environment, provided the CO has an adequate, albeit limited, arsenal of resources.

Michael B. Mensky