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arXiv:2308.02376 · Physical Review Applied

Finite-key security of passive quantum key distribution

Víctor Zapatero, Marcos Curty

Published Jan 11, 2024• 14 citations
arXivHTMLPDF

Abstract

The passive approach to quantum key distribution (QKD) eliminates all optical modulators and random number generators from QKD systems reaching an enhanced simplicity, immunity to modulator side channels, and potentially higher repetition rates. In this work, we provide finite-key security bounds for a fully passive decoy-state Bennett-Brassard 1984 (BB84) protocol, considering a recently presented passive QKD source. With our analysis, the attainable secret-key rate is comparable to that of the perfect parameter-estimation limit, in fact differing from the key rate of the active approach by less than one order of magnitude. This demonstrates the practicality of fully passive QKD solutions.

DOI
10.1103/physrevapplied.21.014018

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The passive approach to quantum key distribution (QKD) eliminates all optical modulators and random number generators from QKD systems reaching an enhanced simplicity, immunity to modulator side channels, and potentially higher repetition rates. In this work, we provide finite-key security bounds for a fully passive decoy-state Bennett-Brassard 1984 (BB84) protocol, considering a recently presented passive QKD source. With our analysis, the attainable secret-key rate is comparable to that of the perfect parameter-estimation limit, in fact differing from the key rate of the active approach by less than one order of magnitude. This demonstrates the practicality of fully passive QKD solutions.
The decoy-state Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol is widely regarded as the de facto standard for practical implementations. On the receiver side, passive basis choice is attractive because it significantly reduces the need for random number generators and eliminates the need for optical modulators. Despite these advantages, a finite-key analytical security proof for the decoy-state BB84 protocol, where the basis is chosen passively with a biased probability, has been lacking. In this work, we present a simple analytical finite-key security proof for this setting, yielding a closed-form secret-key rate formula that can be directly evaluated using experimentally accessible parameters. Numerical simulations show that the key rates of passiveand active-measurement implementations are nearly identical, indicating that passive measurement does not compromise key-generation efficiency in practical QKD systems.
Abstract A passive quantum key distribution (QKD) transmitter generates the quantum states prescribed by a QKD protocol at random, combining a fixed quantum mechanism and a post-selection step. By circumventing the use of active optical modulators externally driven by random number generators, passive QKD transmitters offer immunity to modulator side channels and potentially enable higher frequencies of operation. Recently, the first linear optics setup suitable for passive decoy-state QKD has been proposed. In this work, we simplify the prototype and adopt sharply different approaches for BB84 polarization encoding and decoy-state parameter estimation. In particular, our scheme avoids a probabilistic post-selection step that is central to the former proposal. On top of it, we elaborate a simple and tight custom-made security analysis.
Abstract Passive generation of quantum states in quantum key distribution (QKD) is an elegant solution to utilise an internal source of quantum randomness of the transmitter’s scheme. It can be profitable, for example, in setups running at high repetition rates. However, the original analysis of passive protocol doesn’t consider real-life effects of a laser source, therefore possibly breaches the unconditional security of the scheme. Here we take into account the influence of various laser imperfections and make a refined comparison between active and passive setups. We show that the new bound on the secret key rate of the passive protocol stays similar to the one of an active scheme, thus proving passive generation to be a practical means of constructing QKD systems.
We propose a fully passive linear optical quantum key distribution (QKD) source that implements both random decoy-state and encoding choices with postselection only, thus eliminating all side channels in active modulators. Our source is general purpose and can be used in, e.g., BB84, the six-state protocol, and reference-frame-independent QKD. It can even potentially be combined with measurement-device-independent QKD to achieve robustness against side channels in both detectors and modulators. We also perform a proof-of-principle experimental source characterization to show its feasibility.
The Bennett-Brassard 1984 (BB84) protocol is one of the simplest protocols for implementing quantum key distribution (QKD). In the protocol, the sender and the receiver iteratively choose one of two complementary measurement bases. Regarding the basis choice by the receiver, a passive setup has been adopted in a number of its implementations, including satellite QKD and time-bin encoding. However, conventional theoretical techniques to prove the security of the BB84 protocol are not applicable if the receiver chooses their measurement basis passively, rather than actively, with a biased probability, followed by measurement with threshold detectors. Here we present a fully analytical security proof against coherent attacks for such a decoy-state BB84 protocol with the receiver’s passive basis choice and measurement with threshold detectors. Numerical simulations under practical situations show that the difference in secure key rate between the active and passive implementations of the protocol is negligible except for long communication distances.
Measurement-device-independent quantum key distribution (MDIQKD) can resist all attacks on the detection devices, but there are still some security issues related to the source side. One possible solution is to use the passive protocol to eliminate the side channels introduced by active modulators at the source. Recently, a fully passive QKD protocol was proposed that could simultaneously achieve passive encoding and passive decoy-state modulation using linear optics. In this work, we propose a fully passive MDIQKD scheme that can protect the system from both side channels of source modulators and attacks on the measurement devices, which can significantly improve the implementation security of the QKD systems. We provide a specific passive encoding strategy and a method for decoy-state analysis, followed by simulation results for the secure key rate in the asymptotic scenario. Our work offers a feasible way to improve the implementation security of QKD systems and serves as a reference for achieving passive QKD schemes using realistic devices.