Manipulation of photons in a cavity by dispersive atom-field coupling: Quantum-nondemolition measurements and generation of ‘‘Schrödinger cat’’ states

Abstract

A quantum-nondemolition method to measure the number of photons stored in a high-Q cavity, introduced by Brune et al. [Phys. Rev. Lett. 65, 976 (1990)

Decoherence and Quantum-State Measurement in Quantum Optics

“…Detection of the second atom probes the state produced in C. Since no field was injected into the cavity between the two atoms, it is clear now that the experiment proposed in [23] amounts to a measurement of the Wigner function at the origin of phase space, which is non zero for the pure state |± , vanishes after the decoherence time, and increases again as dissipation takes place, bringing the field to the vacuum state. In the experiment realized by Brune et al [13], both |e and |g lead to dephasings (in opposite directions) of the field in C. In this case, it is easy to show that the Wigner function is again recovered, as long as the onephoton phase shift is φ = π/2 (with opposite signs for e and g), and a dephasing η = π/2 is applied to the second Ramsey zone [24].…”

“…The phase shift is negligible, however, if the atom is in state |g . For a principal quantum number equal to 50 in the state e, and the cavity tuned close to the 50 → 51 circular to circular transition (around 50 GHz), a phase shift of the order of π per photon is produced by an atom crossing the centimeter size cavity with a velocity of about 100 m/s [13].…”

“…An important feature of this scheme is the insensitivity to the detection efficiency of the atomic counters, of the order of 40±15% in recent experiments [13].…”

“…The method for generating the quantum superposition of two coherent states, proposed in [13], and sketched in Fig. 1, involves a beam of circular Rydberg atoms [14] crossing a high-Q cavity C in which a coherent state is previously injected (this is accomplished by coupling the cavity to a classical source -a microwave generator -through a wave guide).…”

Monitoring the Decoherence of Mesoscopic Quantum Superpositions in a Cavity

“…These states are prototypes of Schrödinger cats [13,14]. We will see later how they can be prepared and used to study the dynamics of decoherence.…”