A rack-integrated photon pair source for field deployable quantum internet nodes

Abstract

The losses associated with long-range optical fibre communications are a major challenge for establishing large-scale quantum communication networks. Quantum repeaters provide a solution, requiring field-deployable quantum nodes. One possible way of realising a quantum network node is to combine a photon pair source and a quantum memory. For the photon-pair source, stability, compatibility with both the telecommunications band and the quantum memory, and high efficiency are required. In this work, I designed, assembled, and characterised a rack-integrated cavity-enhanced spontaneous parametric downconversion (cSPDC) narrow-linewidth photon-pair source for a field-deployable quantum repeater node. The source generates photon pairs at 606 nm and 1552 nm from a periodically poled KTP crystal, compatible with Pr3+:Y2SiO5 quantum memory and telecommunications networks. In order to make the photons compatible with the quantum memory, their linewidth is narrowed using a 2.2-meter-long bow-tie cavity. Additionally, to make the source truly field-deployable, it is mounted in a standard 19-inch rack drawer together with the optics and lasers required for its operation. The cavity characterization yielded a linewidth of 1.26 ± 0.02 MHz for the signal photons, compatible with a Pr3+:Y2SiO5 solid-state quantum memory. The rack-integrated source maintained a fibre-coupling efficiency of approximately 64% over a 48-hour measurement, demonstrating its stability. I measured a heralding efficiency of 21%, lower than expected from the cavity design. The main limitations were identified as non-optimal pump focusing and photon collection. Overall, the results demonstrate a compact, stable, and rack-integrated cSPDC source. The source is designed to be compatible with a quantum memory setup that constitutes a field-deployable quantum node, contributing to the development of large-scale quantum communication networks