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Overcoming the rate-distance barrier of quantum key distribution without using quantum repeaters
Marco Lucamarini, Zhiliang Yuan, James F. Dynes, Andrew J. Shields
TL;DR
Existing point-to-point QKD schemes do not surpass the secret key capacity bound, while practical quantum repeaters remain difficult to realize. This paper introduces Twin-Field QKD, where twin optical fields interfere at a central station, and predicts key rates scaling as η^1/2 that surpass repeaterless bounds at long distances.
Problem
Existing QKD schemes cannot surpass the secret key capacity bound, and quantum repeaters that could overcome it remain difficult to realize.
Source: p. 1Method
Twin-Field QKD sends same-phase twin fields over separate channels to interfere at Charlie.
Source: p. 2Results
TF-QKD rates scale as η^1/2, surpassing repeaterless bounds after 200 km ideally and surpassing the ideal repeaterless bound after 340 km with realistic parameters.
Source: p. 1Takeaways & Limitations
The scheme borrows components and techniques from ordinary QKD while suggesting a route to overcome the point-to-point secret key capacity bound without quantum repeaters.
Source: p. 3Public phase disclosure can leak information to Eve, Eq. (3) does not cover her most general attack, and unconditional security remains unproved.
Source: p. 3
Abstract
from arXiv · showhide
Quantum key distribution (QKD) allows two distant parties to share encryption keys with security based on physical laws. Experimentally, it has been implemented with optical means, achieving key rates of 1.26 Megabit/s over 50 kilometres (km) of standard optical fibre and 1.16 bit/hour over 404 km of ultralow-loss fibre in a measurement-device-independent configuration. Increasing the bit rate and range of QKD is a formidable, but important, challenge. A related target, currently considered unfeasible without quantum repeaters, is overcoming the fundamental rate-distance limit of point-to-point QKD. Here we introduce a conceptually new scheme where pairs of phase-randomised optical fields are first generated at two distant locations and then combined at a central measuring station. The fields imparted with the same random phase are "twins" and can be employed to distil a quantum key, as we prove under an explicit security assumption. The key rate of this Twin-Field QKD (TF-QKD) shows the same dependence on distance as a quantum repeater, scaling with the square-root of the channel transmittance, irrespective of whom is in control of the measuring station. Differently from a quantum repeater, however, the new scheme is feasible with current technology and presents manageable levels of noise even on 550 km of standard optical fibre. This is promising to overcome the QKD rate-distance barrier and to greatly extend the range of secure quantum communications.