Fig. 1. (a) Scheme of a classical n-tap transversal MWP filter. (b) Scheme of a QMWP transversal filter. Insets (I)–(III) depict the working principle of the QMWP signal processing based on the coincidence window selection technique.

Fig. 2. (a) Different window displacements in the biphoton coincidence distribution. (b), (c) The reconstructed waveforms from the signal and idler photons based on the coincidence window selection technique. Their center deviations are, respectively: (b1), (c1) γ=0; (b2), (c2) γ=120ps; (b3), (c3) γ=240ps; and (d) the experimentally acquired amplitudes of the recovered RF signals from the signal photon path as a function of the center deviation. For all the results, the selected windows have a width of 48 ps. The GDD of the DCM is chosen as 495ps/nm and the RF modulation is at 2.08 GHz. (e) The extracted phase shift as a function of the window displacement. Three modulation frequencies at 0.2, 4.1, and 6.1 GHz are investigated and shown by black squares, orange dots, and purple triangles, respectively.

Fig. 3. Illustration of the three-tap transversal filter based on applying three selection windows to the biphoton coincidence measurement. (a) The displacements of the three windows are set as −240, 0, and 240 ps, with their widths being identical to 48 ps. (a1)–(a3) Reconfigurable MSLR of the three-tap filter is investigated via setting the weights of the three windows at 0.56:1:0.56, 0.75:1:0.75, and 1:1:1, respectively. (b1)–(b3) The FSR tunability of the three-tap filter is investigated while the spacing between the windows is chosen as 240, 160, and 120 ps, and the weight of the three windows is fixed at 0.56:1:0.56. (c1)–(c3) Illustration of the multitap filter by increasing the number of selection windows four-tap, five-tap, and seven-tap, respectively. (d) Dependence of the MSLR on the weight ratio. (e) The relationship between FSR and the window spacing value. (f) The theoretically achieved maximum tap number as a function of biphoton coincidence width.

Fig. 4. Fourier spectra of the RF-modulated signal photons at 5 GHz (a1) and the RF-modulated idler photons at 1 GHz (a2). Fourier spectra of the signal photon waveform (b1) and the idler photon waveform (b2) after the nonlocal RF signal mapping. (c1) The frequency response of the designed two-tap filter. (c2) Fourier spectrum of the idler photon waveform after the two-tap frequency filtering manipulation. (d1) The frequency response of a designed digital filter designed based on the Chebyshev algorithm. (d2) Fourier spectrum of the idler photon waveform after the digital-filtering manipulation.

Fig. 5. Experimental setup. EOM, electro-optic modulator; DCM, dispersion compensation module; SNSPD, superconductive nanowire single-photon detector; RF signal, radio-frequency signal; and TCSPC, time-correlated single-photon counting.

Fig. 6. (a) Different window displacements in the biphoton coincidence distribution. (b), (c) The reconstructed waveforms from the signal and idler photons based on the coincidence window selection technique. Their center deviations are, respectively: (b1), (c1) γ=0ps; (b2), (c2) γ=200ps; and (b3), (c3) γ=400ps. (d) The experimentally acquired amplitudes of the recovered RF signals from the signal photon path as a function of the center deviation. For all the results, the selected windows have a width of 48 ps. The GDD of the DCM is chosen as 495ps/nm, and the RF modulation is at 0.81 GHz. (e) The extracted phase shift as a function of the window displacement. Three modulation frequencies at 0.2, 2.5, and 4.9 GHz are investigated and shown by black squares, orange dots, and purple triangles, respectively.