329 research outputs found
Decoherence free algebra
We consider the decoherence free subalgebra which satisfies the minimal
condition introduced by Alicki. We show the manifest form of it and relate the
subalgebra with the Kraus representation. The arguments also provides a new
proof for generalized L\"{u}ders theorem.Comment: To appear in Physics Letters A v2.minor chang
Hypervelocity atomic oxygen source for the study of atom-surface interactions
Planned improvements in an electric discharge heated atomic oxygen beam source are described which will provide 6 to 7 kms(-1) beams of atomic oxygen with a flux of 10(16) cm(-2) s(-1) at 50 cm distance from the source aperture. A major advance will be the use of a zone of silence nozzle-skimmer arrangement which is necessitated by the need for high source flux and performance. It is anticipated that a Phase 2 program would provide for the fabrication of a two stage vacuum system which would be suitable for bolting on to a UHV (ultrahigh vacuum) surface study apparatus
Optimal Cloning of Pure States, Judging Single Clones
We consider quantum devices for turning a finite number N of d-level quantum
systems in the same unknown pure state \sigma into M>N systems of the same
kind, in an approximation of the M-fold tensor product of the state \sigma. In
a previous paper it was shown that this problem has a unique optimal solution,
when the quality of the output is judged by arbitrary measurements, involving
also the correlations between the clones. We show in this paper, that if the
quality judgement is based solely on measurements of single output clones,
there is again a unique optimal cloning device, which coincides with the one
found previously.Comment: 16 Pages, REVTe
Approximations of subhomogeneous algebras
Let be a natural number. Recall that a C*-algebra is said to be
-subhomogeneous if all its irreducible representations have dimension at
most . In this short note, we give various approximation properties
characterising -subhomogeneous C*-algebras.Comment: 9 pages; v2 minor improvement in the introduction, 10 page
A Unified and Generalized Approach to Quantum Error Correction
We present a unified approach to quantum error correction, called operator
quantum error correction. This scheme relies on a generalized notion of
noiseless subsystems that is not restricted to the commutant of the interaction
algebra. We arrive at the unified approach, which incorporates the known
techniques -- i.e. the standard error correction model, the method of
decoherence-free subspaces, and the noiseless subsystem method -- as special
cases, by combining active error correction with this generalized noiseless
subsystem method. Moreover, we demonstrate that the quantum error correction
condition from the standard model is a necessary condition for all known
methods of quantum error correction.Comment: 5 page
Quantum Channels and Representation Theory
In the study of d-dimensional quantum channels , an assumption
which is not very restrictive, and which has a natural physical interpretation,
is that the corresponding Kraus operators form a representation of a Lie
algebra. Physically, this is a symmetry algebra for the interaction
Hamiltonian. This paper begins a systematic study of channels defined by
representations; the famous Werner-Holevo channel is one element of this
infinite class. We show that the channel derived from the defining
representation of SU(n) is a depolarizing channel for all , but for most
other representations this is not the case. Since the Bloch sphere is not
appropriate here, we develop technology which is a generalization of Bloch's
technique. Our method works by representing the density matrix as a polynomial
in symmetrized products of Lie algebra generators, with coefficients that are
symmetric tensors. Using these tensor methods we prove eleven theorems, derive
many explicit formulas and show other interesting properties of quantum
channels in various dimensions, with various Lie symmetry algebras. We also
derive numerical estimates on the size of a generalized ``Bloch sphere'' for
certain channels. There remain many open questions which are indicated at
various points through the paper.Comment: 28 pages, 1 figur
Quantum erasure of decoherence
We consider the classical algebra of observables that are diagonal in a given
orthonormal basis, and define a complete decoherence process as a completely
positive map that asymptotically converts any quantum observable into a
diagonal one, while preserving the elements of the classical algebra. For
quantum systems in dimension two and three any decoherence process can be
undone by collecting classical information from the environment and using such
an information to restore the initial system state. As a relevant example, we
illustrate the quantum eraser of Scully et al. [Nature 351, 111 (1991)] as an
example of environment-assisted correction. Moreover, we present the
generalization of the eraser setup for d-dimensional systems, showing that any
von Neumann measurement on a system can be undone by a complementary
measurement on the environment.Comment: 10 pages, 1 figur
Distilling entanglement from arbitrary resources
We obtain the general formula for the optimal rate at which singlets can be
distilled from any given noisy and arbitrarily correlated entanglement
resource, by means of local operations and classical communication (LOCC). Our
formula, obtained by employing the quantum information spectrum method, reduces
to that derived by Devetak and Winter, in the special case of an i.i.d.
resource. The proofs rely on a one-shot version of the so-called "hashing
bound," which in turn provides bounds on the one-shot distillable entanglement
under general LOCC.Comment: 24 pages, article class, no figure. v2: references added, published
versio
Semiquantum key distribution using entangled states
Recently, Boyer et al. presented a novel semiquantum key distribution
protocol [M. Boyer, D. Kenigsberg, and T. Mor, Phys. Rev. Lett. 99, 140501
(2007)], by using four quantum states, each of which is randomly prepared by Z
basis or X basis. Here we present a semiquantum key distribution protocol by
using entangled states in which quantum Alice shares a secret key with
classical Bob. We also show the protocol is secure against eavesdropping.Comment: 6 page
Interference of Quantum Channels
We show how interferometry can be used to characterise certain aspects of
general quantum processes, in particular, the coherence of completely positive
maps. We derive a measure of coherent fidelity, maximum interference visibility
and the closest unitary operator to a given physical process under this
measure.Comment: 4 pages, 5 figures, REVTeX 4, typographical corrections and added
acknowledgemen
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