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Beam-splitter-free, high-rate quantum key distribution inspired by intrinsic quantum mechanical spatial randomness of entangled photons

Ayan Kumar Nai, Gopal Prasad Sahu, Rutuj Gharate, C. M. Chandrashekar, G. K. Samanta

Abstract

Quantum key distribution (QKD) using entangled photon sources (EPS) is a cornerstone of secure communication. Despite rapid advances in QKD, conventional protocols still employ beam splitters (BSs) for passive random basis selection. However, BSs intrinsically suffer from photon loss, imperfect splitting ratios, and polarization dependence, limiting the key rate, increasing the quantum bit error rate (QBER), and constraining scalability, particularly over long distances. By contrast, EPSs based on spontaneous parametric down-conversion (SPDC) intrinsically exhibit quantum randomness in spatial and spectral degrees of freedom, offering a natural replacement for BS-based basis selection. Here, we demonstrate a proof-of-concept QKD scheme that exploits the intrinsic spatial randomness of SPDC without employing beam splitters. The annular SPDC emission ring is divided into four spatial sections, effectively generating two independent EPSs whose photon pairs are distributed to Alice and Bob. Crucially, the measurement basis is not predetermined but is assigned after photon detection by exploiting intrinsic detector timing jitter, thereby concealing the basis information from a potential eavesdropper. This post-detection basis assignment emulates stochastic basis choice while avoiding BS-induced losses and bias. Experimentally, our scheme achieves a 6.4-fold enhancement in sifted key rate, a consistently reduced QBER, and a near-ideal encoding balance between linear and rectilinear bases. Furthermore, the need for four spatial channels can be avoided by employing wavelength demultiplexing to generate two EPSs at distinct wavelength pairs. Harnessing intrinsic spatial/spectral randomness thus enables robust, bias-free, high-rate, and low-QBER QKD, offering a scalable pathway for next-generation quantum networks.

Beam-splitter-free, high-rate quantum key distribution inspired by intrinsic quantum mechanical spatial randomness of entangled photons

Abstract

Quantum key distribution (QKD) using entangled photon sources (EPS) is a cornerstone of secure communication. Despite rapid advances in QKD, conventional protocols still employ beam splitters (BSs) for passive random basis selection. However, BSs intrinsically suffer from photon loss, imperfect splitting ratios, and polarization dependence, limiting the key rate, increasing the quantum bit error rate (QBER), and constraining scalability, particularly over long distances. By contrast, EPSs based on spontaneous parametric down-conversion (SPDC) intrinsically exhibit quantum randomness in spatial and spectral degrees of freedom, offering a natural replacement for BS-based basis selection. Here, we demonstrate a proof-of-concept QKD scheme that exploits the intrinsic spatial randomness of SPDC without employing beam splitters. The annular SPDC emission ring is divided into four spatial sections, effectively generating two independent EPSs whose photon pairs are distributed to Alice and Bob. Crucially, the measurement basis is not predetermined but is assigned after photon detection by exploiting intrinsic detector timing jitter, thereby concealing the basis information from a potential eavesdropper. This post-detection basis assignment emulates stochastic basis choice while avoiding BS-induced losses and bias. Experimentally, our scheme achieves a 6.4-fold enhancement in sifted key rate, a consistently reduced QBER, and a near-ideal encoding balance between linear and rectilinear bases. Furthermore, the need for four spatial channels can be avoided by employing wavelength demultiplexing to generate two EPSs at distinct wavelength pairs. Harnessing intrinsic spatial/spectral randomness thus enables robust, bias-free, high-rate, and low-QBER QKD, offering a scalable pathway for next-generation quantum networks.

Paper Structure

This paper contains 4 sections, 2 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experiment
  3. Results and discussions
  4. Conclusions

Figures (3)

  • Figure 1: Experimental setup and network architecture.a, Conceptual representation of the present BBM92 scheme. b, Division of the spatial distribution of pair photons to form two identical entangled photon sources. c, Schematic of the experimental setup. Laser: 405 nm cw diode laser; HWP: half-wave plate; PBS: polarizing beam splitter; M: mirrors; DCM: dichroic mirror; L: plano-convex lenses; D-PBS: dual-wavelength (405 nm and 810 nm) PBS; D-HWP: dual-wavelength HWP; C: PPKTP crystal in an oven for photon-pair generation; PM: prism-shaped gold-coated mirror; DM: D-shaped mirrors; IF: 3 nm bandwidth interference filter; Coupler: collimator system for fiber coupling; SMF: single-mode fiber; SPCM: single-photon counting module; TDC: time-to-digital converter.
  • Figure 2: Characterization of the entangled photon sources.a, Quantum interference of spatially separated entangled photon sources measured in the horizontal (H, black dots), vertical (V, blue dots), diagonal (D, red dots), and anti-diagonal (A, green dots) polarization bases. Solid lines represent sinusoidal fits to the experimental data. Absolute values of d, real and e, imaginary parts of the reconstructed density matrix of the polarization-entangled Bell state $\ket{\phi^+}$.
  • Figure 3: Performance metrics of the QKD schemes. (a) Sifted key rate and (b) quantum bit error rate (QBER) as functions of pump power for the current BBM92 scheme with Protocol $\#$1 (red circles), Protocol $\#$2 (blue circles) and the conventional BS-based BBM92 scheme (black circles). Solid lines represent linear fits to the experimental data.