137 research outputs found
Detecting Gaussian entanglement via extractable work
We show how the presence of entanglement in a bipartite Gaussian state can be
detected by the amount of work extracted by a continuos variable Szilard-like
device, where the bipartite state serves as the working medium of the engine.
We provide an expression for the work extracted in such a process and
specialize it to the case of Gaussian states. The extractable work provides a
sufficient condition to witness entanglement in generic two-mode states,
becoming also necessary for squeezed thermal states. We extend the protocol to
tripartite Gaussian states, and show that the full structure of inseparability
classes cannot be discriminated based on the extractable work. This suggests
that bipartite entanglement is the fundamental resource underpinning work
extraction.Comment: 12 pages, 8 figure
Optimal quantum repeaters for qubits and qudits
A class of optimal quantum repeaters for qubits is suggested. The schemes are
minimal, i.e. involve a single additional probe qubit, and optimal, i.e.
provide the maximum information adding the minimum amount of noise. Information
gain and state disturbance are quantified by fidelities which, for our schemes,
saturate the ultimate bound imposed by quantum mechanics for randomly
distributed signals. Special classes of signals are also investigated, in order
to improve the information-disturbance trade-off. Extension to higher
dimensional signals (qudits) is straightforward.Comment: Revised version. To appear in PR
Metrology with Unknown Detectors
The best possible precision is one of the key figures in metrology, but this
is established by the exact response of the detection apparatus, which is often
unknown. There exist techniques for detector characterisation, that have been
introduced in the context of quantum technologies, but apply as well for
ordinary classical coherence; these techniques, though, rely on intense data
processing. Here we show that one can make use of the simpler approach of data
fitting patterns in order to obtain an estimate of the Cram\'er-Rao bound
allowed by an unknown detector, and present applications in polarimetry.
Further, we show how this formalism provide a useful calculation tool in an
estimation problem involving a continuous-variable quantum state, i.e. a
quantum harmonic oscillator
A measure of the non-Gaussian character of a quantum state
We address the issue of quantifying the non-Gaussian character of a bosonic
quantum state and introduce a non-Gaussianity measure based on the
Hilbert-Schmidt distance between the state under examination and a reference
Gaussian state. We analyze in details the properties of the proposed measure
and exploit it to evaluate the non-Gaussianity of some relevant single- and
multi-mode quantum states. The evolution of non-Gaussianity is also analyzed
for quantum states undergoing the processes of Gaussification by loss and
de-Gaussification by photon-subtraction. The suggested measure is easily
computable for any state of a bosonic system and allows to define a
corresponding measure for the non-Gaussian character of a quantum operation.Comment: revised and enlarged version, 7 pages, 4 figure
Quantifying the nonlinearity of a quantum oscillator
We address the quantification of nonlinearity for quantum oscillators and
introduce two measures based on the properties of the ground state rather than
on the form of the potential itself. The first measure is a fidelity-based one,
and corresponds to the renormalized Bures distance between the ground state of
the considered oscillator and the ground state of a reference harmonic
oscillator. Then, in order to avoid the introduction of this auxiliary
oscillator, we introduce a different measure based on the non-Gaussianity (nG)
of the ground state. The two measures are evaluated for a sample of significant
nonlinear potentials and their properties are discussed in some detail. We show
that the two measures are monotone functions of each other in most cases, and
this suggests that the nG-based measure is a suitable choice to capture the
anharmonic nature of a quantum oscillator, and to quantify its nonlinearity
independently on the specific features of the potential. We also provide
examples of potentials where the Bures measure cannot be defined, due to the
lack of a proper reference harmonic potential, while the nG-based measure
properly quantify their nonlinear features. Our results may have implications
in experimental applications where access to the effective potential is
limited, e.g., in quantum control, and protocols rely on information about the
ground or thermal state.Comment: 8 pages, 5 figures, published versio
Quantum non-Gaussianity witnesses in the phase space
We address detection of quantum non-Gaussian states, i.e. nonclassical states
that cannot be expressed as a convex mixture of Gaussian states, and present a
method to derive a new family of criteria based on generic linear functionals.
We then specialise this method to derive witnesses based on -parametrized
quasiprobability functions, generalising previous criteria based on the Wigner
function. In particular we discuss in detail and analyse the properties of
Husimi Q-function based witnesses and prove that they are often more effective
than previous criteria in detecting quantum non-Gaussianity of various kinds of
non-Gaussian states evolving in a lossy channel.Comment: 9 pages, 6 figure
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