Security of Optical Beam Splitter in Quantum Key Distribution

Abstract

The optical beam splitter is an essential device used for decoding in quantum key distribution. The impact of optical beam splitters on the security of quantum key distribution was studied, and it was found that the realistic device characteristics closely influence the error rate introduced by the wavelength-dependent attack on optical beam splitters. A countermeasure, combining device selection and error rate over-threshold alarms, is proposed to protect against such attacks. Beam splitters made of mirror coatings are recommended, and the variation of splitting ratio should be restricted to lower than 1 dB at 1260–1700 nm. For the partial attack scenario where the eavesdropper attacks only a portion of the quantum signal, a modified secure key rate formula is proposed to eliminate the revealed information of the attacked portion. Numerical results show that the QKD system adopting this countermeasure exhibits good performance with a secure key rate of over 10 kbps at 100 km and a maximum transmission distance of over 150 km, with only a small difference from the no-attack scenario. Additionally, a countermeasure to monitor the light intensity of different wavelengths is proposed to protect against the wavelength-dependent attack on optical beam splitters.