5,639 research outputs found
Quantum Bit Commitment with a Composite Evidence
Entanglement-based attacks, which are subtle and powerful, are usually
believed to render quantum bit commitment insecure. We point out that the no-go
argument leading to this view implicitly assumes the evidence-of-commitment to
be a monolithic quantum system. We argue that more general evidence structures,
allowing for a composite, hybrid (classical-quantum) evidence, conduce to
improved security. In particular, we present and prove the security of the
following protocol: Bob sends Alice an anonymous state. She inscribes her
commitment by measuring part of it in the + (for ) or (for
) basis. She then communicates to him the (classical) measurement outcome
and the part-measured anonymous state interpolated into other, randomly
prepared qubits as her evidence-of-commitment.Comment: 6 pages, minor changes, journal reference adde
Classical capacity of the lossy bosonic channel: the exact solution
The classical capacity of the lossy bosonic channel is calculated exactly. It
is shown that its Holevo information is not superadditive, and that a
coherent-state encoding achieves capacity. The capacity of far-field,
free-space optical communications is given as an example.Comment: 4 pages, 2 figures (revised version
Determination of quantum-noise parameters of realistic cavities
A procedure is developed which allows one to measure all the parameters
occurring in a complete model [A.A. Semenov et al., Phys. Rev. A 74, 033803
(2006); quant-ph/0603043] of realistic leaky cavities with unwanted noise. The
method is based on the reflection of properly chosen test pulses by the cavity.Comment: 5 pages, 2 figure
High temperature cavity polaritons in epitaxial Er_2O_3 on silicon
Cavity polaritons around two Er^(3+) optical transitions are observed in microdisk resonators fabricated from epitaxial Er_2O_3 on Si(111). Using a pump-probe method, spectral anticrossings and linewidth averaging of the polariton modes are measured in the cavity transmission and luminescence at temperatures above 361 K
Cloning of Gaussian states by linear optics
We analyze in details a scheme for cloning of Gaussian states based on linear
optical components and homodyne detection recently demonstrated by U. L.
Andersen et al. [PRL 94 240503 (2005)]. The input-output fidelity is evaluated
for a generic (pure or mixed) Gaussian state taking into account the effect of
non-unit quantum efficiency and unbalanced mode-mixing. In addition, since in
most quantum information protocols the covariance matrix of the set of input
states is not perfectly known, we evaluate the average cloning fidelity for
classes of Gaussian states with the degree of squeezing and the number of
thermal photons being only partially known.Comment: 8 pages, 7 figure
Statistical Geometry in Quantum Mechanics
A statistical model M is a family of probability distributions, characterised
by a set of continuous parameters known as the parameter space. This possesses
natural geometrical properties induced by the embedding of the family of
probability distributions into the Hilbert space H. By consideration of the
square-root density function we can regard M as a submanifold of the unit
sphere in H. Therefore, H embodies the `state space' of the probability
distributions, and the geometry of M can be described in terms of the embedding
of in H. The geometry in question is characterised by a natural Riemannian
metric (the Fisher-Rao metric), thus allowing us to formulate the principles of
classical statistical inference in a natural geometric setting. In particular,
we focus attention on the variance lower bounds for statistical estimation, and
establish generalisations of the classical Cramer-Rao and Bhattacharyya
inequalities. The statistical model M is then specialised to the case of a
submanifold of the state space of a quantum mechanical system. This is pursued
by introducing a compatible complex structure on the underlying real Hilbert
space, which allows the operations of ordinary quantum mechanics to be
reinterpreted in the language of real Hilbert space geometry. The application
of generalised variance bounds in the case of quantum statistical estimation
leads to a set of higher order corrections to the Heisenberg uncertainty
relations for canonically conjugate observables.Comment: 32 pages, LaTex file, Extended version to include quantum measurement
theor
Minimum-error discrimination between mixed quantum states
We derive a general lower bound on the minimum-error probability for {\it
ambiguous discrimination} between arbitrary mixed quantum states with given
prior probabilities. When , this bound is precisely the well-known
Helstrom limit. Also, we give a general lower bound on the minimum-error
probability for discriminating quantum operations. Then we further analyze how
this lower bound is attainable for ambiguous discrimination of mixed quantum
states by presenting necessary and sufficient conditions related to it.
Furthermore, with a restricted condition, we work out a upper bound on the
minimum-error probability for ambiguous discrimination of mixed quantum states.
Therefore, some sufficient conditions are obtained for the minimum-error
probability attaining this bound. Finally, under the condition of the
minimum-error probability attaining this bound, we compare the minimum-error
probability for {\it ambiguously} discriminating arbitrary mixed quantum
states with the optimal failure probability for {\it unambiguously}
discriminating the same states.Comment: A further revised version, and some results have been adde
Unified Treatment of Heterodyne Detection: the Shapiro-Wagner and Caves Frameworks
A comparative study is performed on two heterodyne systems of photon
detectors expressed in terms of a signal annihilation operator and an image
band creation operator called Shapiro-Wagner and Caves' frame, respectively.
This approach is based on the introduction of a convenient operator
which allows a unified formulation of both cases. For the Shapiro-Wagner
scheme, where , quantum phase and amplitude
are exactly defined in the context of relative number state (RNS)
representation, while a procedure is devised to handle suitably and in a
consistent way Caves' framework, characterized by , within the approximate simultaneous measurements of
noncommuting variables. In such a case RNS phase and amplitude make sense only
approximately.Comment: 25 pages. Just very minor editorial cosmetic change
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