Phase memory across two single-photon interferometers including wavelength conversion

Spontaneous parametric down-conversion (SPDC) in a nonlinear crystal generates two single photons (signal and idler) with random phases. Thus, no first-order interference between them occurs. However, coherence can be induced in a cascaded setup of two crystals if, e.g., the idler modes of both crystals are aligned to be indistinguishable. Due to the effect of phase memory it is found that the first-order interference of the signal beams can be controlled by the phase delay between the pump beams. Even for pump photon delays much larger than the coherence length of the SPDC photons, the visibility is above $90\%$. The high visibilities reported here prove an almost perfect phase memory effect across the two interferometers for the pump and the signal photon modes.

Figure 1

Experimental setup for observing phase memory in single-photon interference. For parametric down-conversion two 4-mm-long BBO crystals were used. The variable pump path length difference $\Delta x_p$ was realized with a high-resolution delay line in the path of pump beam ${\mathrm{p}}_2$. The path length difference between the two signal beams $\Delta x_s$ was realized by a high-resolution (minimum step size of 10 nm) translation stage. ${\mathrm{D}}_{\mathrm{A}}$ and ${\mathrm{D}}_{\mathrm{B}}$ are fiber-coupled avalanche photodiodes (SPCM-AQRH-13 from PerkinElmer).

Figure 2

(a), (b) Measured coincidence rate $R_{AB}$ between detectors ${\mathrm{D}}_{\mathrm{A}}$ and ${\mathrm{D}}_{\mathrm{B}}$ as a function of the signal path length difference $\Delta x_s$ and (c), (d) measured single count rate $R_{\mathrm{A}}$ of detector ${\mathrm{D}}_{\mathrm{A}}$ as a function of the signal path length difference $\Delta x_s$. (a), (c) Coarse-resolution scan over the entire region where interference occurs and (b), (d) high-resolution scan to visualize the fringe period. The solid curves are the best-fitting sinusoidal functions, resulting in a period of 808 nm.

Figure 3

(a), (b) Measured coincidence count rate $R_{AB}$ between detectors ${\mathrm{D}}_{\mathrm{A}}$ and ${\mathrm{D}}_{\mathrm{B}}$ as a function of the pump path length difference $\Delta x_p$ and (c), (d) measured single count rate $R_{\mathrm{A}}$ of detector ${\mathrm{D}}_{\mathrm{A}}$ as a function of the pump path length difference $\Delta x_p$. (a), (c) Scan over the entire coherence region and (b), (d) close-up to visualize the fringe period. The solid curves are the best-fitting sinusoidal functions, resulting in a period of 355 nm.