Better safeguards for sensitive information (Vol. 50, No. 2)

Schema of the encryption channel

Study improves the lower boundary and secret key capacity of an encryption channel

The secure encryption of information units based on a method called quantum key distribution (QKD) involves distributing secret keys between two parties—namely, Alice, the sender, and Bob, the receiver—by using quantum systems as information carriers. However, the most advanced quantum technology, QKD, is currently limited by the channel's capacity to send or share secret bits. In a recent study the authors show how to better approach the secret key capacity by improving the channel's lower boundary. They focus on a particular type of channel, called the noisy thermal amplifier channel, where the input signals are amplified together with noise induced by the thermal environment. The authors calculate the highest-known amount of secret information units, or bits, that Alice and Bob can share via such a channel. This is done by injecting controlled noise—made up of well-defined thermal agitation—into the detection apparatuses. By optimising over this noise, they improve the lower boundary of the capacity in the amplifier channel. The authors also confirm that the distribution of secret keys over this channel may occur at higher rates than the transmission of quantum information itself.

G. Wang, C. Ottaviani, H. Guo, and S. Pirandola, Improving the lower bound to the secret-key capacity of the thermal amplifier channel, Eur. Phys. J. D 73, 17 (2019)