398 research outputs found
Optimal N-to-M Cloning of Quantum Coherent States
The cloning of continuous quantum variables is analyzed based on the concept
of Gaussian cloning machines, i.e., transformations that yield copies that are
Gaussian mixtures centered on the state to be copied. The optimality of
Gaussian cloning machines that transform N identical input states into M output
states is investigated, and bounds on the fidelity of the process are derived
via a connection with quantum estimation theory. In particular, the optimal
N-to-M cloning fidelity for coherent states is found to be equal to
MN/(MN+M-N).Comment: 3 pages, RevTe
Universal Quantum Cloning in Cavity QED
We propose an implementation of an universal quantum cloning machine [UQCM,
Hillery and Buzek, Phys. Rev. A {\bf 56}, 3446 (1997)] in a Cavity Quantum
Electrodynamics (CQED) experiment. This UQCM acts on the electronic states of
atoms that interact with the electromagnetic field of a high cavity. We
discuss here the specific case of the cloning process using either a
one- or a two-cavity configuration
Advancement of estimation fidelity in continuous quantum measurement
We estimate an unknown qubit from the long sequence of n random polarization
measurements of precision Delta. Using the standard Ito-stochastic equations of
the aposteriori state in the continuous measurement limit we calculate the
advancement of fidelity. We show that the standard optimum value 2/3 is
achieved asymptotically for n >> Delta^2 / 96 >> 1. We append a brief
derivation of novel Ito-equations for the estimate state.Comment: 12 pp LaTe
Reversibility of continuous-variable quantum cloning
We analyze a reversibility of optimal Gaussian quantum cloning of a
coherent state using only local operations on the clones and classical
communication between them and propose a feasible experimental test of this
feature. Performing Bell-type homodyne measurement on one clone and anti-clone,
an arbitrary unknown input state (not only a coherent state) can be restored in
the other clone by applying appropriate local unitary displacement operation.
We generalize this concept to a partial LOCC reversal of the cloning and we
show that this procedure converts the symmetric cloner to an asymmetric cloner.
Further, we discuss a distributed LOCC reversal in optimal Gaussian
cloning of coherent states which transforms it to optimal cloning for
. Assuming the quantum cloning as a possible eavesdropping attack on
quantum communication link, the reversibility can be utilized to improve the
security of the link even after the attack.Comment: 7 pages, 5 figure
Experimental detection of entanglement via witness operators and local measurements
In this paper we address the problem of detection of entanglement using only
few local measurements when some knowledge about the state is given. The idea
is based on an optimized decomposition of witness operators into local
operators. We discuss two possible ways of optimizing this local decomposition.
We present several analytical results and estimates for optimized detection
strategies for NPT states of 2x2 and NxM systems, entangled states in 3 qubit
systems, and bound entangled states in 3x3 and 2x4 systems.Comment: 24 pages, 2 figures. Contribution to the proceedings of the
International Conference on Quantum Information in Oviedo, Spain (July 13-18,
2002). Error in W_W1-witness Eq. (35) corrected as well as minor typos.
Reference adde
Optimal purification of single qubits
We introduce a new decomposition of the multiqubit states of the form
and employ it to construct the optimal single qubit
purification procedure. The same decomposition allows us to study optimal
quantum cloning and state estimation of mixed states.Comment: 4 pages, 1 figur
Quantum cloning and the capacity of the Pauli channel
A family of quantum cloning machines is introduced that produce two
approximate copies from a single quantum bit, while the overall input-to-output
operation for each copy is a Pauli channel. A no-cloning inequality is derived,
describing the balance between the quality of the two copies. This also
provides an upper bound on the quantum capacity of the Pauli channel with
probabilities , and . The capacity is shown to be vanishing if
lies outside an ellipsoid whose pole
coincides with the depolarizing channel that underlies the universal cloning
machine.Comment: 5 pages RevTeX, 3 Postscript figure
Nonlinear dynamics of two coupled nano-electromechanical resonators
As a model of coupled nano-electromechanical resonantors we study two
nonlinear driven oscillators with an arbitrary coupling strength between them.
Analytical expressions are derived for the oscillation amplitudes as a function
of the driving frequency and for the energy transfer rate between the two
oscillators. The nonlinear restoring forces induce the expected nonlinear
resonance structures in the amplitude-frequency characteristics with asymmetric
resonance peaks. The corresponding multistable behavior is shown to be an
efficient tool to control the energy transfer arising from the sensitive
response to small changes in the driving frequency. Our results imply that the
nonlinear response can be exploited to design precise sensors for mass or force
detection experiments based on nano-electromechanical resonators.Comment: 19 pages, 2 figure
Distributed phase-covariant cloning with atomic ensembles via quantum Zeno dynamics
We propose an interesting scheme for distributed orbital state quantum
cloning with atomic ensembles based on the quantum Zeno dynamics. These atomic
ensembles which consist of identical three-level atoms are trapped in distant
cavities connected by a single-mode integrated optical star coupler. These
qubits can be manipulated through appropriate modulation of the coupling
constants between atomic ensemble and classical field, and the cavity decay can
be largely suppressed as the number of atoms in the ensemble qubits increases.
The fidelity of each cloned qubit can be obtained with analytic result. The
present scheme provides a new way to construct the quantum communication
network.Comment: 5 pages, 4 figure
Optimal minimal measurements of mixed states
The optimal and minimal measuring strategy is obtained for a two-state system
prepared in a mixed state with a probability given by any isotropic a priori
distribution. We explicitly construct the specific optimal and minimal
generalized measurements, which turn out to be independent of the a priori
probability distribution, obtaining the best guesses for the unknown state as
well as a closed expression for the maximal mean averaged fidelity. We do this
for up to three copies of the unknown state in a way which leads to the
generalization to any number of copies, which we then present and prove.Comment: 20 pages, no figure
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