Quantum Information Processing Using Spin Coherent States

Spin coherent states are the matter equivalent of optical coherent states, where a large number of spins occupy the same quantum state.  Experiments have now advanced to the point that such states, as realized in atomic Bose-Einstein condensates (BECs), can be coherently controlled with long  decoherence times. Here we discuss a method of using spin coherent states, particularly of the BEC form, directly for quantum information processing.  We construct a general framework for quantum algorithms to be executed using spin coherent states.  We illustrate the scheme by an application to quantum algorithms and discuss the effects of decoherence induced by the large number of particles in the BEC.  We also discuss a new quantum teleportation protocol that allows for the transfer of a spin coherent states between two parties, assisted by entanglement.  In the protocol, two macroscopic spin state are entangled, and an initially unknown spin coherent state is transferred to a distant location.  Unlike standard quantum teleportation where a single qubit is transferred, a macroscopic ensemble of spins is teleported in our scheme.  By the use of a special class of entangled states, it is possible to avoid the detrimental effects of decoherence on such macroscopic state on the protocol.