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arXiv:1711.06592 · Journal of Physics B Atomic Molecular and Optical Physics

Quantum secrecy in thermal states

Elizabeth Newton, Anne Ghesquière, Freya L. Wilson, Benjamin T. H. Varcoe, Martin Moseley

Published May 28, 2019• 13 citations
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Abstract

Abstract We propose to perform quantum key distribution using quantum correlations occurring within thermal states produced by low power sources such as LEDs. These correlations are exploited through the Hanbury Brown and Twiss effect. We build an optical central broadcast protocol using a superluminescent diode which allows switching between laser and thermal regimes, enabling us to provide comparable experimental key rates in both regimes. We provide a theoretical analysis and show that quantum secrecy is possible, even in high noise situations.

DOI
10.1088/1361-6455/ab1e91

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Abstract We propose to perform quantum key distribution using quantum correlations occurring within thermal states produced by low power sources such as LEDs. These correlations are exploited through the Hanbury Brown and Twiss effect. We build an optical central broadcast protocol using a superluminescent diode which allows switching between laser and thermal regimes, enabling us to provide comparable experimental key rates in both regimes. We provide a theoretical analysis and show that quantum secrecy is possible, even in high noise situations.
In this paper we consider a scheme for cryptographic key distribution based on a variation of continuous variable quantum key distribution called central broadcast. In the continuous variable central broadcast scheme, security arises from discord present in the Hanbury Brown and Twiss effect from a thermal source. The benefit of this scheme is that it expands the range of frequencies into the microwave regime. Longer wavelengths—where the thermal photon number is higher and correlations remain robust over long distances—may even be preferable to optical wavelengths. Assuming that Alice controls the source but not the distribution of the light (e.g. satellite broadcasts), then we demonstrate that the central broadcast scheme is robust to an entangling cloner attack. We establish the security of the protocol both experimentally and theoretically.
Abstract In this paper we expanded the security of a central broadcast protocol using thermal states to the case in which the eavesdropper controls the source. Quantum secrecy in a continuous variable central broadcast scheme is guaranteed by the quantum correlations present in thermal states arising from the Hanbury Brown and Twiss effect. This work allows for a method of key exchange in which two parties can agree a key as long as both can detect the same source and they are within the spatial coherence length of the source. This is important because it allows quantum secure key exchange with only minimal changes to existing infrastructure.
Secret key exchange relies on the creation of correlated signals, serving as the raw resource for secure communication. Thermal states exhibit Hanbury Brown and Twiss correlations, which offer a promising avenue for generating such signals. In this paper, we present an experimental implementation of a central broadcast thermal-state quantum key distribution (QKD) protocol in the microwave region. Our objective is to showcase a straightforward method of QKD utilizing readily available broadcasting equipment. Unlike conventional approaches to thermal-state QKD, we leverage displaced thermal states. These states enable us to share the output of a thermal source among Alice, Bob, and Eve via both waveguide channels and free space. Through measurement and conversion into bit strings, our protocol produces key-ready bit strings without the need for specialized equipment. By harnessing the inherent noise in thermal broadcasts, our setup facilitates the recovery of distinct bit strings by all parties involved.
Adopting a quantum information perspective, we analyse the correlations in the thermal light beams used to demonstrate the Hanbury Brown and Twiss effect. We find that the total correlations measured by the Renyi mutual information match the normalised intensity correlations in the regime of low source intensity. Genuine quantum correlations in the form of discord are relevant in such regime but get washed out with increasing source intensity. This provides a new angle on the issue about the nature--quantum versus classical--of the effect.
Abstract We analyse a central broadcast continuous variable quantum key distribution protocol in which a beam produced by a thermal source is used to create a secret key between two parties, Alice and Bob. A beam splitter divides the initial beam into a pair of output beams, which are sent to Alice and Bob, with Eve intercepting Bob’s beam. We investigate the protocol in detail, calculating mutual informations through a pair of analytic methods and comparing the results to the outputs of a Monte Carlo simulation of the protocol. In a lossless system, we find that a lower bound on the key rate remains positive in the protocol under a beam splitter attack, provided Bob receives a nonzero proportion of the beam initially sent to him. This suggests that the thermal state protocol could be used experimentally to produce secure keys.
A beam emitted by a displaced thermal source, incident on a beam splitter, is used as the source in a central broadcast quantum key distribution protocol. These displaced thermal states have parallels to signals produced through phase-shift keying, already commonly used in a variety of microwave-based communications equipment. Here, we analyse a thermal state quantum key distribution protocol for which there is no dependency on displacement. Our analysis suggests the ability to carry out QKD using widely available broadcasting equipment. We also consider the effects of the introduction of loss at several points in the protocol, and find that, while increasing loss in Alice's channel eventually results in advantage distillation being required, security is still maintained.