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International Journal of Quantum Information

DECOHERENCE-FREE MULTIPARTITE ATOMIC ENTANGLEMENT IN A CAVITY QED SYSTEM

Matteo Bina, Federico Casagrande, ALFREDO LULLI

Published Jan 1, 2009• 1 citation
Publisher

Abstract

We present analytical results on entanglement and decoherence in a system of N strongly driven two-level atoms resonantly coupled to a dissipative cavity mode. In the specific cases of N = 3 and 4 qubits we show the existence of decoherence-free subspaces for the whole system and/or the atomic subsystem and we discuss examples where a multipartite qubit entanglement remains preserved. Also we illustrate the conditional preparation of cat-like states of the cavity field and the generation of atomic correlations.

DOI
10.1142/s021974990900461x

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We present analytical results on entanglement and decoherence in a system of N strongly driven two-level atoms resonantly coupled to a dissipative cavity mode. In the specific cases of N = 3 and 4 qubits we show the existence of decoherence-free subspaces for the whole system and/or the atomic subsystem and we discuss examples where a multipartite qubit entanglement remains preserved. Also we illustrate the conditional preparation of cat-like states of the cavity field and the generation of atomic correlations.
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We present a fully analytical solution of the dynamics of two strongly driven atoms resonantly coupled to a dissipative cavity field mode. We show that an initial atom-atom entanglement cannot be increased. In fact, the atomic Hilbert space divides into two subspaces, one of which is decoherence free so that the initial atomic entanglement remains available for applications, even in presence of a low enough atomic decay rate. In the other subspace a measure of entanglement, decoherence, and also purity, are described by a similar functional behavior that can be monitored by joint atomic measurements. Furthermore, we show the possible generation of Schr\"odinger-cat-like states for the whole system in the transient regime, as well as of entanglement for the cavity field and the atom-atom subsystems conditioned by measurements on the complementary subsystem.
We present the exact solution of the dynamics of N two-level atoms strongly driven by an external coherent field and equally coupled on resonance to a cavity mode, in the presence of both cavity dissipation and atomic decay. Analytical results are presented for system and subsystem dynamics, showing how environment-induced decoherence leads the system from pure to mixed states. In the limit of negligible atomic decay, where the system is known to exhibit decoherence-free subspaces, we present a detailed discussion of the decoherence function that can be monitored by atomic population measurements.
We describe the unitary dynamics and the entanglement properties of a system where a cavity field mode is resonantly coupled to one or two two-level atoms that are strongly driven in the cavity by a coherent field. The cavity field and one atom prepared in pure states always entangle, except when atoms are maximally polarized. The entanglement is maximum for atoms in the upper or lower state and for interaction times in the strong coupling regime. Two atoms, though correlated via the cavity field, do not entangle if the field is not accessible to measurements as in the microwave domain. However, in the optical regime, a null measurement of the cavity field can conditionally prepare an atom pair in a maximally entangled state.
We consider the resonant interaction of a cavity mode with a two-level atom that is driven by a coherent field while it crosses the cavity. Starting from the cavity field in a coherent state, we show that the state of the system can reach the maximum entanglement after a unitary evolution for long enough interaction times. Also we illustrate how the generation of cavity field superposition states can allow, in the open system dynamics, the observation of their decoherence in atomic correlation measurements, for any initial cavity field state, and even under the combined effects of dissipation, thermal noise, and atomic pumping.