Twin-field QKD and the repeaterless rate-loss limit

Short answer: Twin-field QKD is a quantum key distribution scheme introduced in a 2018 Nature paper. Pairs of phase-randomized optical fields are generated at two distant locations and combined at a central measuring station. The authors say the key rate scales with the square root of the channel transmittance, the same dependence on distance as a quantum repeater, without needing repeaters. Later papers study its security with a finite number of signals, and report simulations, not deployed networks.

The limit it addresses

The 2018 Nature paper says quantum key distribution allows two distant parties to share encryption keys with security based on physical laws. It reports that experiments had achieved key rates of 1.26 megabits per second over 50 kilometres of standard optical fibre, and of 1.16 bits per hour over 404 kilometres of ultralow-loss fibre in a measurement-device-independent configuration. It calls increasing the bit rate and range of QKD a formidable but important challenge.

It describes the fundamental rate-distance limit as the maximum possible secret key rate that two parties can distil at a given distance, quantified by the secret-key capacity of the quantum channel connecting them. A 2019 paper in Nature Communications says that for a direct link, the key rate cannot surpass loss bounds that scale linearly with the single-photon transmissivity of the link, and that quantum repeaters are known to beat this limitation. A 2020 paper in npj Quantum Information says the main limitation is the exponential decrease of transmittance with the channel length in optical fibres.

The twin-field idea

The 2018 paper introduces an alternative scheme in which pairs of phase-randomized optical fields are first generated at two distant locations and then combined at a central measuring station. Fields imparted with the same random phase are called twins, and can be used to distil a quantum key. The authors say the key rate exhibits the same dependence on distance as a quantum repeater, scaling with the square root of the channel transmittance, irrespective of who is in control of the measuring station.

They also say that, unlike schemes involving quantum repeaters, theirs is feasible with current technology and presents manageable levels of noise even on 550 kilometres of standard optical fibre. They call it a promising step towards overcoming the rate-distance limit and greatly extending the range of secure quantum communications. These are claims from the original proposal, made before the later security analyses described below.

Finite-key security

The 2019 Nature Communications paper says that twin-field QKD had been conjectured to beat the limit by using an untrusted central station performing single-photon interference detection, but that efforts to prove it had been confined to the infinite key limit, which neglects the time and cost of monitoring an adversary. It proposes a variant with a simple monitoring method and an information-theoretic security proof for finite communication time. The authors simulate the key rate and say the protocol beats the linear bound in a reasonable running time of sending 10 to the power 12 pulses.

The 2020 npj Quantum Information paper extends the security of a variant it calls particularly suited for experimental implementation to the finite-key regime. It says the protocol can overcome the fundamental bounds on point-to-point QKD with about 10 to the power 10 transmitted signals, and that in many practical regimes its analysis offers higher key rates than alternative variants. It describes early demonstrations as suggesting that twin-field QKD can beat the current rate-versus-distance records.

What this does not tell you

This article summarises abstracts and introductions of three papers. The finite-key figures come from analysis and simulation, and this article does not describe any network or product. Implementation flaws can affect any QKD system, as covered in our article on QKD implementation attacks.

FAQ

What is twin-field QKD?

A scheme from a 2018 Nature paper where phase-randomized optical fields from two distant locations are combined at a central measuring station to distil a key.

How does the key rate scale?

With the square root of the channel transmittance, like a quantum repeater, according to the 2018 paper.

Does it need quantum repeaters?

The authors say no, and that it is feasible with current technology.

Is the security proven for real, finite runs?

Two later papers give finite-key analyses and report simulated key rates that beat the linear bound with about 10 to the power 12 pulses (2019 paper) and about 10 to the power 10 signals (2020 paper).

Does a central station have to be trusted?

The 2018 paper says the rate scaling holds irrespective of who is in control of the measuring station.