32 research outputs found
A quantum-mechanical study of optical regenerators based on nonlinear-loop mirrors
We present a quantum-mechanical analysis of a nonlinear interferometer that
achieves optical switching via cross-phase modulation resulting from the Kerr
effect. We show how it performs as a very precise optical regenerator, highly
improving the transmitted bit-error rate in the presence of loss.Comment: 420KB tar file, including 5 eps figures. To appear on IEEE Photonics
Technology Letter
Adaptive Bayesian and frequentist data processing for quantum tomography
The outcome statistics of an informationally complete quantum measurement for
a system in a given state can be used to evaluate the ensemble expectation of
any linear operator in the same state, by averaging a function of the outcomes
that depends on the specific operator. Here we introduce two novel
data-processing strategies, non-linear in the frequencies, which lead to faster
convergence to theoretical expectations.Comment: 12 pages, 2 figures, revised versio
Optimal estimation of ensemble averages from a quantum measurement
We consider the general measurement scenario in which the ensemble average of
an operator is determined via suitable data-processing of the outcomes of a
quantum measurement described by a POVM. After reviewing the optimization of
data processing that minimizes the statistical error of the estimation, we
provide a compact formula for the evaluation of the estimation error.Comment: 4 pages, contribution for the proceedings of the QCMC06 at Tsukuba,
Japan. qcmc06.st
Quantum cellular automata and free quantum field theory
In a series of recent papers it has been shown how free quantum field theory
can be derived without using mechanical primitives (including space-time,
special relativity, quantization rules, etc.), but only considering the easiest
quantum algorithm encompassing a countable set of quantum systems whose network
of interactions satisfies the simple principles of unitarity, homogeneity,
locality, and isotropy. This has opened the route to extending the axiomatic
information-theoretic derivation of the quantum theory of abstract systems to
include quantum field theory. The inherent discrete nature of the informational
axiomatization leads to an extension of quantum field theory to a quantum
cellular automata theory, where the usual field theory is recovered in a regime
where the discrete structure of the automata cannot be probed. A simple
heuristic argument sets the scale of discreteness to the Planck scale, and the
customary physical regime where discreteness is not visible is the relativistic
one of small wavevectors. In this paper we provide a thorough derivation from
principles that in the most general case the graph of the quantum cellular
automaton is the Cayley graph of a finitely presented group, and showing how
for the case corresponding to Euclidean emergent space (where the group resorts
to an Abelian one) the automata leads to Weyl, Dirac and Maxwell field dynamics
in the relativistic limit. We conclude with some perspectives towards the more
general scenario of non-linear automata for interacting quantum field theory.Comment: 10 pages, 2 figures, revtex style. arXiv admin note: substantial text
overlap with arXiv:1601.0483
Superbroadcasting of conjugate quantum variables
We consider the problem of broadcasting arbitrary states of radiation modes
from N to M>N copies by a map that preserves the average value of the field and
optimally reduces the total noise in conjugate variables. For N>=2 the
broadcasting can be achieved perfectly, and for sufficiently noisy input states
one can even purify the state while broadcasting--the so-called
superbroadcasting. For purification (i.e. M<=N), the reduction of noise is
independent of M. Similar results are proved for broadcasting with
phase-conjugation. All the optimal maps can be implemented by linear optics and
linear amplification.Comment: 7 pages, 1 eps figures. Accepted for publication on Europhysics
Letter
The Quantum No-Stretching: A geometrical interpretation of the no-cloning theorem
We consider the ideal situation in which a space rotation is transferred from
a quantum spin j to a quantum spin l different from j. Quantum-information
theoretical considerations lead to the conclusion that such operation is
possible only for lj. For l>j the optimal stretching transformation is derived.
We show that for qubits the present no-stretching theorem is equivalent to the
usual no-cloning theorem
Superbroadcasting of continuous variables mixed states
We consider the problem of broadcasting quantum information encoded in the
average value of the field from N to M>N copies of mixed states of radiation
modes. We derive the broadcasting map that preserves the complex amplitude,
while optimally reducing the noise in conjugate quadratures. We find that from
two input copies broadcasting is feasible, with the possibility of simultaneous
purification (superbroadcasting). We prove similar results for purification
(M<=N) and for phase-conjugate broadcasting.Comment: 11 pages, 1 figure, revtex4 style, revised versio
Incompatibility of observables, channels and instruments in information theories
Every theory of information, including classical and quantum, can be studied
in the framework of operational probabilistic theories--where the notion of
test generalizes that of quantum instrument, namely a collection of quantum
operations summing to a channel, and simple rules are given for the composition
of tests in parallel and in sequence. Here we study the notion of compatibility
for tests of an operational probabilistic theory. Following the quantum
literature, we first introduce the notion of strong compatibility, and then we
illustrate its ultimate relaxation, that we deem weak compatibility. It is
shown that the two notions coincide in the case of observation tests--which are
the counterpart of quantum POVMs--while there exist weakly compatible channels
that are not strongly compatible. We prove necessary and sufficient conditions
for a theory to exhibit incompatible tests. We show that a theory admits of
incompatible tests if and only if some information cannot be extracted without
disturbance.Comment: 11 pages, revtex4; bibliography is now displayed correctl