933 research outputs found
Quantum Markov Channels for Qubits
We examine stochastic maps in the context of quantum optics. Making use of
the master equation, the damping basis, and the Bloch picture we calculate a
non-unital, completely positive, trace-preserving map with unequal damping
eigenvalues. This results in what we call the squeezed vacuum channel. A
geometrical picture of the effect of stochastic noise on the set of pure state
qubit density operators is provided. Finally, we study the capacity of the
squeezed vacuum channel to transmit quantum information and to distribute EPR
states.Comment: 18 pages, 4 figure
Atom correlations and spin squeezing near the Heisenberg limit: finite system size effect and decoherence
We analyze a model for spin squeezing based on the so-called counter-twisting
Hamiltonian, including the effects of dissipation and finite system size. We
discuss the conditions under which the Heisenberg limit, i.e. phase sensitivity
, can be achieved. A specific implementation of this model based
on atom-atom interactions via quantized photon exchange is presented in detail.
The resulting excitation corresponds to the creation of spin-flipped atomic
pairs and can be used for fast generation of entangled atomic ensembles, spin
squeezing and apllications in quantum information processing. The conditions
for achieving strong spin squeezing with this mechanism are also analyzed.Comment: 15 pages, 8 figure
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