The quantum microwave photonics in radio-over-fiber (QMWP-RoF) systems has been recently demonstrated with a time-energy entangled biphoton source as the optical carrier combined with the single-photon detection technique. The results showed that the QMWP-RoF can realize the nonlocal recovery of the RF modulation from the unmodulated signal photons. Moreover, the RF modulation on the dispersed idler photons can be distilled. In this letter, we further investigate the SNRs of the recovered RF modulation as a function of the temporal selection window on the coincidence distribution of the biphotons. According to the investigation, the highest SNR of the nonlocally recovered RF modulation from the non-dispersed signal photons is achieved when the selection temporal width on the photon approaches the FWHM of the coincidence distribution, which may experience broadening due to the dispersion. On the other hand, the highest SNR of the distilled RF modulation from the dispersed idler photons is achieved at a fixed temporal width regardless of the dispersion effect. The results provide a guideline for optimizing the QMWP-RoF system under different dispersive conditions, which can better illustrate the resistance of the dispersion.
The dispersive wavelength to time mapping with the entangled photon source is an effective way of measuring the spectral information of the entangled photon pairs. This approach avoids the usage of spectral filtering equipment like the monochromator, which reduces the measuring time and the system’s complexity. The wavelength-to-time mapping method can be divided into local mapping and nonlocal mapping depending on whether the measurement utilizes the frequency correlation of the entangled photon pairs. For local mapping, the spectral information of signal photons is directly mapped to the time domain through dispersion without utilizing the frequency correlation between the photon pair. For nonlocal mapping, the signal photons with spectral information are directly detected. And the corresponding idler photons are dispersed. With the help of the frequency correlation between the photon pair, the spectral information on signal photons can be recorded in count measurement. In this letter, the two types of mapping results are theoretically and experimentally compared. The theoretical result indicates that the two types of mapping results are the same when the pump light of the entangled source is ideal monochromatic with infinite linewidth. However, when using a real pump light with finite linewidth, the theoretical and experimental result of the two types of mapping is different. The difference in the result indicates the potential influence of the mapping method, which can further help to select a more suitable mapping method for different measuring conditions.
KEYWORDS: Dispersion, Signal processing, Single photon detectors, Modulation, Signal detection, Superconducting detectors, Nanowires, Knowledge management, Time correlated photon counting
A nonlocal wavelength-to-time mapping method based on the entangled photon sources is proposed for nonlocal wavelength measurement. In this approach, the spectrally modulated optical carrier from the signal path is detected the single-photon detector. The dispersive medium is applied on the idler path before the idler photons reaches another SPD. With the help of GVD, the frequency correlation is transformed to the correlation of arrival time, when observed in coincidence count. As a result, the spectral pattern of the signal photon can be nonlocally transformed into temporal shape, even though there is no dispersion in the signal path. A proof-of-concept experiment demonstrates the wavelength measurement around 1560 nm, with a resolution of 0.2 nm.
A new type of single-photon spectrograph combining a tunable optical filter and a dispersive element is presented for measurement of the spectral properties of the two-photon state. In comparison with the previous single-photon spectrograph which is merely based on the dispersive Fourier transformation (DFT) technique, this scheme avoids the need for additional wavelength calibration and the electronic laser trigger for coincidence measurement; therefore, its application is extended to continuous wave (CW) pumped two-photon sources. The achievable precision of the spectrum measurement has also been discussed in theory and demonstrated experimentally with a CW pumped periodically poled lithium niobate (PPLN) waveguide-based spontaneous parametric down conversion photon source. Such a device is expected to be a versatile tool for the characterization of the frequency entangled two-photon state.
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