Compact and efficient quantum frequency conversion of a fiber-pigtailed single-photon source

TL;DR

The paper tackles the challenge of connecting near-infrared quantum-dot single-photon sources to telecom channels by implementing a compact, fully fiber-based quantum frequency conversion interface. It combines a fiber-pigtailed SPS emitting at $925.7\,\mathrm{nm}$ with a DFG process in a 4 cm PPLN/WG pumped at $2272\,\mathrm{nm}$ to yield photons at $1560\,\mathrm{nm}$ with an end-to-end efficiency of $\eta_{ext}=48.4\%$, while preserving $g^{(2)}(0)$ and indistinguishability near $80\%$. The system exhibits tunability over a ~10 nm range via pump and temperature control, and the photons maintain high purity and coherence after conversion, demonstrating a practical path toward interconnected quantum networks. This fiber-integrated approach offers a compact, transportable solution suitable for field deployment, supporting applications in QKD and quantum repeaters by bridging disparate wavelength regimes without sacrificing quantum integrity.

Abstract

Quantum frequency converters are key enabling technologies in photonic quantum information science to bridge the gap between quantum emitters and telecom photons. Here, we report a coherent frequency converter scheme combining a fiber-coupled nonlinear optical Lithium Niobate waveguide with a fiber-pigtailed single-photon source based on semiconductor quantum dots. Single and indistinguishable photons are converted from 925.7 nm to the telecommunication C-band, with a 48.4% end-to-end efficiency and full preservation of single-photon purity and indistinguishability. The integration of the two fiber-based modules achieving top-level performance represents an important step toward the practical interconnection of future quantum information processing systems operating at different wavelengths.

Figures (4)

• Figure 1: (a) Experimental setup of the frequency conversion interface. The signal and pump beams are going through the HWP and PBS to control the polarisation and power. The SMF are used to clean the spatial modes. Overlapping is performed using a WDM coupled into the PPLN/WG. The dashed lines correspond to free-space propagation. We filter the converted photons using telecom DWDM. (b) Phase matching of conversion process with $T_{PPLN} = 43.4$°C. FWMH are extracted, for the signal (FWHM$_{NIR} = 0.40 \pm 0.02$ nm) and the conversion (FWHM$_{conv} = 1.00 \pm 0.02$ nm).
• Figure 2: Phase matching curves with different central signal wavelength. The temperature is adjusted to conserve the central conversion wavelength of 1560 nm. For these four curves: 0.37 nm $<$ FWMH$_{NIR}$$<$ 0.49 nm, with the PPLN temperature from 33.0 °C to 82.5 °C.
• Figure 3: External conversion efficiency as a function of coupled pump power. The data (black) are fitted (grey zone) and compared with the theoretical curve (blue). Orange dots : Signal-to-noise ratio of the interface, depending on the coupled pump power.
• Figure 4: Characterization of single photon properties conservation. Left column : Detection setups. $g^{(2)}(0)$ is measured using an HBT interferometer, recording coincidences on a time-to-digital converter (TDC) and receiving signals from single-photon detectors (SPD). The $V_{HOM}$ is extracted by using an UMZI, using a HWP to configure indistinguishable case (co-polarization) and distinguishable case (cross-polarization). Middle column : Results in NIR band, at the output of the QD. Right column : Results after conversion, at $1560$ nm.