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Quantum-inspired imaging techniques have been proven to be effective for LiDAR with the advances of single photon detectors and computational algorithms. However, the quantum-limited performance is still far from its ultimate limit set by the quantum fluctuations of signal and noise photons. In this work, we propose and demonstrate LiDAR from the detection perspective for approaching quantum-limited performance. A photon-number-resolving detector is introduced to accurately reconstruct the echo signals in a wide photon-flux range and an active photon number filter is further developed to overcome the heavy background noise. The Fisher information of this LiDAR based on a coherent source is only 0.04 dB below the quantum limit in intensity detection when mean signal photon number is 10. An improvement of 41.17 dB in the signal-to-background ratio is achieved with the proposed LiDAR in daytime, and the accuracy of intensity estimation is consistent with the theoretical prediction, which helps to improve the quality of reconstructed images. This work provides a fundamental strategy for constructing a noise tolerant LiDAR approaching quantum-limited precision.

Photon number resolving (PNR) is an important capacity for detectors working in quantum and classical applications. Although a conventional superconducting nanowire single-photon detector (SNSPD) is not a PNR detector, by arranging nanowires in a series array and multiplexing photons over space, such series PNR-SNSPD can gain quasi-PNR capacity. However, the accuracy and maximum resolved photon number are both limited by the signal-to-noise (SNR) ratio of the output pulses. Here, we introduce a matched filter, which is an optimal filter in terms of SNR for SNSPD pulses. Experimentally, compared to conventional readout using a room-temperature amplifier, the normalized spacing between pulse amplitudes from adjacent photon number detections increased by a maximum factor of 2.1 after the matched filter. Combining with a cryogenic amplifier to increase SNR further, such spacing increased by a maximum factor of 5.3. In contrast to a low pass filter, the matched filter gave better SNRs while maintaining good timing jitters. Minimum timing jitter of 55 ps was obtained experimentally. Our results suggest that the matched filter is a useful tool for improving the performance of the series PNR-SNSPD and the maximum resolved photon number can be expected to reach 65 or even large.

Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses. Light: Science & Applications (2015) 4, e253; doi:10.1038/lsa.2015.26; published online 27 February 2015

In the title complex, 2-methyl-1-(4-nitrobenzyl)pyridinium bis(1,2-dicyanoethene-1,2-dithiolato)nickelate(III), (C(13)H(13)N(2)O(2))[Ni(C(4)N(2)S(2))(2)], the most prominent general structural feature of the complex is the completely segregated columnar stacks of anions and cations. Within the cation column, there may be stacking interactions between adjacent nitro groups and benzene rings.