Figure 4
From: Engineering integrated photonics for heralded quantum gates
(a) Setup for measuring two-photon interference. Degenerate photon pairs at 820 nm are created via spontaneous parametric down-conversion (SPDC) in a nonlinear β-barium-borate (BBO) crystal pumped by a 410 nm frequency-doubled Ti:Sapphire laser. Manual polarisation controllers (POL) enable alignment of SPDC polarisation with the axes of polarisation-maintaining fibers coupled to the test device. We detect photons in coincidence using avalanche photo diodes (APDs). (b) Representative observed non-classical interference patterns, showing anti-coalescent and coalescent interference for two different output mode combinations as well as fits to the data with Gaussian and sinc components. The error bars shown are calculated from Poissonian counting statistics. (c) Two photon interference results for the best hcz circuit. We compare predictions (left) from the coherently-characterised circuit against measured two-photon interference visibilities (centre). The right panel shows visibilities for the ideal circuit Uhcz; most of the difference between this panel and the other two is due to the deviation in the best circuit from the ideal reflectivity for BS4 and unwanted net phase ϕN, as shown in Fig. 3. For the residuals after subtracting the measured visibilities from the predictions the mean and standard deviation are − 0.002 and 0.061, while after subtracting the measured values from the ideal values they are − 0.022 and 0.282 respectively.
