Observing the Progressive Decoherence of the “Meter” in a Quantum Measurement

Abstract

A mesoscopic superposition of quantum states involving radiation fields with classically distinct phases was created and its progressive decoherence observed. The experiment involved Rydberg atoms interacting one at a time with a few photon coherent field trapped in a high Q microwave cavity. The mesoscopic superposition was the equivalent of an "atom 1 measuring apparatus" system in which the "meter" was pointing simultaneously towards two different directions -a "Schrödinger cat." The decoherence phenomenon transforming this superposition into a statistical mixture was observed while it unfolded, providing a direct insight into a process at the heart of quantum measurement. [S0031-9007(96)01848-0] The transition between the microscopic and macroscopic worlds is a fundamental issue in quantum measurement theory [1]. In an ideal model of measurement, the coupling between a macroscopic apparatus ("meter") and a microscopic system ("atom") results in their entanglement and produces a quantum superposition state of the "meter 1 atom" system. Such a superposition is however never observed. Schrödinger has illustrated vividly this problem, replacing the meter by a "cat" [2] and considering the dramatic superposition of dead and alive animal "states." Although such a striking image can only be a metaphor, quantum superpositions involving "meter states" are often called "Schrödinger cats." Following von Neumann [3], it is postulated that an irreversible reduction process takes the quantum superposition into a statistical mixture in a "preferred" basis, corresponding to the eigenvalues of the observable measured by the meter. From then on, the information contents in the system can be described classically. The nature of this reduction has been much debated, with recent theories stressing the role of quantum decoherence [4,5]. According to these approaches, the meter coordinate is always coupled to a large reservoir of microscopic variables inducing a fast dissipation of macroscopic coherences.The simplest model of a quantum measurement involves a two-level atom (e, g) coupled to a quantum oscillator (meter or cat). An oscillator in a coherent state [6] is indeed defined by a c number a, represented by a vector in phase space (jaj p n where n is the mean number of oscillator quanta). Quantum fluctuations make the tip of this vector uncertain, with a circular gaussian distribution of radius unity [ Fig. 1(a)]. Consider the ideal measurement where the "atom-meter" interaction entangles the phase of the oscillator (6f) to the state of the atom, leading to the combined stateWhen the "distance" D 2 p n sin f between the meter states is larger than 1, a Schrödinger cat is obtained [ Fig. 1(b)].Decoherence is modeled by coupling the oscillator to a reservoir, which damps its energy in a characteristic time T r . When D ¿ 1, decoherence is found to occur within a time scale 2T r ͞D 2 [7,8]. This result illustrates the basic feature of the quantum to classical transition [4]. Mesoscopic superpositions made of a few quanta are ex...

Physics: Quantum all the way

“…Can we actually see quantum effects leaking away into the environment? Serge Haroche and his colleagues at the Ecole Normale Supérieure in Paris began to put the idea to the test in 1996 by studying bunches of photons held in a type of light trap called an optical cavity 6 . They passed a rubidium atom through the cavity in a superposition of two states.…”

Minimum decoherence cat-like states in Gaussian noisy channels

“…The theme of decoherence of cat-like states spurred relevant theoretical works [5,6,7,8], especially aimed to select schemes of quantum control and feedback stabilizing coherent superpositions against decoherence [7]. Recent promising experimental results and perspectives continue to keep a widespread interest in this subject [9,10].…”

Environment-Induced Decoherence and the Transition from Quantum to Classical

“…A remarkable characteristic of the current debates on the nature of the quantum to classical transition and on the problem of quantum measurement is that for the first time in history there have been actual experiments probing the boundary between the quantum and the classical domains in a controlled way [6][7][8][9][10]. Controlled decoherence experiments (which are very difficult because nature provides us with classical or quantum systems but not with objects whose interaction with the environment can be controlled at will) were recently carried on for the first time and help us in understanding the nature of this process.…”