123 research outputs found
Transforming quantum operations: quantum supermaps
We introduce the concept of quantum supermap, describing the most general
transformation that maps an input quantum operation into an output quantum
operation. Since quantum operations include as special cases quantum states,
effects, and measurements, quantum supermaps describe all possible
transformations between elementary quantum objects (quantum systems as well as
quantum devices). After giving the axiomatic definition of supermap, we prove a
realization theorem, which shows that any supermap can be physically
implemented as a simple quantum circuit. Applications to quantum programming,
cloning, discrimination, estimation, information-disturbance trade-off, and
tomography of channels are outlined.Comment: 6 pages, 1 figure, published versio
Optimization of quantum universal detectors
The expectation value of an arbitrary operator O can be obtained via a
universal measuring apparatus that is independent of O, by changing only the
data-processing of the outcomes. Such a ``universal detector'' performs a joint
measurement on the system and on a suitable ancilla prepared in a fixed state,
and is equivalent to a positive operator valued measure (POVM) for the system
that is ``informationally complete''. The data processing functions generally
are not unique, and we pose the problem of their optimization, providing some
examples for covariant POVM's, in particular for SU(d) covariance group.Comment: 8 pages, no figures. Proceedingsof the 8th International Conference
on Squeezed States and Uncertainty Relations ICSSUR' 2003, Puebla, Mexico -
June 9-13, 200
Informationally complete measurements on bipartite quantum systems: comparing local with global measurements
Informationally complete measurements allow the estimation of expectation
values of any operator on a quantum system, by changing only the
data-processing of the measurement outcomes. In particular, an informationally
complete measurement can be used to perform quantum tomography, namely to
estimate the density matrix of the quantum state. The data-processing is
generally nonunique, and can be optimized according to a given criterion. In
this paper we provide the solution of the optimization problem which minimizes
the variance in the estimation. We then consider informationally complete
measurements performed over bipartite quantum systems focusing attention on
universally covariant measurements, and compare their statistical efficiency
when performed either locally or globally on the two systems. Among global
measurements we consider the special case of Bell measurements, which allow to
estimate the expectation of a restricted class of operators. We compare the
variance in the three cases: local, Bell, and unrestricted global--and derive
conditions for the operators to be estimated such that one type of measurement
is more efficient than the other. In particular, we find that for factorized
operators and Bell projectors the Bell measurement always performs better than
the unrestricted global measurement, which in turn outperforms the local one.
For estimation of the matrix elements of the density operator, the relative
performances depend on the basis on which the state is represented, and on the
matrix element being diagonal or off-diagonal, however, with the global
unrestricted measurement generally performing better than the local one.Comment: 8 pages, no figure
Quantum Circuits Architecture
We present a method for optimizing quantum circuits architecture. The method
is based on the notion of "quantum comb", which describes a circuit board in
which one can insert variable subcircuits. The method allows one to efficiently
address novel kinds of quantum information processing tasks, such as
storing-retrieving, and cloning of channels.Comment: 10 eps figures + Qcircuit.te
The Thirring quantum cellular automaton
We analytically diagonalize a discrete-time on-site interacting fermionic
cellular automaton in the two-particle sector. Important features of the
solutions sensibly differ from those of analogous Hamiltonian models. In
particular, we found a wider variety of scattering processes, we have bound
states for every value of the total momentum, and there exist bound states also
in the free case, where the coupling constant is null.Comment: 4 pages+references, Revtex style, 2 figures, supplemental material
included as appendi
About the use of entanglement in the optical implementation of quantum information processing
We review some applications of entanglement to improve quantum measurements
and communication, with the main focus on the optical implementation of quantum
information processing. The evolution of continuos variable entangled states in
active optical fibers is also analyzed.Comment: 8pages, invited contribution to Quant. Interf. IV (ICTP Trieste 2002
Efficient use of quantum resources for the transmission of a reference frame
We propose a covariant protocol for transmitting reference frames encoded on
spins, achieving sensitivity without the need of a pre-established
reference frame and without using entanglement between sender and receiver. The
protocol exploits the use of equivalent representations, which were overlooked
in the previous literature.Comment: 4 pages, no figures; added new references and improved introduction.
Accepted for publication on PR
The Quantum Cocktail Party
We consider the problem of decorrelating states of coupled quantum systems.
The decorrelation can be seen as separation of quantum signals, in analogy to
the classical problem of signal-separation rising in the so-called
cocktail-party context. The separation of signals cannot be achieved perfectly,
and we analyse the optimal decorrelation map in terms of added noise in the
local separated states. Analytical results can be obtained both in the case of
two-level quantum systems and for Gaussian states of harmonic oscillators.Comment: 4 pages, 2figures, revtex
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