178 research outputs found
A Lorentz-invariant look at quantum clock synchronization protocols based on distributed entanglement
Recent work has raised the possibility that quantum information theory
techniques can be used to synchronize atomic clocks nonlocally. One of the
proposed algorithms for quantum clock synchronization (QCS) requires
distribution of entangled pure singlets to the synchronizing parties. Such
remote entanglement distribution normally creates a relative phase error in the
distributed singlet state which then needs to be purified asynchronously. We
present a fully relativistic analysis of the QCS protocol which shows that
asynchronous entanglement purification is not possible, and, therefore, that
the proposed QCS scheme remains incomplete. We discuss possible directions of
research in quantum information theory which may lead to a complete, working
QCS protocol.Comment: 5 pages; typeset in RevTe
Robustness of Decoherence-Free Subspaces for Quantum Computation
It was shown recently [D.A. Lidar et al., Phys. Rev. Lett. 81, 2594 (1998)]
that within the framework of the semigroup Markovian master equation,
decoherence-free (DF) subspaces exist which are stable to first order in time
to a perturbation. Here this result is extended to the non-Markovian regime and
generalized. In particular, it is shown that within both the semigroup and the
non-Markovian operator sum representation, DF subspaces are stable to all
orders in time to a symmetry-breaking perturbation. DF subspaces are thus ideal
for quantum memory applications. For quantum computation, however, the
stability result does not extend beyond the first order. Thus, to perform
robust quantum computation in DF subspaces, they must be supplemented with
quantum error correcting codes.Comment: 16 pages, no figures. Several changes, including a clarification of
the derivation of the Lindblad equation from the operator sum representation.
To appear in Phys. Rev
Interaction-free generation of entanglement
In this paper, we study how to generate entanglement by interaction-free
measurement. Using Kwiat et al.'s interferometer, we construct a two-qubit
quantum gate that changes a particle's trajectory according to the other
particle's trajectory. We propose methods for generating the Bell state from an
electron and a positron and from a pair of photons by this gate. We also show
that using this gate, we can carry out the Bell measurement with the
probability of 3/4 at the maximum and execute a controlled-NOT operation by the
method proposed by Gottesman and Chuang with the probability of 9/16 at the
maximum. We estimate the success probability for generating the Bell state by
our procedure under imperfect interaction.Comment: 18 pages, Latex2e, 11 eps figures, v2: minor corrections and one
reference added, v3: a minor correctio
Maximizing the entanglement of two mixed qubits
Two-qubit states occupy a large and relatively unexplored Hilbert space. Such
states can be succinctly characterized by their degree of entanglement and
purity. In this letter we investigate entangled mixed states and present a
class of states that have the maximum amount of entanglement for a given linear
entropy.Comment: 4 pages, 3 figure
Disentanglement and Inseparability correlation : in two-qubit system
Started from local universal isotropic disentanglement, a threshold
inequality on reduction factors is proposed, which is necessary and sufficient
for this type of disentanglement processes. Furthermore, we give the conditions
realizing ideal disentanglement processes provided that some information on
quantum states is known. In addition, based on fully entangled fraction, a
concept called inseparability correlation is presented. Some properties on
inseparability correlation coefficient are studied.Comment: 10 Pages, 2 Figures, REVTeX; to appear in PR
Reducing the communication complexity with quantum entanglement
We propose a probabilistic two-party communication complexity scenario with a
prior nonmaximally entangled state, which results in less communication than
that is required with only classical random correlations. A simple all-optical
implementation of this protocol is presented and demonstrates our conclusion.Comment: 4 Pages, 2 Figure
Quantum Gambling Using Two Nonorthogonal States
We give a (remote) quantum gambling scheme that makes use of the fact that
quantum nonorthogonal states cannot be distinguished with certainty. In the
proposed scheme, two participants Alice and Bob can be regarded as playing a
game of making guesses on identities of quantum states that are in one of two
given nonorthogonal states: if Bob makes a correct (an incorrect) guess on the
identity of a quantum state that Alice has sent, he wins (loses). It is shown
that the proposed scheme is secure against the nonentanglement attack. It can
also be shown heuristically that the scheme is secure in the case of the
entanglement attack.Comment: no essential correction, 4 pages, RevTe
Analysis of Generalized Grover's Quantum Search Algorithms Using Recursion Equations
The recursion equation analysis of Grover's quantum search algorithm
presented by Biham et al. [PRA 60, 2742 (1999)] is generalized. It is applied
to the large class of Grover's type algorithms in which the Hadamard transform
is replaced by any other unitary transformation and the phase inversion is
replaced by a rotation by an arbitrary angle. The time evolution of the
amplitudes of the marked and unmarked states, for any initial complex amplitude
distribution is expressed using first order linear difference equations. These
equations are solved exactly. The solution provides the number of iterations T
after which the probability of finding a marked state upon measurement is the
highest, as well as the value of this probability, P_max. Both T and P_max are
found to depend on the averages and variances of the initial amplitude
distributions of the marked and unmarked states, but not on higher moments.Comment: 8 pages, no figures. To appear in Phys. Rev.
Experimental realization of the one qubit Deutsch-Jozsa algorithm in a quantum dot
We perform quantum interference experiments on a single self-assembled
semiconductor quantum dot. The presence or absence of a single exciton in the
dot provides a qubit that we control with femtosecond time resolution. We
combine a set of quantum operations to realize the single-qubit Deutsch-Jozsa
algorithm. The results show the feasibility of single qubit quantum logic in a
semiconductor quantum dot using ultrafast optical control.Comment: REVTex4, 4 pages, 3 figures. Now includes more details about the
dephasing in the quantum dots. The introduction has been reworded for
clarity. Minor readability fixe
Decoherence as Decay of the Loschmidt Echo in a Lorentz Gas
Classical chaotic dynamics is characterized by the exponential sensitivity to
initial conditions. Quantum mechanics, however, does not show this feature. We
consider instead the sensitivity of quantum evolution to perturbations in the
Hamiltonian. This is observed as an atenuation of the Loschmidt Echo, ,
i.e. the amount of the original state (wave packet of width ) which is
recovered after a time reversed evolution, in presence of a classically weak
perturbation. By considering a Lorentz gas of size , which for large is
a model for an {\it unbounded} classically chaotic system, we find numerical
evidence that, if the perturbation is within a certain range, decays
exponentially with a rate determined by the Lyapunov exponent
of the corresponding classical dynamics. This exponential decay
extends much beyond the Eherenfest time and saturates at a time
, where is the effective dimensionality of the Hilbert space. Since quantifies the increasing uncontrollability of the quantum phase
(decoherence) its characterization and control has fundamental interest.Comment: 3 ps figures, uses Revtex and epsfig. Major revision to the text, now
including discussion and references on averaging and Ehrenfest time. Figures
2 and 3 content and order change
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