Superlinear threshold detectors in quantum cryptography

TL;DR

This paper introduces superlinear threshold detectors in quantum cryptography, demonstrating how their properties can be exploited for undetectable eavesdropping, and analyzes vulnerabilities in common detector types.

Contribution

It generalizes the detector control attack and quantifies superlinearity in superconducting and avalanche photodiode detectors, revealing potential security flaws in quantum key distribution.

Findings

Superlinear detectors enable undetectable eavesdropping.

Avalanche photodiodes become superlinear near the end of the gate.

Eavesdropping attacks can use less than 120 photons per pulse.

Abstract

We introduce the concept of a superlinear threshold detector, a detector that has a higher probability to detect multiple photons if it receives them simultaneously rather than at separate times. Highly superlinear threshold detectors in quantum key distribution systems allow eavesdropping the full secret key without being revealed. Here, we generalize the detector control attack, and analyze how it performs against quantum key distribution systems with moderately superlinear detectors. We quantify the superlinearity in superconducting single-photon detectors based on earlier published data, and gated avalanche photodiode detectors based on our own measurements. The analysis shows that quantum key distribution systems using detector(s) of either type can be vulnerable to eavesdropping. The avalanche photodiode detector becomes superlinear towards the end of the gate, allowing eavesdropping using trigger pulses containing less than 120 photons per pulse. Such an attack would be virtually impossible to catch with an optical power meter at the receiver entrance.