High-performance single-photon generation with commercial-grade optical fiber

High-quality quantum sources are of paramount importance for the implementation of quantum technologies. We present here a heralded single-photon source based on commercial-grade polarization-maintaining optical fiber. The heralded photons exhibit a purity of at least 0.84 and an unprecedented heralding efficiency into a single-mode fiber of $85\%$. The birefringent phase-matching condition of the underlying four-wave mixing process can be controlled mechanically to optimize the wavelength tuning needed for interfacing multiple sources, as is required for large-scale entanglement generation.

Figure 1

Schematic setup of the HOM interference experiment. The pump beam from our Ti:sapphire oscillator was split and sent to the two PM source fibers. We used half-wave plates (HWP) to align the pump polarization with the high-index axis of each fiber. Polarizing beamsplitters (PBS) at the fiber outputs separated the generated photon pairs from the orthogonally polarized pump. A dichroic mirror (DM) then divided signal and idler before long-pass and bandpass filters (LPF, BPF) suppressed residual pump light. The idler photons from both sources interfered at a 50/50 fiber-integrated single-mode beamsplitter with half- and quarter-wave plates (QWP) at the inputs ensuring polarization overlap. The relative time delay ($\Delta \tau$) between the interfering photons could be controlled with a motor-driven delay line in front of source fiber B. Fiber A was placed in a polarization controller to allow fine-tuning of the generated wavelengths. We used silicon single-photon detectors and a field-programmable gate array (FPGA) to record coincidences.

Figure 2

HOM interference dips. (a) Recorded fourfold coincidences as a function of time delay between the interfering idler photons. Only broadband filters were applied to suppress residual pump light. Error bars have been calculated by taking the square root of each data point, assuming Poissonian count statistics. (b) Illustration of the spectral amplitude distribution of signal and idler. The secondary lobes along the $-{45}^{°}$ axis lead to spectral entanglement and reduce the heralded state’s purity. The dashed vertical lines represent the FWHM bandwidth of the filters added for the subsequent measurement. (c) Fourfold coincidences with additional filters in the heralding arms. The heralded state’s purity is improved from 0.73 to 0.84 due to suppression of the spectral side peaks. The reduced overall count rate can be attributed to limited filter transmission and slightly reduced pump power.