Determine the mechanism of state-dependent electromagnetic emissions

Determine the mechanism responsible for the state-dependent differences in electromagnetic spectra and time-domain waveforms emitted by the prototype time-bin decoy BB84 quantum key distribution transmitter.

Background

The study observes that the electromagnetic emissions of the prototype quantum key distribution transmitter vary with the transmitted quantum state, particularly at the clock frequency and its third harmonic. These variations could enable an eavesdropper to infer basis and bit-value information from externally measured electromagnetic leakage.

The paper suggests that digital-control circuits may generate the state-dependent radiation, while the contribution of the modulation drivers may be limited because the transmitter uses a highly symmetric driving scheme. However, the authors do not establish the physical mechanism producing the observed spectral and waveform differences, leaving its identification unresolved.