204 research outputs found
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
Subspace local quantum channels
A special class of quantum channels, named subspace local (SL), are defined
and investigated. The proposed definition of subspace locality of quantum
channels is an attempt to answer the question of what kind of restriction
should be put on a channel, if it is to act `locally' with respect to two
`locations', when these naturally correspond to a separation of the total
Hilbert space in an orthogonal sum of subspaces, rather than a tensor product
decomposition. It is shown that the set of SL channels decomposes into four
disjoint families of channels. Explicit expressions to generate all channels in
each family is presented. It is shown that one of these four families, the
local subspace preserving (LSP) channels, is precisely the intersection between
the set of subspace preserving channels and the SL channels. For a subclass of
the LSP channels, a special type of unitary representation using ancilla
systems is presented.Comment: References adde
Surface Acoustic Wave Single-Electron Interferometry
We propose an experiment to observe interference of a single electron as it
is transported along two parallel quasi-one-dimensional channels trapped in a
single minimum of a travelling periodic electric field. The experimental device
is a modification of the surface acoustic wave (SAW) based quantum processor.
Interference is achieved by creating a superposition of spatial wavefunctions
between the two channels and inducing a relative phase shift via either a
transverse electric field or a magnetic field. The interference can be used to
estimate the decoherence time of an electron in this type of solid-state
device
Delocalization power of global unitary operations on quantum information
We investigate how originally localized two pieces of quantum information
represented by a tensor product of two unknown qudit states are delocalized by
performing two-qudit global unitary operations. To characterize the
delocalization power of global unitary operations on quantum information, we
analyze the necessary and sufficient condition to deterministically relocalize
one of the two pieces of quantum information to its original Hilbert space by
using only LOCC. We prove that this LOCC one-piece relocalization is possible
if and only if the global unitary operation is local unitary equivalent to a
controlled-unitary operation. The delocalization power and the entangling power
characterize different non-local properties of global unitary operations.Comment: 14 pages, 1 figur
Decision problems with quantum black boxes
We examine how to distinguish between unitary operators, when the exact form
of the possible operators is not known. Instead we are supplied with "programs"
in the form of unitary transforms, which can be used as references for
identifying the unknown unitary transform. All unitary transforms should be
used as few times as possible. This situation is analoguous to programmable
state discrimination. One difference, however, is that the quantum state to
which we apply the unitary transforms may be entangled, leading to a richer
variety of possible strategies. By suitable selection of an input state and
generalized measurement of the output state, both unambiguous and minimum-error
discrimination can be achieved. Pairwise comparison of operators, comparing
each transform to be identified with a program transform, is often a useful
strategy. There are, however, situations in which more complicated strategies
perform better. This is the case especially when the number of allowed
applications of program operations is different from the number of the
transforms to be identified
Identifying a Two-State Hamiltonian in the Presence of Decoherence
Mapping the system evolution of a two-state system allows the determination
of the effective system Hamiltonian directly. We show how this can be achieved
even if the system is decohering appreciably over the observation time. A
method to include various decoherence models is given and the limits of this
technique are explored. This technique is applicable both to the problem of
calibrating a control Hamiltonian for quantum computing applications and for
precision experiments in two-state quantum systems. For simple models of
decoherence, this method can be applied even when the decoherence time is
comparable to the oscillation period of the system.Comment: 8 pages, 6 figures. Minor corrections, published versio
Direct estimations of linear and non-linear functionals of a quantum state
We present a simple quantum network, based on the controlled-SWAP gate, that
can extract certain properties of quantum states without recourse to quantum
tomography. It can be used used as a basic building block for direct quantum
estimations of both linear and non-linear functionals of any density operator.
The network has many potential applications ranging from purity tests and
eigenvalue estimations to direct characterization of some properties of quantum
channels. Experimental realizations of the proposed network are within the
reach of quantum technology that is currently being developed.Comment: This paper supersedes the paper quant-ph/0112073, titled "Universal
Quantum Estimator". We emphasise the estimation of linear and non-linear
functionals of a quantum stat
Identifying an Experimental Two-State Hamiltonian to Arbitrary Accuracy
Precision control of a quantum system requires accurate determination of the
effective system Hamiltonian. We develop a method for estimating the
Hamiltonian parameters for some unknown two-state system and providing
uncertainty bounds on these parameters. This method requires only one
measurement basis and the ability to initialise the system in some arbitrary
state which is not an eigenstate of the Hamiltonian in question. The scaling of
the uncertainty is studied for large numbers of measurements and found to be
proportional to one on the square-root of the number of measurements.Comment: Minor corrections, Accepted for publication in Physical Review
Evidence for Efimov quantum states in an ultracold gas of cesium atoms
Systems of three interacting particles are notorious for their complex
physical behavior. A landmark theoretical result in few-body quantum physics is
Efimov's prediction of a universal set of bound trimer states appearing for
three identical bosons with a resonant two-body interaction.
Counterintuitively, these states even exist in the absence of a corresponding
two-body bound state. Since the formulation of Efimov's problem in the context
of nuclear physics 35 years ago, it has attracted great interest in many areas
of physics. However, the observation of Efimov quantum states has remained an
elusive goal. Here we report the observation of an Efimov resonance in an
ultracold gas of cesium atoms. The resonance occurs in the range of large
negative two-body scattering lengths, arising from the coupling of three free
atoms to an Efimov trimer. Experimentally, we observe its signature as a giant
three-body recombination loss when the strength of the two-body interaction is
varied. We also detect a minimum in the recombination loss for positive
scattering lengths, indicating destructive interference of decay pathways. Our
results confirm central theoretical predictions of Efimov physics and represent
a starting point with which to explore the universal properties of resonantly
interacting few-body systems. While Feshbach resonances have provided the key
to control quantum-mechanical interactions on the two-body level, Efimov
resonances connect ultracold matter to the world of few-body quantum phenomena.Comment: 18 pages, 3 figure
Physics-based mathematical models for quantum devices via experimental system identification
We consider the task of intrinsic control system identification for quantum
devices. The problem of experimental determination of subspace confinement is
considered, and simple general strategies for full Hamiltonian identification
and decoherence characterization of a controlled two-level system are
presented.Comment: 15 pages, 8 figures, published in proceedings of workshop on
Physics-based mathematical models of low-dimensional semi-conductor
nanostructures (18-23 November, 2007, Banff International Research Station,
Alberta, Canada
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