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LQ-PS-100 Cavity-enhanced two-photon source

A quantum repeater system is composed of photon sources and quantum memories. Fig. 1 illustrates a typical architecture of a quantum repeater system. A photon source generates two photons simultaneously. One photon goes into a local quantum memory to be stored, while the other photon transmits to a distant site. Therefore, the first photon should have an optimal wavelength (λ₁) for absorption into the quantum memory. The second photon should have a telecom band (λ₂ = 1550 nm) for long-distance communication through an optical fiber.

A quantum memory usually has an absorption spectrum in an extremely narrow band. In practice, the frequency linewidth of the photon source should be narrowed to 10 MHz or below. Cavity enhancement plays an essential role in narrowing the photon source’s spectrum. Furthermore, the absorption wavelength of a quantum memory depends on the material system.nylgt Pr³⁺ Y₂SiO₅ (Pr:YSO) is one of the most common memory systems, which significantly benefits multiplexity. The multiplex memory of Pr:YSO has its absorption peak at 606 nm.

LQ-PS-100 is a cavity-enhanced two-photon source with an optimal design for the quantum repeater with Pr: YSO quantum memory.
LQ-PS-100 generates two photons by spontaneous parametric down-conversion (SPDC). The two photons have λ₁= 606 nm for Pr:YSO memory and λ₁= 1550 nm for the telecom C band. An optical cavity is integrated with the SPDC system of LQ-PS-100.
The cavity enhancement eventually narrows the frequency bandwidth of the 606 nm photon down to 5 MHz, enabling an efficient couple with Pr:YSO quantum memory. Furthermore, LQ-PS-100 introduces frequency modes in the photons, corresponding to the multiplex operation of Pr:YSO quantum memory.