The bosonic quantum channels have recently attracted a growing interest, motivated by the hope that they open a tractable approach to the generally hard problem of evaluating quantum channel capacities. These studies, however, have always been restricted to memoryless channels. Here, it is shown that the classical capacity of a bosonic Gaussian channel with memory can be significantly enhanced if entangled symbols are used instead of product symbols. For example, the capacity of a photonic channel with 70%-correlated thermal noise of one third the shot noise is enhanced by about 11% when using 3.8-dB entangled light with a modulation variance equal to the shot noise.PACS numbers: 03.67.-a, 03.65.-w A main goal of quantum information theory is to evaluate the information capacities of quantum communication channels. In particular, an important question is to determine how much classical information can be processed asymptotically via a quantum channel. This problem has been solved, today, only for a few quantum channels, and it has been addressed only recently for bosonic channels, i.e., continuous-variable quantum channels acting on a bosonic field such as the electromagnetic field [1]. The classical capacity of a purely lossy bosonic channel was solved exactly very recently [2], while the case of noisy bosonic channels is already more involved. Actually, the classical capacity of the Gaussian bosonic channel, i.e., a continuous-variable quantum channel undergoing a Gaussian-distributed thermal noise, has been derived in [3] although this result only holds provided that the optimal input ensemble is a tensor product of Gaussian states, as conjectured by several authors but not rigorously proven today (see e.g. [4] for recent progress on this problem). All these studies, however, have been restricted to memoryless bosonic channels.In this Letter, we investigate the capacity of a bosonic Gaussian channel that exhibits memory. This study is motivated by the recent finding that, for some appropriate extension of the depolarizing channel with correlated noise, entangled qubit pairs can enhance the 2-shot classical capacity [5]. Here, we consider channels with a thermal noise that has a finite bandwidth. The resulting memory effect is modeled by assuming that the noise affecting two subsequent uses of the channel follows a bivariate Gaussian distribution with a non-vanishing correlation coefficient, measuring the degree of memory of the channel. We prove that if the memory is non-zero and if the input energy is constrained, then the channel capacity can be significantly enhanced by using entangled symbols instead of product symbols, in contrast with the common knowledge that entanglement is of no use for information transfer via a quantum channel. The relation between the degree of memory and the resulting optimal input entanglement is analyzed.Bosonic Gaussian channels. Let us define a memoryless bosonic Gaussian channel T acting on a mode of the electromagnetic field associated with the annihilation and creation ope...

a b s t r a c tThe technological obsolescence of a unit is characterized by the existence of challenger units displaying identical functionalities, but with higher performances. This paper aims to define and model in a realistic way, possible maintenance policies of a system including replacement strategies when one type of challenger unit is available. The comparison of these possible strategies is performed based on a Monte Carlo estimation of the costs they incur.

Gaussian quantum channels have recently attracted a growing interest, since they may lead to a tractable approach to the generally hard problem of evaluating quantum channel capacities. However, the analysis performed so far has always been restricted to memoryless channels. Here, we consider the case of a bosonic Gaussian channel with memory, and show that the classical capacity can be significantly enhanced by employing entangled input symbols instead of product symbols.

a b s t r a c tThe technological obsolescence of a unit is characterised by the existence of challenger units displaying identical functionalities, but with higher performances. Though this issue is commonly encountered in practice, it has received little consideration in the literature. Previous exploratory works have treated the problem of replacing old-technology items by new ones, for identical components facing a unique new generation of items. This paper aims to define, in a realistic way, possible replacement policies when several types of challenger units are available and when the performances of these newly available units improve with time.Since no fully generic model can exist in maintenance optimisation, a modular modelling of the problem, allowing easy adaptations to features corresponding to specific applications is highly desirable. This work therefore proposes a modular Petri net model for this problem, underlying a Monte Carlo (MC) estimation of the costs incurred by the different possible replacement strategies under consideration.

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