48 research outputs found
Fidelity for displaced squeezed states and the oscillator semigroup
The fidelity for two displaced squeezed thermal states is computed using the
fact that the corresponding density operators belong to the oscillator
semigroup.Comment: 3 pages, REVTEX, no figures, submitted to Journal of Physics A, May
5, 199
Statistical distinguishability between unitary operations
The problem of distinguishing two unitary transformations, or quantum gates,
is analyzed and a function reflecting their statistical distinguishability is
found. Given two unitary operations, and , it is proved that there
always exists a finite number such that and are perfectly distinguishable, although they were not in the single-copy
case. This result can be extended to any finite set of unitary transformations.
Finally, a fidelity for one-qubit gates, which satisfies many useful properties
from the point of view of quantum information theory, is presented.Comment: 6 pages, REVTEX. The perfect distinguishability result is extended to
any finite set of gate
Bures distance between two displaced thermal states
The Bures distance between two displaced thermal states and the corresponding
geometric quantities (statistical metric, volume element, scalar curvature) are
computed. Under nonunitary (dissipative) dynamics, the statistical distance
shows the same general features previously reported in the literature by
Braunstein and Milburn for two--state systems. The scalar curvature turns out
to have new interesting properties when compared to the curvature associated
with squeezed thermal states.Comment: 3 pages, RevTeX, no figure
Non-adaptive Measurement-based Quantum Computation and Multi-party Bell Inequalities
Quantum correlations exhibit behaviour that cannot be resolved with a local
hidden variable picture of the world. In quantum information, they are also
used as resources for information processing tasks, such as Measurement-based
Quantum Computation (MQC). In MQC, universal quantum computation can be
achieved via adaptive measurements on a suitable entangled resource state. In
this paper, we look at a version of MQC in which we remove the adaptivity of
measurements and aim to understand what computational abilities still remain in
the resource. We show that there are explicit connections between this model of
computation and the question of non-classicality in quantum correlations. We
demonstrate this by focussing on deterministic computation of Boolean
functions, in which natural generalisations of the Greenberger-Horne-Zeilinger
(GHZ) paradox emerge; we then explore probabilistic computation, via which
multipartite Bell Inequalities can be defined. We use this correspondence to
define families of multi-party Bell inequalities, which we show to have a
number of interesting contrasting properties.Comment: 13 pages, 4 figures, final version accepted for publicatio
Chow's theorem and universal holonomic quantum computation
A theorem from control theory relating the Lie algebra generated by vector
fields on a manifold to the controllability of the dynamical system is shown to
apply to Holonomic Quantum Computation. Conditions for deriving the holonomy
algebra are presented by taking covariant derivatives of the curvature
associated to a non-Abelian gauge connection. When applied to the Optical
Holonomic Computer, these conditions determine that the holonomy group of the
two-qubit interaction model contains . In particular, a
universal two-qubit logic gate is attainable for this model.Comment: 13 page
Fidelity for Multimode Thermal Squeezed States
In the theory of quantum transmission of information the concept of fidelity
plays a fundamental role. An important class of channels, which can be
experimentally realized in quantum optics, is that of Gaussian quantum
channels. In this work we present a general formula for fidelity in the case of
two arbitrary Gaussian states. From this formula one can get a previous result
(H. Scutaru, J. Phys. A: Mat. Gen {\bf 31}, 3659 (1998)), for the case of a
single mode; or, one can apply it to obtain a closed compact expression for
multimode thermal states.Comment: 5 pages, RevTex, submitted to Phys. Rev.
Measuring the Quantum State of a Large Angular Momentum
We demonstrate a general method to measure the quantum state of an angular
momentum of arbitrary magnitude. The (2F+1) x (2F+1) density matrix is
completely determined from a set of Stern-Gerlach measurements with (4F+1)
different orientations of the quantization axis. We implement the protocol for
laser cooled Cesium atoms in the 6S_{1/2}(F=4) hyperfine ground state and apply
it to a variety of test states prepared by optical pumping and Larmor
precession. A comparison of input and measured states shows typical
reconstruction fidelities of about 0.95.Comment: 4 pages, 6 figures, submitted to PR
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
Irreversibility in asymptotic manipulations of entanglement
We show that the process of entanglement distillation is irreversible by
showing that the entanglement cost of a bound entangled state is finite. Such
irreversibility remains even if extra pure entanglement is loaned to assist the
distillation process.Comment: RevTex, 3 pages, no figures Result on indistillability of PPT states
under pure entanglement catalytic LOCC adde
Asymmetric universal entangling machine
We give a definition of asymmetric universal entangling machine which
entangles a system in an unknown state to a specially prepared ancilla. The
machine produces a fixed state-independent amount of entanglement in exchange
to a fixed degradation of the system state fidelity. We describe explicitly
such a machine for any quantum system having levels and prove its
optimality. We show that a -dimensional ancilla is sufficient for reaching
optimality. The introduced machine is a generalization to a number of widely
investigated universal quantum devices such as the symmetric and asymmetric
quantum cloners, the symmetric quantum entangler, the quantum information
distributor and the universal-NOT gate.Comment: 28 pages, 3 figure