Curator's Take
This article introduces a new free‑space QKD scheme that encodes information in the geometric separation of a single‑photon source, turning the spatial superposition itself into a continuous modulation variable. By avoiding complex phase or polarization modulators, the approach could simplify hardware for satellite and ground‑to‑ground links while offering resilience to beam wander and turbulence. It builds on recent high‑dimensional and free‑space QKD demonstrations, suggesting a scalable path toward practical quantum networks. The proposal remains at the asymptotic security level, so finite‑key analysis and experimental validation will be crucial before deployment.
— Mark Eatherly
Summary
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.