Friederike Johlinger

Recently, the first integrated Measurement Device Independent Quantum Key Distribution (MDI QKD) system has been implemented here in Bristol (Semenenko, 2020). To build on this result and work towards improved security and key rates, a new indium phosphide (InP) transmitter chip has been designed for a second-generation MDI QKD implementation. The new chip contains two laser sources, including a distributed feedback laser to allow for faster pulsing and high-speed phase modulators with a bandwidth of up to 30 GHz. With the new lasers and phase modulators a higher pulse rate will be achieved, leading to better key rates. Additionally, an on-chip photodiode can be used to monitor incoming light. This makes the chip much more resilient against hacking attacks, such as a Trojan Horse or Laser Damage Attacks. Since MDI QKD is intrinsically protected against detector attacks, this means that this new MDI QKD system will show great security overall.

We have come a long way since the first implementation of a quantum key distribution (QKD) system and various attacks have been demonstrated, such as the Trojan Horse Attack (THA). In this attack, the eavesdropper Eve gains information by analysing the back-reflections of light she shines into the QKD system. Almost all attacks have been demonstrated on devices based on fibre components. However, the first chip-scale QKD devices are being developed now, utilising the small size of integrated optics, the stability of the optical system and the ease at which it can be mass-produced, showing great commercial potential. Here we discuss a THA on chip-scale QKD. First, points of reflection in a QKD transmitter chip were found via a reflectometry. Based on this we give an experimental set-up to implement a THA which would give Eve access to 50% of the key. Full experimental results of this attack are expected in the next two to three months.

In the age of measurement-device-independent quantum key distribution (MDI QKD) and twin- field QKD (TF QKD), the source units of these QKD schemes may become a new ``Achilles' heel" of the whole system. An adversary, Eve, can conduct various attacks on the sources by injecting lasers, whose power is limited by the laser-induced damage threshold of the quantum channel. Such an amount of power may modify the characteristics of components in a source. In this work, we study possible components to protect the source from the light-injected attacks, i.e., Trojan-horse attack, the laser-seeding attack, and the laser-damage attack. Experimental testing shows that fiber-optics isolators and circulators are good passive countermeasures because they sacrifice themselves' isolation under a high-power laser to protect other components behind them. Moreover, we find that illuminated by the high-power laser, integrated photonics QKD chips only lose the transmission of the coupler before any other change happens for the other components in the chips.