Observing quantum coherence from photons scattered in free-space

Quantum coherence is crucial for various quantum technologies. Current free-space quantum channels primarily use polarization encoding, which is disrupted by scattering. Spatial mode encoding is also unsuitable for scattered photons. Time-bin encoding, while robust to scattering, has not been previously demonstrated for non-line-of-sight (NLOS) free-space channels. This research addresses this gap by utilizing time-bin encoding with a multimode imaging interferometer and single-photon detector array to transfer and recover quantum coherence from scattered photons. The applications explored include NLOS quantum communication and enhanced low-light imaging.

Existing free-space quantum communication methods primarily rely on polarization encoding, which is highly sensitive to scattering. While higher-order spatial modes have been explored, they are also significantly impacted by scattering. Time-bin encoding offers resilience against scattering and has been used in fiber-optic communication but its application to NLOS free-space communication remains unexplored until now. Previous free-space time-bin experiments employed direct line-of-sight communication, addressing atmospheric turbulence through techniques like converting the distorted beam back into a single mode, adding complexity and loss, or using expensive adaptive optics. This paper innovatively addresses the challenges of NLOS free-space quantum communication using time-bin encoding and scattering.

The experiment uses two unbalanced Michelson interferometers: a converter to create time-bin states and an analyzer to measure them. A diffusive scattering surface (white paper) is used to scatter photons. A single-photon detector array (SPDA) with 8x8 pixels captures the scattered photons, individually time-tagged with high temporal precision (120 ps). The interference visibility is measured across the detector array. The scattering angle is varied to study the robustness of coherence to scattering. For quantum communication demonstration, specific phases are set at the converter and analyzer, and photon counts are recorded at the SPDA pixels. Low-light imaging is demonstrated by illuminating an object with photons carrying a specific phase signature, and reconstructing the image by correlating the received count pattern with the reference pattern. A high background light source is added to simulate noisy environment.

The researchers achieved a high time-bin visibility of approximately 95% for scattered photons over a wide scattering angle range (-45° to +45°). This demonstrates the successful transfer and recovery of quantum coherence through scattering. The multimode imaging interferometer coupled with the SPDA allows for simultaneous imaging and coherence measurement, with each pixel independently resolving the coherence. Non-line-of-sight quantum communication was successfully demonstrated, with visibility comparable to previous line-of-sight experiments. The method was used to enhance the contrast of low-light imaging in a high-background environment by correlating the observed count pattern with the reference phase signature. The SPDA's spatial resolution helps suppress background noise in quantum communication and enhances robustness against certain quantum attacks.

The high visibility achieved even with scattered photons demonstrates the robustness of the time-bin encoding method and the effectiveness of the multimode imaging interferometer in mitigating the effects of scattering. The successful demonstration of NLOS quantum communication opens new possibilities for secure communication in environments with obstructions. The ability to enhance low-light imaging contrast has implications for various applications including LIDAR and biomedical imaging. The use of the SPDA improves the robustness of the system, enabling background suppression and providing better resistance against certain quantum attacks.

This work presents a significant advancement in free-space quantum communication and sensing. The successful transfer and recovery of quantum coherence from scattered photons opens exciting avenues for NLOS quantum communication and improved low-light imaging techniques. Future research could explore extending the range and improving the efficiency of the system, as well as investigating applications in other quantum sensing and imaging modalities.

The current experimental setup has a limited range of approximately 2 meters. While coherence has been shown to be robust over much longer distances in line-of-sight experiments, extending the range in NLOS scenarios warrants further investigation. The type of scattering surface used might also influence the results, and further studies are needed to explore different scattering materials. The signal-to-noise ratio in the low-light imaging experiment could be improved further.