A Deterministic and Nondestructively-Verifiable Photon Number Source

We present a deterministic approach based on continuous measurement and real-time quantum feedback control to prepare arbitrary photon number states of a cavity mode. The procedure passively monitors the number state actually achieved in each feedback stabilized measurement trajectory, thus providing a nondestructively verifiable photon source. The feasibility of a possible cavity QED implementation in the many-atom good-cavity coupling regime is analyzed

Stability of macroscopic entanglement under decoherence

We investigate the lifetime of macroscopic entanglement under the influence of decoherence. For GHZ-type superposition states we find that the lifetime decreases with the size of the system (i.e. the number of independent degrees of freedom) and the effective number of subsystems that remain entangled decreases with time. For a class of other states (e.g. cluster states), however, we show that the lifetime of entanglement is independent of the size of the system.Comment: 5 pages, 1 figur

Multiparticle entanglement purification for graph states

We introduce a class of multiparticle entanglement purification protocols that allow us to distill a large class of entangled states. These include cluster states, GHZ states and various error correction codes all of which belong to the class of two-colorable graph states. We analyze these schemes under realistic conditions and observe that they are scalable, i.e. the threshold value for imperfect local operations does not depend on the number of parties for many of these states. When compared to schemes based on bipartite entanglement purification, the protocol is more efficient and the achievable quality of the purified states is larger. As an application we discuss an experimental realization of the protocol in optical lattices which allows one to purify cluster states.Comment: 4 pages, 2 figures; V2: some typos corrected; V3: published versio