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arXiv:2309.17066 · IEEE Transactions on Information Theory

Optical Fibers With Memory Effects and Their Quantum Communication Capacities

Francesco Anna Mele, Giacomo De Palma, Marco Fanizza, Vittorio Giovannetti, Ludovico Lami

Published Sep 3, 2024• 7 citations
arXivHTMLPDF

Abstract

If the transmissivity of an optical fibre falls below a critical value, its use as a reliable quantum channel is known to be drastically compromised. However, if the memoryless assumption does not hold — e.g. when input signals are separated by a sufficiently short time interval — the validity of this limitation is put into question. In this work we introduce a model of optical fibre that can describe memory effects for long transmission lines. We then solve its quantum capacity, two-way quantum capacity, and secret-key capacity exactly. By doing so, we show that — due to the memory cross-talk between the transmitted signals — reliable quantum communication is attainable even for highly noisy regimes where it was previously considered impossible.

DOI
10.1109/tit.2024.3450501

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If the transmissivity of an optical fibre falls below a critical value, its use as a reliable quantum channel is known to be drastically compromised. However, if the memoryless assumption does not hold — e.g. when input signals are separated by a sufficiently short time interval — the validity of this limitation is put into question. In this work we introduce a model of optical fibre that can describe memory effects for long transmission lines. We then solve its quantum capacity, two-way quantum capacity, and secret-key capacity exactly. By doing so, we show that — due to the memory cross-talk between the transmitted signals — reliable quantum communication is attainable even for highly noisy regimes where it was previously considered impossible.
In the absence of quantum repeaters, quantum communication proved to be nearly impossible across optical fibers longer than ≳20 km due to the drop of transmissivity below the critical threshold of 1/2. However, if the signals fed into the fiber are separated by a sufficiently short time interval, memory effects must be taken into account. In this Letter, we show that by properly accounting for these effects it is possible to devise schemes that enable unassisted quantum communication across arbitrarily long optical fibers at a fixed positive qubit transmission rate. We also demonstrate how to achieve entanglement-assisted communication over arbitrarily long distances at a rate of the same order of the maximum achievable in the unassisted noiseless case.
Is quantum communication possible over an optical fiber with transmissivity $\ensuremath{\lambda}\ensuremath{\le}1/2$? The answer is well known to be negative if the environment with which the incoming signal interacts is initialized in a thermal state. However, Lami et al. [Phys. Rev. Lett. 125, 110504 (2020)] found the quantum capacity to be always bounded away from zero for all $\ensuremath{\lambda}>0$, a phenomenon dubbed ``die-hard quantum communication'' (D-HQCOM), provided that the initial environment state can be chosen appropriately, depending on $\ensuremath{\lambda}$. Here we show an even stronger version of D-HQCOM in the context of entanglement-assisted classical communication: entanglement assistance and control of the environment enable communication with performance at least equal to that of the ideal case of absence of noise, even if $\ensuremath{\lambda}>0$ is arbitrarily small. These two phenomena of D-HQCOM have technological potential provided that we are able to control the environment. How can we achieve this? Our second main result answers this question. Here we provide a fully consistent protocol to activate the phenomena of D-HQCOM without directly accessing the environment state. This is done by sending over the channel ``trigger signals,'' i.e., signals which do not encode information, prior to the actual communication, with the goal of modifying the environment in an advantageous way. This is possible due to the memory effects which arise when the sender feeds signals separated by a sufficiently short temporal interval. Our results may offer a concrete scheme to communicate across arbitrarily long optical fibers, without using quantum repeaters. As a by-product of our analysis, we derive a simple Kraus representation of the thermal attenuator exploiting the associated Lindblad master equation.
Using quantum channels to transmit classical information has been proven to be advantageous in several scenarios. These channels have been assumed to be memoryless, meaning that consecutive transmissions of information are uncorrelated. However, as shown experimentally, such correlations do exist, and thereby retain memory of previous information. This memory complicates the protection of entangled-information transmission from decoherence. We have recently addressed these fundamental questions by developing a generalized master equation for multipartite entangled systems coupled to finite-temperature baths and subject to arbitrary external perturbations whose role is to provide dynamical protection from decay and decoherence. Here we explore and extend the foregoing strategy to quantum optical communication schemes wherein polarization-entangled photons traverse a bit-flip channel with temporal and spatial memory, such that the two channels experience cross-decoherence. We introduce a novel approach to the protection of the entangled information from decoherence in such schemes. It is based on selectively modulating the photon polarizations in each channel. We show that by applying selective modulation, one can independently control the symmetry and spatial memory attributes of the channel. We then explore the effects of these attributes on the channel capacity. Remarkably, we show that there is a nontrivial interplay between the effects of asymmetry and memory on the channel capacity.
This paper defines a fundamentally new approach of capacity recovery of very noisy, practically completely useless optical quantum channels. The transmission of information through classical channels and quantum channels differs in many ways. The capacity recovery of very noisy communication channels cannot be imagined for classical systems, and this effect has no analogue in classical systems. The capacity recovery of very noisy quantum channels makes it possible to use two very noisy optical-fiber based quantum channels with a positive joint capacity at the output. Our method gives an algorithmic solution to the capacity recovery problem, and provides an efficient algorithmic solution for finding recoverable very noisy optical quantum channels.
Quantum communication promises to outperform its classical counterparts and enable protocols previously impossible. Specifically, quantum key distribution (QKD) allows a cryptographic key to be shared between distant parties with provable security. Much work has been performed on theoretical and experi- mental aspects of QKD, and the push is on to make it commercially viable and integrable with existing technologies. To this end I have performed simulations and experiments on QKD and other quantum protocols in regimes previously unexplored. \nThe first experiment involves QKD via distributed entanglement through the standard telecommunications optical fibre network. I show that entanglement is preserved, even when the photons used are a shorter wavelength than the design of the optical fibre calls for. This surprising result is then used to demonstrate QKD over installed optical fibre, even with co-propagating classical traffic. Because the quantum and classical signals are sufficiently separated in wavelength, little cross-talk is observed, leading to high compatibility between this type of QKD and existing telecommunications infrastructure. \nSecondly, I demonstrate the key components of fully-modulated decoy-state QKD over the highest-loss channel to date, using a novel photon source based on weak coherent (laser) pulses. This system has application in a satellite uplink of QKD, which would enable worldwide secure communication. The uplink allows the complex quantum source to be kept on the ground while only simple receivers are in space, but suffers from high link loss due to atmospheric turbulence, necessitating the use of specific photon detectors and highly tailored photon pulses. My results could be applied in a near term satellite mission.