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Journal of Physics Conference Series

Does an onlooker stop an evolving quantum system?

P. E. Toschek

Published Oct 1, 2007• 0 citations
Publisher

Abstract

The evolution of mechanics has followed the critical analysis of gedanken experiments. Many of these concrete speculations can become implemented today in the laboratory – thanks to now available techniques. A key experiment is concerned with the time evolution of a system under repeated or continuing observation. Here, three problems overlap: 1. The microphysical measurement by a macroscopic device, 2. the system's temporal evolution, and 3. the emergence of macroscopic reality out of the microcosmos. A well-known calculation shows the evolution of a system being slowed down, or even obstructed, when the system is merely observed.An experiment designed to demonstrate this quantum Zeno and performed in the late eighties on an ensemble of identical atomic ions confirmed its description, but turned out inconclusive with respect to the very origin of the impediment of evolution. During the past years, experiments on individualelectrodynamically stored and laser-cooled ions have been performed that unequivocally demonstrate the observed system's evolution being impeded. Strategy and results exclude any physical reaction on the measured object, but reveal the effect of the gain of information as put forward by the particular correlation of the ion state with the detected signal. They shed light on the process of measurement as well as on the evolution and allow an epistemological interpretation.

DOI
10.1088/1742-6596/84/1/012019

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The evolution of mechanics has followed the critical analysis of gedanken experiments. Many of these concrete speculations can become implemented today in the laboratory – thanks to now available techniques. A key experiment is concerned with the time evolution of a system under repeated or continuing observation. Here, three problems overlap: 1. The microphysical measurement by a macroscopic device, 2. the system's temporal evolution, and 3. the emergence of macroscopic reality out of the microcosmos. A well-known calculation shows the evolution of a system being slowed down, or even obstructed, when the system is merely observed.An experiment designed to demonstrate this quantum Zeno and performed in the late eighties on an ensemble of identical atomic ions confirmed its description, but turned out inconclusive with respect to the very origin of the impediment of evolution. During the past years, experiments on individualelectrodynamically stored and laser-cooled ions have been performed that unequivocally demonstrate the observed system's evolution being impeded. Strategy and results exclude any physical reaction on the measured object, but reveal the effect of the gain of information as put forward by the particular correlation of the ion state with the detected signal. They shed light on the process of measurement as well as on the evolution and allow an epistemological interpretation.
Reiterated or continuous measurement on a quantum system impedes the quantum object's evolution. This "quantum Zeno" effect (QZE), contemplated for decades, has been conventionally attributed to the reaction of the measuring device on the quantum object being measured, in the sense of the Heisenberg microscope. However, even reactionless "quantum non-demolition" measurements seem to qualify for that inhibition to take place and may constitute a "quantum Zeno paradox" (QZP). An experimental proof of QZE had been attempted in the past on clouds of ions confined in a Paul trap. In the meantime it has been shown, however, that the anticipated impediment of the quantum evolution by a sheer gain of information, i.e. by reactionless measurement cannot be, in principle, proven with an ensemble. Recently, experiments have been performed with the use of a single quantum system: an 172Yb+ ion, laser-driven on its E2 line S1/2 - D5/2, or an 171Yb+ ion, microwave-driven on its ground-state hyperfine resonance. The results of these experiments demonstrate that mere gain of information on the quantum system, while lacking dynamic action, modifies the system's evolution as it is predicted by quantum mechanics, based on the system's preparation. This seems plausible if we attribute reality to the results of the measurements. If, in contrast, reality is claimed for the prediction, two-fold discontinuity must be admitted: loss of coherence with potential measurements, and renormalization from the results of actual measurements.
Summary form only given. The temporal evolution of a quantum system is retarded by repeated measurements of the observable involved in the evolution. This impediment, the "quantum Zeno effect", has been proven indistinguishable from the effect of dephasing the system's wave function, except in an individual quantum system. We have demonstrated the retardation of the light-driven quantum evolution on the line S/sub 1/2/-D/sub 6/2/ of a single cooled /sup 172/Yb ion placed in the node of the electric field of a 2-mm Paul trap.
Reiterated measurement blocks the quantum evolution of the measured object. An attempt at experimental demonstration of this postulate was reported a decade ago; it applied series of light pulses during the microwave-induced evolution of an ensemble of trapped ions, followed by a measurement of the ensemble’s state. The results agreed with quantum mechanical predictions. However, the blockade of the quantum evolution by acts of measurement cannot, in principle , be verified on an ensemble, since such a measurement yields, as the result, an expectation value, not an eigenvalue, and the micro-state of the system remains unknown. Moreover, each radiative intervention requires subsequent detection of the emerging state in order to qualify as a measurement. A recent experiment has used a single quantum entity (an ion) and has included repeated probing of the ion’s state alternating with the laser-driven evolution. Here, each measurement signals the eigen state of the system. The statistics of correlated results contains complete information on the evolution of the ion, in particular on the blocking effect of reiterated measurements. Recently, an extended relaxationless version has included coherently driving such an ion on the ground-state hyperfine transition by a microwave.
The quantum Zeno effect is the inhibition of the free evolution of a system when that system is subject to a measurement process. The authors of one recent experiment study, W. M. Itano et al., [Phys. Rev. A 41, 2295 (1990)] claimed a demonstration of the quantum Zeno effect. This was done by performing discontinuous atomic-level measurements on a coherently evolving two-level atomic stystem. The measurement was implemented using a pulsed laser strong enough to saturate the two-level atomic system. In this paper, we examine the simpler case where the measurement laser is continuously on and does not saturate the two-level atomic system. In contrast to previous treatments, we show certain parameter regimes wherein an effective state reduction does occur. We ascertain the experimental parameter regimes under which a measurement can be said to have been performed. We show that, in some regimes, fully efficient photon detection does not give good information about the atomic-level populations and hence no measurement has been performed. Throughout, we emphasize that the design of an atomic level-measurement device requires more than the analysis of a mechanism for photon detection or of coherence loss. We gain insight into the operation of a particular measurement process by following the selective evolution of various systems using the quantum-trajectories approach.
Rapid experimental progress has recently allowed the use of light to prepare macroscopic mechanical objects into nearly pure quantum states. This research field of quantum optomechanics opens new doors toward testing quantum mechanics, and possibly other laws of physics, in new regimes. In the first part of this paper, I will review a set of techniques of quantum measurement theory that are often used to analyze quantum optomechanical systems. Some of these techniques were originally designed to analyze how a classical driving force passes through a quantum system, and can eventually be detected with optimal signal-to-noise ratio --- while others focus more on the quantum state evolution of a mechanical object under continuous monitoring. In the second part of this paper, I will review a set of experimental concepts that will demonstrate quantum mechanical behavior of macroscopic objects --- quantum entanglement, quantum teleportation, and the quantum Zeno effect. Taking the interplay between gravity and quantum mechanics as an example, I will review a set of speculations on how quantum mechanics can be modified for macroscopic objects, and how these speculations --- and their generalizations --- might be tested by optomechanics.