569 research outputs found
Critical behavior in ultra-strong-coupled oscillators
We investigate the strong coupling regime of a linear - coupled
harmonic oscillator system, by performing a direct diagonalization of the
hamiltonian. It is shown that the - coupled hamiltonian can be
equivalently described by a Mach-Zehnder-type interferometer with a quadratic
unitary operation in each of its arms. We show a sharp transition of the
unitary operation from an elliptical phase rotator to an elliptical squeezer as
the coupling gets stronger, which leads to the continuous generation of
entanglement, even for a significantly thermal state, in the ultra-strong
coupled regime. It is also shown that this critical regime cannot be achieved
by a classical Hookian coupling. Finally, the effect of a finite-temperature
environment is analyzed, showing that entanglement can still be generated from
a thermal state in the ultra-strong coupled regime, but is destroyed rapidly
Probing anharmonicity of a quantum oscillator in an optomechanical cavity
We present a way of measuring with high precision the anharmonicity of a
quantum oscillator coupled to an optical field via radiation pressure. Our
protocol uses a sequence of pulsed interactions to perform a loop in the phase
space of the mechanical oscillator, which is prepared in a thermal state. We
show how the optical field acquires a phase depending on the anharmonicity.
Remarkably, one only needs small initial cooling of the mechanical motion to
probe even small anharmonicities. Finally, by applying tools from quantum
estimation theory, we calculate the ultimate bound on the estimation precision
posed by quantum mechanics and compare it with the precision obtainable with
feasible measurements such as homodyne and heterodyne detection on the cavity
field. In particular we demonstrate that homodyne detection is nearly optimal
in the limit of a large number of photons of the field and we discuss the
estimation precision of small anharmonicities in terms of its signal-to-noise
ratio.Comment: 8 pages, 2 figures, RevTeX
Reliable source of conditional non-Gaussian states from single-mode thermal fields
We address both theoretically and experimentally the generation of pulsed
non-Gaussian states from classical Gaussian ones by means of conditional
measurements. The setup relies on a beam splitter and a pair of linear
photodetectors able to resolve up to tens of photons in the two outputs. We
show the reliability of the setup and the good agreement with the theory for a
single-mode thermal field entering the beam splitter and present a thorough
characterization of the photon statistics of the conditional states.Comment: 18 pages, 12 figure
Use of social media by academic librarians in Iraq
Purpose â The purpose of this study is to identify the nature and extent of the âinformation gapâ in Iraq and explore perspectives on international academic library collaboration using social media that may assist in reducing this gap. The study investigates the information gap in Iraq in the context of the country's recent history of political and social upheaval. Design/methodology/approach â The study uses qualitative data collected through interviews with academic library managers, librarians and researchers in both Australia and Iraq. Findings â The findings demonstrate that currently academic librarians in Iraq and Australia use social media differently. The degradation of critical research infrastructure and reliable communication means that Iraqi librarians rely heavily on informal communication channels. The study finds that use of social media offers will enhance collaboration by Iraqi academic librarians and reduce the information gap. Research limitations/implications â There are some limitations in terms of selection sampling, approaching Iraqi participants and average quality of connections. It is suggested that snowballing and chain sampling should be used. Social implications â The ethical challenges that are faced by the Iraqi community with social cultural systems are in the early stages of valuing higher education and a scholarly communication system based on unfamiliar models. Originality/value â This study represents the first attempt to assess the information gap that exists in the wake of recent political and social upheaval, and to explore ways in which Iraqi librarians use social media to redress this gap
Conditional measurements on multimode pairwise entangled states from spontaneous parametric downconversion
We address the intrinsic multimode nature of the quantum state of light
obtained by pulsed spontaneous parametric downconversion and develop a
theoretical model based only on experimentally accessible quantities. We
exploit the pairwise entanglement as a resource for conditional multimode
measurements and derive closed formulas for the detection probability and the
density matrix of the conditional states. We present a set of experiments
performed to validate our model in different conditions that are in excellent
agreement with experimental data. Finally, we evaluate nonGaussianity of the
conditional states obtained from our source with the aim of discussing the
effects of the different experimental parameters on the efficacy of this type
of conditional state preparation
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
Tripartite quantum state mapping and discontinuous entanglement transfer in a cavity QED open system
We describe the transfer of quantum information and entanglement from three
flying (radiation) to three localized (atomic) qubits via cavity modes
resonantly coupled to the atoms, in the presence of a common reservoir. Upon
addressing the full dynamics of the resulting nine-qubit open system, we find
that once the cavities are fed, fidelity and transferred entanglement are
optimal, while their peak values exponentially decrease due to dissipative
processes. The external radiation is then turned off and quantum correlations
oscillate between atomic and cavity qubits. For a class of mixtures of W and
GHZ input states we deal with a discontinuous exchange of entanglement among
the subsystems, facing the still open problem of entanglement sudden death and
birth in a multipartite system.Comment: 7 pages, 6 figures, 2 table
Nonequilibrium readiness and precision of Gaussian quantum thermometers
The dimensionality of a thermometer is key in the design of quantum thermometry schemes. In general, the phenomenology that is typical of qubit-based quantum thermometry does not apply to infinite-dimensional ones. We analyze the dynamical and metrological features of nonequilibrium Gaussian quantum thermometers: On one hand, we highlight how quantum entanglement can enhance the readiness of composite Gaussian thermometers; on the other hand, we show that nonequilibrium conditions do not guarantee the best sensitivities in temperature estimation, thus suggesting the reassessment of some of the working principles underpinning quantum thermometry
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