Fig. 3: Schematic of the experimental setup.
From: Boosted Bell-state measurements for photonic quantum computation
The setup consists of a pulsed 775 nm pump laser (2 ps pulse duration, 76 MHz repetition rate), which is split using a balanced beam splitter (BS) to pump two Sagnac-type Bell-state sources. The sources are based on spontaneous parametric downconversion (SPDC) in periodically poled potassium titanyl phosphate (ppKTP) and generate polarization-entangled photon pairs at λ = 1550 nm. For setting the polarization of the pump laser, half-wave plates (HWP) are used. The generated photonic Bell states are then coupled into single-mode fibers and sent to the fused fiber 4 × 4 splitter. The relative phase between the sagnac sources is randomized using two quarter-wave plates and a motorized HWP. To ensure that the polarization of the input photons is maintained, fiber polarization paddles are used. In the measurement stage, the photons are sorted by polarization using polarizing beam splitters (PBS) where each output mode is connected to a 1 × 7 splitter with seven superconducting nanowire single-photon detectors (SNSPD) to allow for probabilistic photon number resolution. For the implementation of this detection scheme, a total of 56 SNSPDs and time-to-digital converters (TDC) are needed.
