We have recently constructed a fiber-optic “plug and play” quantum cryptography system as shown in Fig.1, wherein a Faraday-Mirror was used to compensate the polarization mode dispersion and phase drifts. It worked at 1550 nm and was based on the B92 protocol[1]. We have achieved stable quantum key distribution in a standard telecom fiber with different working-distances of 10, 25 and 50 km, respectively, by using some specifically developed techniques to guarantee long-term stability of our system, such as polarization-independent phase-modulation for stable phase encoding and decoding, hybrid coincidence for efficient detection of single photons with coincidence-gated quenching of the single-photon detectors followed by coincident counting of the avalanche signals, and synchronized detection of single-photon pulses by coincidence counting gated with proper delay and width. These unique techniques greatly enhanced the fringe visibility of single-photon interference. And we have also demonstrated the application of our system in cryptographic communication by sending coded pictures from Alice to Bob using the sifted quantum keys.
This paper proposes a candidate framework for free-space quantum key distribution (QKD) based on geometrical-configuration modulation (GM). In the minimal implementation considered here, Alice coherently splits a single photon emitted from one source into two spatial output modes with a tunable separation, and uses the source separation
R as the GM variable that defines the prepared single-photon spatial superposition state. Bob records the single-photon detection coordinate in the far field or Fourier plane, providing the correlated data used for soft-input information reconciliation. Based on this physical mechanism, we first establish an
R−x protocol model in which the source separation
R and the single-photon detection coordinate
x are random variables, and further propose an
R−Δx extension based on the difference variable
Δx between adjacent accepted detection events to mitigate slowly varying center drift in free-space links. The framework specifies state preparation, far-field conditional probabilities, soft-input information generation, parameter estimation, reconciliation, and asymptotic candidate key-rate formulas. A complete composable security analysis further requires derive an explicit computable upper bound on Eve's information from experimentally observed parameters, together with finite-key analysis and experimental validation under free-space conditions. The proposed candidate framework (GM-QKD) provides a modulation approach based on spatial degrees of freedom in which the source geometry serves as the modulation variable.
Abstract In theory, quantum key distribution (QKD) provides unconditional security; however, its practical implementations are susceptible to exploitable vulnerabilities. This investigation tackles the constraints in practical QKD implementations using weak coherent pulses. The conventional approach of using decoy pulses is improved by integrating it with the coincidence detection (CD) protocol. Additionally, an easy‐to‐implement technique is employed to compute asymptotic key rates for the protocol secure under specific photon number splitting (PNS) attacks. Furthermore, an experimental implementation of the protocol, where it is demonstrated that monitoring coincidences in the decoy state protocol leads to enhanced key rates under realistic experimental conditions, is carried out.