38,528 research outputs found
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
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
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
Optimal Bidding in the Mexican Treasury Securities Primary Auctions: Results from a Structural Econometrics Approach
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
Use of natural resins in repairing damaged timber beams â An experimental investigation
Different techniques including the application of steel elements, composite materials and polymeric resins have been used in the past to repair damaged timber beams. However, there is a growing need to replace these materials with those with minimal environmental impact. In addition, stringent requirements of conservation authorities on the compatibility between repair and parent materials have also necessitated search for innovative repair materials for timber beams. Therefore, an increasing shift of focus towards the use of materials derived from natural sources in repairing and reinforcing timber structures is currently experienced. This paper presents the results of an exploratory study on the use of natural resins (rosin and bone glue) in repairing oak timber beams. 15 oak timber beams with cross section dimensions of 67 x 67 mm and 1100 mm in length were tested in four-point bending to failure. Undamaged, damaged (unrepaired) and damaged but repaired timber beams (with rosin and bone glue) were tested. The effectiveness of the repair material and technique was analysed based on the bending capacity and mid span deflection at failure. The initial results show negligible effectiveness of rosin in repairing timber beams. In fact, about 16% reduction (average) in load carrying capacity with a corresponding 5% decrease (average) in maximum displacement was recorded. Relatively higher level of effectiveness was recorded with the use of bone glue (about 10 % average increase in load carrying capacity). However, over 30% corresponding average increase in the maximum displacement was also recorded. Further work investigating different repair techniques and other natural resins is presently underway
Many-body orbital paramagnetism in doped graphene sheets
The orbital magnetic susceptibility (OMS) of a gas of noninteracting massless
Dirac fermions is zero when the Fermi energy is away from the Dirac point.
Making use of diagrammatic perturbation theory, we calculate exactly the OMS of
massless Dirac fermions to first order in the Coulomb interaction demonstrating
that it is finite and positive. Doped graphene sheets are thus unique systems
in which the OMS is completely controlled by many-body effects.Comment: 4 pages, 2 figures, submitte
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