Abstract: Optical homodyne detection has been widely used in continuous-variable (CV) quantum information processing for measuring field quadrature. Here we explore the possibility of operating a conjugate homodyne detection system in “photon counting” mode to implement discrete-variable (DV) quantum key distribution (QKD) protocols. A conjugate homodyne detection system, which consists of a beam splitter followed by two optical homodyne detectors, can simultaneously measure a pair of conjugate quadratures of the incoming quantum state. This allows single-shot photon number measurement without the need of a phase reference between the sender and the receiver. While the above photon-number measurement is intrinsically noisy, the security analysis could be improved by taking advantage of two special features of the detection scheme: First, the fundamental detection noise associated with vacuum fluctuations cannot be manipulated by an external adversary and thus can be trusted. Second, the photon number distribution at the receiver’s end can be reconstructed for an ensemble of quantum states. As an example, we study the security of the BB84 QKD using conjugate homodyne detection and evaluate its performance through numerical simulations. This study may open the door to new QKD protocols, complementary to the well-established DV-QKD based on single-photon detection and CV-QKD based on coherent detection.
*Journal reference: B. Qi, Phys. Rev. A 103, 012606 (2021)