Rack-integrated quantum dot-based source of single and entangled photons at telecom C-band

As quantum technology transitions from laboratory experiments to real-world infrastructure, the development of robust, portable light sources is critical. This paper presents a rack-integrated system that houses an epitaxial semiconductor quantum dot (QD) capable of producing high-quality single photons and entangled photon pairs at telecom C-band wavelengths. By integrating a compact cryostat, excitation lasers, and precise optical filtering into a transportable rack, the authors demonstrate a system that can be moved and operated outside of a traditional, highly controlled quantum optics lab.

The setup is divided into three functional floors: an excitation floor for laser control, a cryostat floor for the quantum dot sample, and a collection floor for signal processing. The system utilizes a closed-cycle cryostat to maintain the quantum dot at 4K without requiring external water cooling. To ensure high performance, the researchers employed two different excitation methods: longitudinal acoustic (LA) phonon-assisted excitation for single-photon generation and two-photon excitation (TPE) for the generation of polarization-entangled photon pairs via the biexciton-exciton cascade.

The integrated source demonstrates state-of-the-art performance, achieving a fiber-coupled coincidence rate for entangled photon pairs that sets a new record for telecom C-band sources. The system maintains high single-photon purity, with zero-delay second-order correlation values as low as 0.009 for biexciton emission. Furthermore, the setup includes automated polarization control and remote-controllable optomechanics, allowing the source to be integrated into existing metropolitan fiber infrastructures. The authors successfully validated the system's viability by demonstrating its operation over a 36 km deployed fiber link.