Exciting new paper out from the Crypto Lab!
Continuous-variable quantum key distribution (CVQKD) devices are susceptible to information leakage due to flaws in the process of modulation.
In our CVQKD implementation, the transmitter/Alice encodes information about the key in the amplitude & phase quadratures using optical single sideband modulation (OSSB) with an in-phase and quadrature (IQ) modulator, while the receiver/Bob decodes that information using quadrature detection, e.g., radio-frequency heterodyne measurements. OSSB coding is a well-known technique in classical optical communication, and has the potential to revolutionize broadband CVQKD protocols by offering spectral efficiency and very low excess noise performance.
However, the suppression in practical OSSB setups is limited, implying a leakage of the coded information via the suppressed sideband, as illustrated in the figure. An eavesdropper/Eve, using an appropriate optical filter in the quantum channel, diverts the suppressed components of modulated signal to herself and performs the same measurements as Bob. One can observe the correlations between the measurement outcomes from the desired and suppressed bands through the constellation diagrams in the figure.
In this article, we have experimentally demonstrated and theoretically analyzed the security vulnerability due to such a modulation leakage. We have shown that the final secret key may actually not be secure anymore if Alice and Bob do not properly take the leakage into account.
Reference: https://iopscience.iop.org/article/10.1088/2058-9565/ac0d4c