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arXiv:1805.12004 · Scientific Reports

Measurement-Device-Independent Twin-Field Quantum Key Distribution

Hua‐Lei Yin, Yao Fu

Published Feb 28, 2019• 90 citations
arXivHTMLPDF

Abstract

The ultimate aim of quantum key distribution (QKD) is improving the transmission distance and key generation speed. Unfortunately, it is believed to be limited by the secret-key capacity of quantum channel without quantum repeater. Recently, a novel twin-field QKD (TF-QKD) is proposed to break through the limit, where the key rate is proportional to the square-root of channel transmittance. Here, by using the vacuum and one-photon state as a qubit, we show that the TF-QKD can be regarded as a measurement-device-independent QKD (MDI-QKD) with single-photon Bell state measurement. Therefore, the MDI property of TF-QKD can be understood clearly. Importantly, the universal security proof theories can be directly used for TF-QKD, such as BB84 encoding, six-state encoding and reference-frame-independent scheme. Furthermore, we propose a feasible experimental scheme for the proof-of-principle experimental demonstration.

DOI
10.1038/s41598-019-39454-1

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The ultimate aim of quantum key distribution (QKD) is improving the transmission distance and key generation speed. Unfortunately, it is believed to be limited by the secret-key capacity of quantum channel without quantum repeater. Recently, a novel twin-field QKD (TF-QKD) is proposed to break through the limit, where the key rate is proportional to the square-root of channel transmittance. Here, by using the vacuum and one-photon state as a qubit, we show that the TF-QKD can be regarded as a measurement-device-independent QKD (MDI-QKD) with single-photon Bell state measurement. Therefore, the MDI property of TF-QKD can be understood clearly. Importantly, the universal security proof theories can be directly used for TF-QKD, such as BB84 encoding, six-state encoding and reference-frame-independent scheme. Furthermore, we propose a feasible experimental scheme for the proof-of-principle experimental demonstration.
Twin field quantum key distribution (TF-QKD) has attracted the attention of the research community thanks to its higher secure key rate which scales as square root of the channel transmittance. The twin field secure key rate thus crosses the repeaterless secure key rate distance bound, showing that the protocol can cater to large channel lengths. Here, we present a twin-field-like QKD protocol based on single photon interference of differential phase shift (DPS) encoded states and prove the security of the scheme. Our protocol builds upon an existing measurement device independent (MDI) QKD system that has a DPS encoding and modifies the sifting step to one based on single photon interference. We demonstrate security of our DPS-twin-field (DPS-TF) protocol against collective attacks and quantify the secure key rate. We find that with the current state-of-the-art technology, both TF-QKD and MDI-QKD protocols with the DPS encoding work almost identically in terms of the channel length, although the achievable secure key rate of the DPS-TF QKD is higher compared to MDI-QKD.
Quantum cryptography has a strong security that classical cryptography cannot match, and quantum key distribution (QKD) is the most important method of quantum cryptography. Reference-frame-independent measurement-device-independent QKD (RFI-MDI-QKD) protocol can avoid the drift of the reference frame and all side channels of measurement devices. However, RFI MDI-QKD usually employs a weak coherent state (WCS) as the light source, which limits the transmission distance of secure keys. Here, by using heralded single-photon sources, we investigate an improved decoy-state RFI-MDI-QKD scheme. We perform numerical simulations for the scheme under different scenarios, such as considering the finite size and different drift angles. Compared with the scheme with WCS, our scheme can significantly improve the transmission distance of RFI-MDI-QKD.
Recently some alternatives to the measurement-device-independent (MDI) quantum key distribution (QKD) based on the single-photon Bell-state measurement (SBSM) were proposed. Although these alternatives are not precisely as secure as the MDI QKD, they possess the advantage of a high key rate of the traditional BB84-like protocol and avoid the technical complexity of two-photon interference required in the MDI QKD. However, the setups of these proposed schemes are rather complicated compared to commonly used BB84 systems. Here we propose a simple implementation of SBSM-based QKD that is directly built on the existing realization of the BB84 QKD. Our proposal exhibits the hidden connection between the SBSM-based QKD and traditional phase-encoding QKD protocols. This finding discloses the physics behind these two different types of QKD protocols. In addition, we experimentally demonstrate the feasibility of our protocol.
Two time-reversal quantum key distribution (QKD) schemes are the quantum entanglement based device-independent (DI)-QKD and measurement-device-independent (MDI)-QKD. The recently proposed twin field (TF)-QKD, also known as phase-matching (PM)-QKD, has improved the key rate bound from O(η) to O[Formula: see text] with η the channel transmittance. In fact, TF-QKD is a kind of MDI-QKD but based on single-photon detection. In this paper, we propose a different PM-QKD based on single-photon entanglement, referred to as single-photon entanglement-based phase-matching (SEPM)-QKD, which can be viewed as a time-reversed version of the TF-QKD. Detection loopholes of the standard Bell test, which often occur in DI-QKD over long transmission distances, are not present in this protocol because the measurement settings and key information are the same quantity which is encoded in the local weak coherent state. We give a security proof of SEPM-QKD and demonstrate in theory that it is secure against all collective attacks and beam-splitting attacks. The simulation results show that the key rate enjoys a bound of O[Formula: see text] with respect to the transmittance. SEPM-QKD not only helps us understand TF-QKD more deeply, but also hints at a feasible approach to eliminate detection loopholes in DI-QKD for long-distance communications.
Variations of phase-matching measurement-device-independent quantum key distribution (PM-MDI QKD) protocols have been investigated before, but it was recently discovered that this type of protocol (under the name of twin-field QKD) can beat the linear scaling of the repeaterless bound on secret key capacity. We propose a variation of PM-MDI QKD protocol, which reduces the sifting cost and uses non-phase-randomized coherent states as test states. We provide a security proof in the infinite key limit. Our proof is conceptually simple and gives tight key rates. We obtain an analytical key rate formula for the loss-only scenario, confirming the square root scaling and also showing the loss limit. We simulate the key rate for realistic imperfections and show that PM-MDI QKD can overcome the repeaterless bound with currently available technology.
Reference-frame-independent quantum key distribution (RFI-QKD) provides a practical way to generate secret keys between two remote parties without sharing common reference frames. On the other hand, measurement-device-independent QKD (MDI-QKD) offers a high level of security, as it is immune to all quantum hacking attempts to measurement devices. The combination of these two QKD protocols, i.e., RFI-MDI-QKD, is one of the most fascinating QKD protocols, since it holds advantages of both practicality and security. For further practicality of RFI-MDI-QKD, it is beneficial to reduce the implementation complexity. Here, we show that RFI-MDI-QKD can be implemented using fewer quantum states than those of its original proposal. We find that, in principle, the number of quantum states for one of the parties can be reduced from six to three without compromising security. Compared to conventional RFI-MDI-QKD where both parties transmit six quantum states, it significantly simplifies the implementation of the QKD protocol. We also verify the feasibility of the scheme with a proof-of-principle experiment.