A well-protected and characterized source in a quantum key distribution system is needed for its security. Unfortunately, the source is vulnerable to light-injection attacks, such as Trojan-horse, laser-seeding, and laser-damage attacks, in which an eavesdropper actively injects bright light to hack the source unit. The hacking laser could be a high-power one that can modify properties of components via the laser-damage attack and also further help the Trojan-horse and other light-injection attacks. Here we propose a countermeasure against the light-injection attacks, consisting of an additional sacrificial component placed at the exit of the source. This component should either withstand high-power incoming light while attenuating it to a safe level that cannot modify the rest of the source, or get destroyed into a permanent high-attenuation state that breaks up the line. We demonstrate experimentally that off-the-shelf fiber-optic isolators and circulators have these desired properties, at least under attack by a continuous-wave high-power laser.

The gain properties of multicomponent fibers with a heavily ${\rm{Er}^{3 + }}$-doped phosphate-based core in a silica cladding were investigated in an all-fiber amplifier configuration that included spliced joints of the investigated fibers with silica ones. The maximum value of small-signal gain reached about 38 dB. The maximum small-signal gain per unit length of 3.1 dB/cm was demonstrated in 10 cm long 3 wt.% ${\rm{Er}^{3 + }}$-doped fiber.

This paper presents a study of the properties of erbium-doped optical fibres with enhanced gamma-ray resistance at a high dose rate. The fibres have a silica core containing different dopants. The gamma-ray resistance of the fibres is due to additional doping of their core with cerium and germanium ions, as well as to aluminium concentration optimisation. We assess the effect of fibre composition on the radiation-induced loss at a wavelength of 1310 nm and the output power and weighted average wavelength of superluminescent fibre sources based on erbium-doped fibre under pulsed gamma irradiation (dose per pulse, ∼ 20 Gy). The fibre doped with aluminium, germanium and cerium oxides is shown to be optimal for use in gamma-ray-resistant broadband superluminescent fibre sources.

We report a new type of vulnerability based on a physical principle that has not been previously exploited in quantum hacking—optical pumping of a laser in practical implementations of quantum key distribution (QKD) systems. We show that it is possible to increase the pulse energy of a source laser diode not only by injection-locking it with external light near its emission wavelength of 1550 nm, but also by optically pumping it at a much shorter wavelength. We experimentally demonstrate a 10% increase in pulse energy when exposing the laser diode to continuous-wave (cw) laser light at 1310 nm with a power of 1.6 mW via its fiber pigtail. This effect may allow an eavesdropper to steal the secret key. A possible countermeasure is to install broadband optical filters and isolators at the source output and to characterize them during security certification.