title: How to measure the decoherence of a micromaser field under well controlled conditions.
abstract: We discuss a possible realization of a quantum register with controllable decoherence in terms of /0> and /1> photon number states of a micromaser field. It is shown how to create in the Jaynes-Cummings model a superposition state of /0> and /1> photon number states inside a closed micromaser cavity. The loss of phase coherence between these two states can subsequently be measured by a second probe atom monitoring the decoherence of the field. A technique is proposed for forming the superposition of number states /0> and /1> using the time structure of the Rabi oscillation. The proposed method avoids problems with stray fields at the cavity holes, which disturb the coherence of the atomic superposition, and offers a way to study how the coupling strength to the environment influences the decoherence rate, displaying the robustness of physical qubits and the fidelity of quantum computations.
authors.affiliation: Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, D-85748 Garching, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Rinner
authors.forename: H
authors.initials: H
authors.lastname: Walther
authors.forename: E
authors.initials: E
authors.lastname: Werner