521 research outputs found
Continuous-variable teleportation: a new look
In contrast to discrete-variable teleportation, a quantum state is
imperfectly transferred from a sender to a remote receiver in a
continuous-variable setting. We recall the ingenious scheme proposed by
Braunstein and Kimble for teleporting a one-mode state of the quantum radiation
field. By analyzing this protocol, we have previously proven the factorization
of the characteristic function of the output state. This indicates that
teleportation is a noisy process that alters, to some extent, the input state.
Teleportation with a two-mode Gaussian EPR state can be described in terms of
the superposition of a distorting field with the input one. Here we analyze the
one-mode Gaussian distorting-field state. Some of its most important properties
are determined by the statistics of a positive EPR operator in the two-mode
Gaussian resource state. We finally examine the fidelity of teleportation of a
coherent state when using an arbitrary resource state.Comment: Contribution to the special issue of Romanian Journal of Physics
dedicated to the centenary of Serban Titeica (1908-1985), the founder of the
school of theoretical physics in Romani
Continuous-variable teleportation in the characteristic-function description
We give a description of the continuous-variable teleportation protocol in
terms of the characteristic functions of the quantum states involved. The
Braunstein--Kimble protocol is written for an unbalanced homodyne measurement
and arbitrary input and resource states. We show that the output of the
protocol is a superposition between the input one-mode field and a classical
one induced by measurement and classical communication. We choose to describe
the input state distortion through teleportation by the average photon number
of the measurement-induced field. Only in the case of symmetric resource states
we find a relation between the optimal added noise and the minimal EPR
correlations used to define inseparability.Comment: 12 page
Bures distance as a measure of entanglement for symmetric two-mode Gaussian states
We evaluate a Gaussian entanglement measure for a symmetric two-mode Gaussian
state of the quantum electromagnetic field in terms of its Bures distance to
the set of all separable Gaussian states. The required minimization procedure
was considerably simplified by using the remarkable properties of the Uhlmann
fidelity as well as the standard form II of the covariance matrix of a
symmetric state. Our result for the Gaussian degree of entanglement measured by
the Bures distance depends only on the smallest symplectic eigenvalue of the
covariance matrix of the partially transposed density operator. It is thus
consistent to the exact expression of the entanglement of formation for
symmetric two-mode Gaussian states. This non-trivial agreement is specific to
the Bures metric.Comment: published versio
Optimal purifications and fidelity for displaced thermal states
We evaluate the Uhlmann fidelity between two one-mode displaced thermal
states as the maximal probability transition between appropriate purifications
of the given states. The optimal purifications defining the fidelity are proved
to be two-mode displaced Gaussian states.Comment: published versio
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