Preparation Of Entanglement Research Articles

Preparation of Fock states and quantum entanglement via stimulated Raman adiabatic passage using a four-level atom

We study the behaviour of an atom-cavity system exposed to a stimulated Raman adiabatic passage (STIRAP) process in a four-level system, with a coupling scheme which generate two degenerate dark states. We find that the non-adiabatic interaction of the two dark states guarantees that the cavity Fock states can always be generated by both intuitively and counterintuitively ordered pulses. Furthermore, we propose a method to entangle two atoms. Depending on the ordering of the pulses two orthogonal entangled states can be prepared. Since these entangled states do not have component of the excited states included, the technique is robust against the detrimental consequences of spontaneous emission.

Entanglement Swapping as Event-Ready Entanglement Preparation

In this contribution the preparation of event-ready entanglement in polarised photons by means of entanglement swapping is studied. This protocol makes use of Gaussian photon-sources. Since the states resulting from this protocol are equivalent to those obtained by mixing two photons in a 50:50 beam splitter, the nature of the emerging entanglement is controversial. Several methods to enhance the fidelity of the outgoing state are studied. It is found that at least quantum non-demolition measurements or a quantum computer of some kind is needed to create event-ready entanglement with entanglement swapping.

Entanglement and state preparation

When a subset of particles in an entangled state is measured, the state of the subset of unmeasured particles is determined by the outcome of the measurement. This first measurement may be thought of as a state preparation for the remaining particles. This type of measurement is important in quantum computing, quantum information theory, and in the preparation of entangled states such as the Greenberger-Horne--Zeilinger state. In this paper, we examine how the duration of the first measurement effects the state of the unmeasured subsystem. We discuss the case for which the particles are photons, but the theory is sufficiently general that it can be converted to a discussion of any type of particle. The state of the unmeasured subsytem will be a pure or mixed state depending on the nature of the measurement. In the case of quantum teleportation we show that there is an eigenvalue equation which must be satisfied for accurate teleportation. This equation provides a limitation to the states that can be accurately teleported.