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Journal of the Optical Society of America B

Preserving nonclassical correlations in strongly unbalanced conditions

Alessia Allevi, Maria Bondani

Published Nov 7, 2019• 13 citations
Publisher

Abstract

It is well known that optical losses represent the main obstacle to the real exploitation of quantum optical systems for quantum technology. Here we investigate to what extent the presence of unbalanced losses between the two parties of a mesoscopic twin-beam state can prevent or not the observation of nonclassical correlations. Moreover, we focus on the survival of nonclassicality in the presence of asymmetric lossy channels modeled according to specific statistical distributions.

DOI
10.1364/josab.36.003275

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It is well known that optical losses represent the main obstacle to the real exploitation of quantum optical systems for quantum technology. Here we investigate to what extent the presence of unbalanced losses between the two parties of a mesoscopic twin-beam state can prevent or not the observation of nonclassical correlations. Moreover, we focus on the survival of nonclassicality in the presence of asymmetric lossy channels modeled according to specific statistical distributions.
In the past twenty years many experiments have demonstrated that quantum states of light can be used for secure data transfer, despite the presence of many noise sources. In this paper we investigate, both theoretically and experimentally, the role played by a statistically-distributed asymmetric amount of loss in the degradation of nonclassical photon-number correlations between the two parties of multimode twin-beam states in the mesoscopic intensity regime. To be as close as possible to realistic scenarios, we consider two different statistical distributions of such a loss, a Gaussian distribution and a log-normal one. The results achieved in the two cases show to what extent the involved parameters, both those connected to loss and those describing the employed states of light, preserve nonclassicality.
Quantum properties of light, which are crucial resources for quantum technologies, are quite fragile in nature and can be degraded and even concealed by the environment. We show, both theoretically and experimentally, that mesoscopic twin-beam states of light can preserve their nonclassicality even in the presence of major losses and different types of noise, thus suggesting their potential usefulness to encode information in quantum communication protocols. We develop a comprehensive general analytical model for a measurable nonclassicality criterion and find thresholds on noise and losses for the survival of entanglement in the twin beam.
Nowadays, the transmission of quantum information, especially for the distribution of cryptographic keys, is required on a global scale. The main obstacle to overcome in free-space communication is the presence of turbulence, which causes both spatial and temporal deformations of the light signals that code information. Here we investigate the extent at which the transmission of mesoscopic twin-beam states through asymmetric noisy channels degrades the nonclassical nature of the photon-number correlations between signal and idler. We consider three nonclassicality criteria, all written in terms of measurable quantities, and demonstrate, both theoretically and experimentally, that the asymmetry introduced by losses affects the three criteria in different ways.
Quantum resources can improve the security of information transmission between two parties. So far, Quantum Communication protocols have been implemented at the single-photon level by means of entangled states [1]. In contrast to this domain, where the presence of losses can reduce the transmission rate of information [2], in the mesoscopic one the optical pulses contain sizeable numbers of photons, thus resulting more robust against any kind of external degradation [3]. In a recent work of ours, we have demonstrated that the transmission of one arm of a twin-beam (TWB) state through a lossy and noisy channel does not prevent the observation of nonclassical correlations between the two parties [4]. Based on these successful results, here we consider a more realistic scenario, in which a portion of TWB produced in aβ\betaβ-Barium Borate (BBO) crystal is sent through water-filled tubes, while the other one undergoes free-space propagation [5]. We investigate the role played by the length of the tubes, the number of optical elements (OE), and the divergence of the beams through the different media. We demonstrate that, by properly acting on the light beams, we can still observe nonclassical correlations at moderate distances by using the noise reduction factor,RRR, as a criterion for nonclassicality. The experimental implementations involve commercial photon-number-resolving (PNR) detectors. In particular, hybrid photodetectors and Silicon Photomultipliers have been used and compared.
The problem of secure underwater communication can take advantage of the exploitation of quantum resources and novel quantum technologies. At variance with the current experiments performed at the single photon level, here we propose a different scenario involving mesoscopic twin-beam states of light and two classes of commercial photon-number-resolving detectors. We prove that twin-beam states remain nonclassical even if the signal propagates in tubes filled with water, while the idler is transmitted in free space. We also demonstrate that from the study of the nonclassicality information about the loss and noise sources affecting the transmission channels can be successfully extracted.
The robustness of nonclassical correlations exhibited by mesoscopic twin-beam states of light makes them useful for applications in the context of quantum technologies, such as for underwater quantum communication. To this aim, it is crucial to optimize the production of such quantum states of light, and study their level of nonclassicality in terms of different nonclassicality criteria. In this work, we realize an efficient source of well-populated twin-beam states in the red spectral region by pumping parametric down conversion with a natively sub-picosecond laser system. The nonclassical character of such states is investigated in terms of both the noise reduction factor and high-order correlation functions. The obtained results encourage the exploitation of this source in further optical schemes for practical applications.