Quantum microwave photonics (QMWP) is an innovative approach that combines energy–time entangled biphoton sources as the optical carrier with time-correlated single-photon detection for high-speed radio frequency (RF) signal recovery. This groundbreaking method offers unique advantages, such as nonlocal RF signal encoding and robust resistance to dispersion-induced frequency fading. We explore the versatility of processing the quantum microwave photonic signal by utilizing coincidence window selection on the biphoton coincidence distribution. The demonstration includes finely tunable RF phase shifting, flexible multitap transversal filtering (with up to 14 taps), and photonically implemented RF mixing, leveraging the nonlocal RF mapping characteristic of QMWP. These accomplishments significantly enhance the capability of microwave photonic systems in processing ultraweak signals, opening up new possibilities for various applications.
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.
Based on frequency entangled sources and arrival time measurements, a best time synchronization stability result of fewer than 100 fs has been reported over 20km fiber links, verifying the superiority of quantum time synchronization. In this paper, a two-way quantum time transfer over 50 km fiber links with the same frequency standard was implemented, with a short-term stability of 2.63 ps at an averaging time of 18 s and a long-term stability of 79.2 fs at 73700 s. The system accuracy in terms of the 50km fiber transfer length is measured as 524.75 ps, the agreement of which with the theoretical simulation illustrates that improving the spectral consistency of the two entangled sources will significantly improve the accuracy. This result shows that the fiber-optic two-way quantum time transfer can be successfully extended to a metropolitan fiber link distance of around 50 km or longer, not only maintaining the transfer stability well below picosecond but also promising further improvements in synchronization accuracy.
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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