6,222 research outputs found
Selective cloning of Gaussian states by linear optics
We investigate the performances of a selective cloning machine based on
linear optical elements and Gaussian measurements, which allows to clone at
will one of the two incoming input states. This machine is a complete
generalization of a 1 to 2 cloning scheme demonstrated by U. L. Andersen et al.
[Phys. Rev. Lett. vol. 94, 240503 (2005)]. The input-output fidelity is studied
for generic Gaussian input state and the effect of non-unit quantum efficiency
is also taken into account. We show that if the states to be cloned are
squeezed states with known squeezing parameter, then the fidelity can be
enhanced using a third suitable squeezed state during the final stage of the
cloning process. A binary communication protocol based on the selective cloning
machne is also discussed.Comment: 6 pages, 6 figure
Degaussification of twin-beam and nonlocality in the phase space
We show that inconclusive photon subtraction (IPS) on twin-beam produces
non-Gaussian states that violate Bell's inequality in the phase-space. The
violation is larger than for the twin-beam itself irrespective of the IPS
quantum efficiency. The explicit expression of IPS map is given both for the
density matrix and the Wigner function representations.Comment: 7 pages, 6 figure
De-Gaussification by inconclusive photon subtraction
We address conditional de-Gaussification of continuous variable states by
inconclusive photon subtraction (IPS) and review in details its application to
bipartite twin-beam state of radiation. The IPS map in the Fock basis has been
derived, as well as its counterpart in the phase-space. Teleportation assisted
by IPS states is analyzed and the corresponding fidelity evaluated as a
function of the involved parameters. Nonlocality of IPS states is investigated
by means of different tests including displaced parity, homodyne detection,
pseudospin, and displaced on/off photodetection. Dissipation and thermal noise
are taken into account, as well as non unit quantum efficiency in the detection
stage. We show that the IPS process, for a suitable choice of the involved
parameters, improves teleportation fidelity and enhances nonlocal properties.Comment: 17 pages, 30 figure
Photon subtracted states and enhancement of nonlocality in the presence of noise
We address nonlocality of continuous variable systems in the presence of
dissipation and noise. Three nonlocality tests have been considered, based on
the measurement of displaced-parity, field-quadrature and pseudospin-operator,
respectively. Nonlocality of twin beam has been investigated, as well as that
of its non-Gaussian counterparts obtained by inconclusive subtraction of
photons. Our results indicate that: i) nonlocality of twin beam is degraded but
not destroyed by noise; ii) photon subtraction enhances nonlocality in the
presence of noise, especially in the low-energy regime.Comment: 12 pages, 7 figure
Homodyning the of Gaussian states
We suggest a method to reconstruct the zero-delay-time second-order
correlation function of Gaussian states using a single homodyne
detector. To this purpose, we have found an analytic expression of
for single- and two-mode Gaussian states in terms of the elements of their
covariance matrix and the displacement amplitude. In the single-mode case we
demonstrate our scheme experimentally, and also show that when the input state
is nonclassical, there exist a threshold value of the coherent amplitude, and a
range of values of the complex squeezing parameter, above which . For amplitude squeezing and real coherent amplitude, the threshold turns
out to be a necessary and sufficient condition for the nonclassicality of the
state. Analogous results hold also for two-mode squeezed thermal states.Comment: 7 pages, 6 figure
Squeezing-enhanced phase-shift-keyed binary communication in noisy channels
We address binary phase-shift-keyed communication channels based on Gaussian
states and prove that squeezing improves state discrimination at fixed energy
of the channel, also in the presence of phase diffusion. We then assess
performances of homodyne detection against the ultimate quantum limits to
discrimination, and show that homodyning achieves optimality in large noise
regime. Finally, we consider noise in the preparation of the seed signal
(before phase encoding) and show that also in this case squeezing may improve
state discrimination in realistic conditions.Comment: 6 pages, 5 figure
Qubit thermometry for micromechanical resonators
We address estimation of temperature for a micromechanical oscillator lying
arbitrarily close to its quantum ground state. Motivated by recent experiments,
we assume that the oscillator is coupled to a probe qubit via Jaynes-Cummings
interaction and that the estimation of its effective temperature is achieved
via quantum limited measurements on the qubit. We first consider the ideal
unitary evolution in a noiseless environment and then take into account the
noise due to non dissipative decoherence. We exploit local quantum estimation
theory to assess and optimize the precision of estimation procedures based on
the measurement of qubit population, and to compare their performances with the
ultimate limit posed by quantum mechanics. In particular, we evaluate the
Fisher information (FI) for population measurement, maximize its value over the
possible qubit preparations and interaction times, and compare its behavior
with that of the quantum Fisher information (QFI). We found that the FI for
population measurement is equal to the QFI, i.e., population measurement is
optimal, for a suitable initial preparation of the qubit and a predictable
interaction time. The same configuration also corresponds to the maximum of the
QFI itself. Our results indicate that the achievement of the ultimate bound to
precision allowed by quantum mechanics is in the capabilities of the current
technology.Comment: 9 pages, 5 figures, revised version, to appear on PR
Hybrid quantum key distribution using coherent states and photon-number-resolving detectors
We put forward a hybrid quantum key distribution protocol based on coherent
states, Gaussian modulation, and photon-number-resolving (PNR) detectors, and
show that it may enhance the secret key generation rate (KGR) compared to
homodyne-based schemes. Improvement in the KGR may be traced back to the
dependence of the two-dimensional discrete output variable on both the input
quadratures, thus overcoming the limitations of the original protocol. When
reverse reconciliation is considered, the scheme based on PNR detectors
outperforms the homodyne one both for individual and collective attacks. In the
presence of direct reconciliation, the PNR strategy is still the best one
against individual attacks, but for the collective ones the homodyne-based
scheme is still to be preferred as the channel transmissivity decreases.Comment: 5 pages, 5 figures. We extended our analysis to reverse
reconciliation and to collective attack
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