Decoy state quantum key distribution with imperfect source

TL;DR

This paper investigates the security implications of imperfect decoy state generation in quantum key distribution, demonstrating how source imperfections can be exploited and proposing calibration methods to improve security.

Contribution

It models the security breach caused by source imperfections in decoy state QKD and offers practical calibration solutions to mitigate security risks.

Findings

Imperfect source modulation introduces distinguishability in decoy states.

Photon-number-splitting attack can compromise security due to source imperfections.

Calibration of receiver transmittance can restore secure key rates.

Abstract

The decoy state protocol has been considered to be one of the most important methods to protect the security of quantum key distribution (QKD) with a weak coherent source. Here we test two experimental approaches to generating the decoy states with different intensities: modulation of the pump current of a semiconductor laser diode, and external modulation by an optical intensity modulator. The former approach shows a side-channel in the time domain that allows an attacker to distinguish s signal state from a decoy state, breaking a basic assumption in the protocol. We model a photon-number-splitting attack based on our experimental data, and show that it compromises the system's security. Then, based on the work of K. Tamaki et al. [New J. Phys. 18, 065008 (2016)], we obtain two analytical formulas to estimate the yield and the error rate of single-photon pulses when the signal and decoy states are distinguishable. The distinguishability reduces the secure key rate below that of a perfect decoy-state protocol. To mitigate this reduction, we propose to calibrate the transmittance of the receiver (Bob's) unit. We apply our method to three QKD systems and estimate their secure key rates.