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Laser Physics

On sufficient statistics in quantum cryptography: quantum key distribution with mismatching detector efficiency

Kirill A. Balygin, S. N. Molotkov

Published Feb 18, 2021• 0 citations
Publisher

Abstract

Abstract Accounting for the different quantum efficiency of detectors is fundamentally important for ensuring the cryptographic security of practical systems of quantum cryptography. A method for accounting for different quantum efficiency of detectors is proposed. The method is, in fact, analytical and applicable to any quantum key distribution protocols.

DOI
10.1088/1555-6611/abe23e

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Abstract Accounting for the different quantum efficiency of detectors is fundamentally important for ensuring the cryptographic security of practical systems of quantum cryptography. A method for accounting for different quantum efficiency of detectors is proposed. The method is, in fact, analytical and applicable to any quantum key distribution protocols.
Quantum key distribution (QKD) protocols with threshold detectors are driving high-performance QKD demonstrations.The corresponding security proofs usually assume that all physical detectors have the same detection efficiency.However, the efficiencies of the detectors used in practice might show a mismatch depending on the manufacturing and setup of these detectors.A mismatch can also be induced as the different spatialtemporal modes of an incoming signal might couple differently to a detector.Here we develop a method that allows to provide security proofs without the usual assumption.Our method can take the detection-efficiency mismatch into account without having to restrict the attack strategy of the adversary.Especially, we do not rely on any photon-number cutoff of incoming signals such that our security proof is directly applicable to practical situations.We illustrate our method for a receiver that is designed for polarization encoding and is sensitive to a number of spatial-temporal modes.In our detector model, the absence of quantum interference between any pair of spatial-temporal modes is assumed.For a QKD protocol with this detector model, we can perform a security proof with characterized efficiency mismatch and without photon-number cutoff assumption.Our method also shows that in the absence of efficiency mismatch in our detector model, the key rate increases if the loss due to detection inefficiency is assumed to be outside of the adversary's control, as compared to the view where for a security proof this loss is attributed to the action of the adversary.
Subject of study. The effect of different quantum efficiencies of single-photon detectors in quantum key distribution (QKD) systems implementing the BB84 protocol with passive basis choice is investigated. Aim of study. The aim of this study is to determine the secret key generation rate and security of the final key in quantum cryptography systems as functions of detector efficiency mismatch. Additionally, an analytical equation is derived for estimating the secret key generation rate in quantum communication systems with passive basis choice while accounting for different detector efficiencies. Method. The secret key is calculated by dividing the received measurement data according to the measurement bases, followed by performing separate post-processing for each dataset, and finally summing the resulting estimates of the secret key. Main results. An analytical method for estimating the secret key generation rate in systems with passive basis choice and mismatched detector efficiencies is developed. A satellite-to-ground quantum communication channel is modeled using the parameters of the Zvenigorod Observatory in Russia. The proposed theory for accounting for the detector efficiency mismatch is applied to a model of a satellite QKD system, and the determined length of the secret key generated during a single communication session with the satellite is found to be 300 kbit. Practical significance. The obtained results account for the effect of detector efficiency mismatch when estimating the length of the secret key in QKD systems, thereby increasing their reliability in practical implementations.
One of the challenges in practical quantum key distribution is dealing with efficiency mismatch between different threshold single-photon detectors. There are known bounds for the secret key rate for the BB84 protocol with detection-efficiency mismatch provided that the eavesdropper sends no more than one photon to the legitimate receiver. Here we improve these bounds and give tight bounds for the secret key rate with a constant detection-efficiency mismatch under the same single-photon assumption. We propose a method based on the analytical minimization of the relative entropy of coherence, which can be used in other problems in quantum key distribution. Also we propose an adaptation of the decoy state method to prove the security in the case of weak coherent pulses on the source side.
In theory, quantum key distribution (QKD) offers unconditional security based on the laws of physics. However, as demonstrated in recent quantum hacking theory and experimental papers, detection efficiency loophole can be fatal to the security of practical QKD systems. Here, we describe the physical origin of detection efficiency mismatch in various domains including spatial, spectral, and time domains and in various experimental set-ups. More importantly, we prove the unconditional security of QKD even with detection efficiency mismatch. We explicitly show how the key generation rate is characterized by the maximal detection efficiency ratio between the two detectors. Furthermore, we prove that by randomly switching the bit assignments of the detectors, the effect of detection efficiency mismatch can be completely eliminated.
In theory, quantum key distribution (QKD) offers unconditional security based on the laws of physics. However, as demonstrated in recent quantum hacking theory and experimental papers, detection efficiency loophole can be fatal to the security of practical QKD systems. Here, we describe the physical origin of detection efficiency mismatch in various domains including spatial, spectral, and time domains and in various experimental set-ups. More importantly, we prove the unconditional security of QKD even with detection efficiency mismatch. We explicitly show how the key generation rate is characterized by the maximal detection efficiency ratio between the two detectors. Furthermore, we prove that by randomly switching the bit assignments of the detectors, the effect of detection efficiency mismatch can be completely eliminated.