1,615 research outputs found

### Interferometric modulation of quantum cascade interactions

We consider many-body quantum systems dissipatively coupled by a cascade
network, i.e. a setup in which interactions are mediated by unidirectional
environmental modes propagating through a linear optical interferometer. In
particular we are interested in the possibility of inducing different effective
interactions by properly engineering an external dissipative network of
beam-splitters and phase-shifters. In this work we first derive the general
structure of the master equation for a symmetric class of translation-invariant
cascade networks. Then we show how, by tuning the parameters of the
interferometer, one can exploit interference effects to tailor a large variety
of many-body interactions.Comment: 12 pages, 10 figure

### The capacity of coherent-state adaptive decoders with interferometry and single-mode detectors

A class of Adaptive Decoders (AD's) for coherent-state sequences is studied,
including in particular the most common technology for optical-signal
processing, e.g., interferometers, coherent displacements and photon-counting
detectors. More generally we consider AD's comprising adaptive procedures based
on passive multi-mode Gaussian unitaries and arbitrary single-mode destructive
measurements. For classical communication on quantum phase-insensitive Gaussian
channels with a coherent-state encoding, we show that the AD's optimal
information transmission rate is not greater than that of a single-mode
decoder. Our result also implies that the ultimate classical capacity of
quantum phase-insensitive Gaussian channels is unlikely to be achieved with the
considered class of AD's.Comment: v3: final version; 6 pages; 2 figure

### Multi-Phase Hadamard receivers for classical communication on lossy bosonic channels

A scheme for transferring classical information over a lossy bosonic channel
is proposed by generalizing the proposal presented in Phys. Rev. Lett. 106,
240502 (2011) by Guha. It employs codewords formed by products of coherent
states of fixed mean photon number with multiple phases which, through a
passive unitary transformation, reduce to a Pulse-Position Modulation code with
multiple pulse phases. The maximum information rate achievable with optimal,
yet difficult to implement, detection schemes is computed and shown to saturate
the classical capacity of the channel in the low energy regime. An easy to
implement receiver based on a conditional Dolinar detection scheme is also
proposed finding that, while suboptimal, it allows for improvements in an
intermediate photon-number regime with respect to previous proposals.Comment: final version: minor changes; 8+3 pages and 5 figure

### Interferometric Quantum Cascade Systems

In this work we consider quantum cascade networks in which quantum systems
are connected through unidirectional channels that can mutually interact giving
rise to interference effects. In particular we show how to compute master
equations for cascade systems in an arbitrary interferometric configuration by
means of a collisional model. We apply our general theory to two specific
examples: the first consists in two systems arranged in a Mach-Zender-like
configuration; the second is a three system network where it is possible to
tune the effective chiral interactions between the nodes exploiting
interference effects.Comment: 15 pages, 5 figure

### Slow dynamics and thermodynamics of open quantum systems

We develop a perturbation theory of quantum (and classical) master equations
with slowly varying parameters, applicable to systems which are externally
controlled on a time scale much longer than their characteristic relaxation
time. We apply this technique to the analysis of finite-time isothermal
processes in which, differently from quasi-static transformations, the state of
the system is not able to continuously relax to the equilibrium ensemble. Our
approach allows to formally evaluate perturbations up to arbitrary order to the
work and heat exchange associated to an arbitrary process. Within first order
in the perturbation expansion, we identify a general formula for the efficiency
at maximum power of a finite-time Carnot engine. We also clarify under which
assumptions and in which limit one can recover previous phenomenological
results as, for example, the Curzon-Ahlborn efficiency.Comment: 10 page

