Optical Power Limiter Pushes Attack-Proof Quantum Communication Forward

(Photonics) Researchers from the National University of Singapore (NUS) developed two methods to enable quantum key distribution (QKD) communications to avoid side-channel attacks — instances in which attackers exploit weaknesses in the setup of the information system to eavesdrop on the exchange of secret keys.
The first method, which the researchers say is theoretical, is an ultrasecure cryptography protocol that can be deployed in any communication network that needs long-term security. The second, described as experimental, is a first-of-its-kind device that defends QKD systems against bright light pulse attacks by creating a power threshold.
“It’s a simple variation of the original protocol that started this field, but it can only be tackled now thanks to significant developments in mathematical tools,” said professor Valerio Scarani, who was one of the inventors of the type of method and co-author of the paper describing the current work. He is from the NUS Department of Physics and Centre for Quantum Technologies.
In their current work, the NUS researchers developed an optical device to address the issue, based on thermo-optical defocusing effects to limit the energy of the incoming light. Energy from the bright light changes the refractive index of the transparent plastic material embedded in the device and sends a fraction of the light out of the quantum channel. This enforces a power-limiting threshold.
The team’s power limiter can be seen as an optical equivalent of an electric fuse, team members said, except that it is reversible and does not burn when the energy threshold is breached. It is highly cost-effective and can be easily manufactured with off-the-shelf components. It also does not require any power, so it can be easily added to any quantum cryptography system to strengthen its implementation security.