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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...

We have observed the Rabi oscillation of circular Rydberg atoms in the vacuum and in small coherent fields stored in a high Q cavity. The signal exhibits discrete Fourier components at frequencies proportional to the square root of successive integers. This provides direct evidence of field quantization in the cavity. The weights of the Fourier components yield the photon number distribution in the field. This investigation of the excited levels of the atom-cavity system reveals nonlinear quantum features at extremely low field strengths.