### Coherent-state discrimination via non-heralded probabilistic amplification

A scheme for the detection of low-intensity optical coherent signals was
studied which uses a probabilistic amplifier operated in the non-heralded
version, as the underlying non-linear operation to improve the detection
efficiency. This approach allows us to improve the statistics by keeping track
of all possible outcomes of the amplification stage (including failures). When
compared with an optimized Kennedy receiver, the resulting discrimination
success probability we obtain presents a gain up to ~1.85% and it approaches
the Helstrom bound appreciably faster than the Dolinar receiver, when employed
in an adaptive strategy. We also notice that the advantages obtained can be
ultimately associated with the fact that, in the high gain limit, the
non-heralded version of the probabilistic amplifier induces a partial dephasing
which preserves quantum coherence among low energy eigenvectors while removing
it elsewhere. A proposal to realize such transformation based on an optical
cavity implementation is presented.Comment: Final version: 6 pages and 4 figure

### Quantum optomechanical piston engines powered by heat

We study two different models of optomechanical systems where a temperature
gradient between two radiation baths is exploited for inducing self-sustained
coherent oscillations of a mechanical resonator. Viewed from a thermodynamic
perspective, such systems represent quantum instances of self-contained thermal
machines converting heat into a periodic mechanical motion and thus they can be
interpreted as nano-scale analogues of macroscopic piston engines. Our models
are potentially suitable for testing fundamental aspects of quantum
thermodynamics in the laboratory and for applications in energy efficient
nanotechnology.Comment: 10 pages, 6 figure

### On the time optimal thermalization of single mode Gaussian states

We consider the problem of time optimal control of a continuous bosonic
quantum system subject to the action of a Markovian dissipation. In particular,
we consider the case of a one mode Gaussian quantum system prepared in an
arbitrary initial state and which relaxes to the steady state due to the action
of the dissipative channel. We assume that the unitary part of the dynamics is
represented by Gaussian operations which preserve the Gaussian nature of the
quantum state, i.e. arbitrary phase rotations, bounded squeezing and unlimited
displacements. In the ideal ansatz of unconstrained quantum control (i.e. when
the unitary phase rotations, squeezing and displacement of the mode can be
performed instantaneously), we study how control can be optimized for speeding
up the relaxation towards the fixed point of the dynamics and we analytically
derive the optimal relaxation time. Our model has potential and interesting
applications to the control of modes of electromagnetic radiation and of
trapped levitated nanospheres.Comment: 10 pages, 1 figur

### Experiments testing macroscopic quantum superpositions must be slow

We consider a thought experiment where the preparation of a macroscopically
massive or charged particle in a quantum superposition and the associated
dynamics of a distant test particle apparently allow for superluminal
communication. We give a solution to the paradox which is based on the
following fundamental principle: any local experiment, discriminating a
coherent superposition from an incoherent statistical mixture, necessarily
requires a minimum time proportional to the mass (or charge) of the system. For
a charged particle, we consider two examples of such experiments, and show that
they are both consistent with the previous limitation. In the first, the
measurement requires to accelerate the charge, that can entangle with the
emitted photons. In the second, the limitation can be ascribed to the quantum
vacuum fluctuations of the electromagnetic field. On the other hand, when
applied to massive particles our result provides an indirect evidence for the
existence of gravitational vacuum fluctuations and for the possibility of
entangling a particle with quantum gravitational radiation.Comment: 12 pages, 1 figur

### Quantum state majorization at the output of bosonic Gaussian channels

Quantum communication theory explores the implications of quantum mechanics
to the tasks of information transmission. Many physical channels can be
formally described as quantum Gaussian operations acting on bosonic quantum
states. Depending on the input state and on the quality of the channel, the
output suffers certain amount of noise. For a long time it has been
conjectured, but never proved, that output states of Gaussian channels
corresponding to coherent input signals are the less noisy ones (in the sense
of a majorization criterion). In this work we prove this conjecture.
Specifically we show that every output state of a phase insensitive Gaussian
channel is majorized by the output state corresponding to a coherent input. The
proof is based on the optimality of coherent states for the minimization of
strictly concave output functionals. Moreover we show that coherent states are
the unique optimizers.Comment: 7 pages, 1 figure. Published versio

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