7,380 research outputs found
Thermofield-based chain mapping approach for open quantum systems
We consider a thermofield approach to analyze the evolution of an open
quantum system coupled to an environment at finite temperature. In this
approach, the finite temperature environment is exactly mapped onto two virtual
environments at zero temperature. These two environments are then unitarily
transformed into two different chains of oscillators, leading to a one
dimensional structure that can be numerically studied using tensor network
techniques
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
Classical capacity of Gaussian thermal memory channels
The classical capacity of phase-invariant Gaussian channels has been recently
determined under the assumption that such channels are memoryless. In this work
we generalize this result by deriving the classical capacity of a model of
quantum memory channel, in which the output states depend on the previous input
states. In particular we extend the analysis of [C. Lupo, et al., PRL and PRA
(2010)] from quantum limited channels to thermal attenuators and thermal
amplifiers. Our result applies in many situations in which the physical
communication channel is affected by nonzero memory and by thermal noise.Comment: 14 pages, 8 figure
Optimal quantum state discrimination via nested binary measurements
A method to compute the optimal success probability of discrimination of N
arbitrary quantum states is presented, based on the decomposition of any
N-outcome measurement into sequences of nested two-outcome ones. In this way
the optimization of the measurement operators can be carried out in successive
steps, optimizing first the binary measurements at the deepest nesting level
and then moving on to those at higher levels. We obtain an analytical
expression for the maximum success probability after the first optimization
step and examine its form for the specific case of N=3,4 states of a qubit. In
this case, at variance with previous proposals, we are able to provide a
compact expression for the success probability of any set of states, whose
numerical optimization is straightforward; the results thus obtained highlight
some lesser-known features of the discrimination problem.Comment: v2: added references to previous works closely related to Sec. II;
8+3 pages; 3 figure
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