681 research outputs found
On experimental procedures for entanglement verification
We give an overview of different types of entanglement that can be generated
in experiments, as well as of various protocols that can be used to verify or
quantify entanglement. We propose several criteria that, we argue, should be
applied to experimental entanglement verification procedures. Explicit examples
demonstrate that not following these criteria will tend to result in
overestimating the amount of entanglement generated in an experiment or in
infering entanglement when there is none. We distinguish protocols meant to
refute or eliminate hidden-variable models from those meant to verify
entanglement.Comment: 15 page
Decoherence and the conditions for the classical control of quantum systems
We find the conditions for one quantum system to function as a classical
controller of another quantum system: the controller must be an open system and
rapidly diagonalised in the basis of the controller variable that is coupled to
the controlled system. This causes decoherence in the controlled system that
can be made small if the rate of diagonalisation is fast. We give a detailed
example based on the quantum optomechanical control of a mechanical resonator.
The resulting equations are similar in structure to recently proposed models
for consistently combining quantum and classical stochastic dynamics
Entanglement of internal and external angular momenta of a single atom
We consider the exchange of spin and orbital angular momenta between a
circularly polarized Laguerre-Gaussian beam of light and a single atom trapped
in a two-dimensional harmonic potential. The radiation field is treated
classically but the atomic center-of-mass motion is quantized. The spin and
orbital angular momenta of the field are individually conserved upon
absorption, and this results in the entanglement of the internal and external
degrees of freedom of the atom. We suggest applications of this entanglement in
quantum information processing.Comment: 4 pages, 2 figure
Photons in polychromatic rotating modes
We propose a quantum theory of rotating light beams and study some of its
properties. Such beams are polychromatic and have either a slowly rotating
polarization or a slowly rotating transverse mode pattern. We show there are,
for both cases, three different natural types of modes that qualify as
rotating, one of which is a new type not previously considered. We discuss
differences between these three types of rotating modes on the one hand and
non-rotating modes as viewed from a rotating frame of reference on the other.
We present various examples illustrating the possible use of rotating photons,
mostly for quantum information processing purposes. We introduce in this
context a rotating version of the two-photon singlet state.Comment: enormously expanded: 12 pages, 3 figures; a new, more informative,
but less elegant title, especially designed for Phys. Rev.
The Effect of Stochastic Noise on Quantum State Transfer
We consider the effect of classical stochastic noise on control laser pulses
used in a scheme for transferring quantum information between atoms, or quantum
dots, in separate optical cavities via an optical connection between cavities.
We develop a master equation for the dynamics of the system subject to
stochastic errors in the laser pulses, and use this to evaluate the sensitivity
of the transfer process to stochastic pulse shape errors for a number of
different pulse shapes. We show that under certain conditions, the sensitivity
of the transfer to the noise depends on the pulse shape, and develop a method
for determining a pulse shape that is minimally sensitive to specific errors.Comment: 10 pages, 9 figures, to appear in Physical Review
Error free quantum communication through noisy channels
We suggest a method to perform a quantum logic gate between distant qubits by
off-resonant field-atom dispersive interactions. The scheme we present is shown
to work ideally even in the presence of errors in the photon channels used for
communication. The stability against errors arises from the paradoxical
situation that the transmitted photons carry no information about the state of
the qubits. In contrast to a previous proposal for ideal communication [Phys.
Rev. Lett. 78, 4293 (1997)] our proposal only involves single atoms in the
sending and receiving devices.Comment: 6 pages, including 2 figure
The power of random measurements: measuring Tr(\rho^n) on single copies of \rho
While it is known that Tr(\rho^n) can be measured directly (i.e., without
first reconstructing the density matrix) by performing joint measurements on n
copies of the same state rho, it is shown here that random measurements on
single copies suffice, too. Averaging over the random measurements directly
yields estimates of Tr(\rho^n), even when it is not known what measurements
were actually performed (so that one cannot reconstruct \rho)
The cryptographic power of misaligned reference frames
Suppose that Alice and Bob define their coordinate axes differently, and the
change of reference frame between them is given by a probability distribution
mu over SO(3). We show that this uncertainty of reference frame is of no use
for bit commitment when mu is uniformly distributed over a (sub)group of SO(3),
but other choices of mu can give rise to a partially or even asymptotically
secure bit commitment.Comment: 4 pages Latex; v2 has a new referenc
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