Free-space optical communication (FSO) can achieve fast, secure, and license-free communication without physical cables, providing a cost-effective, energy-efficient, and flexible solution when fiber connection is unavailable. To achieve FSO on demand, portable FSO devices are essential for flexible and fast deployment, where the key is achieving compact structure and plug-and-play operation. Here, we develop a miniaturized FSO system and realize 9.16 Gbps FSO in a 1 km link, using commercial single-mode-fiber-coupled optical transceiver modules without optical amplification. Fully automatic four-stage acquisition, pointing, and tracking systems are developed, which control the tracking error within 3 μrad, resulting in an average link loss of 13.7 dB. It is the key for removing optical amplification; hence FSO is achieved with direct use of commercial transceiver modules in a bidirectional way. Each FSO device is within an overall size of 45 cm × 40 cm × 35 cm, and 9.5 kg weight, with power consumption of ∼10 W. The optical link up to 4 km is tested with average loss of 18 dB, limited by the foggy test environment. With better weather conditions and optical amplification, longer FSO can be expected. Such a portable and automatic FSO system will produce massive applications of field-deployable high-speed wireless communication in the future.
Here, we demonstrate a field-deployable free-space optical link for quantum key distribution (QKD) between a ground station and radio-controlled electric vehicle (RCEV). Compact and high-performance acquisition, pointing and tracking (APT) system is developed for small tracking error down to 3.8 μrad and average link loss of 16 dB. Such link is within the requirement of normal QKD system. With automatic acquisition and coarse tracking, the APT system can automatically establish the free-space optical link in 10 minutes. It is the first step towards mobile quantum secure communication network. Based on RCEV, the quantum optical link can be reconfigured at different link distances. This result greatly improves the flexibility of the link construction for the future QKD applications.
The dielectric filter is a commonly used optical filter, benefiting from its low production cost and compact size. While the bandwidth of the dielectric filters made by traditional fabrication technique can hardly reach sub-nanometer. Here, we present a method for fabricating a sub-nanometer dielectric filter using the Fabry-Pérot (FP) cavity. This FP dielectric filter features a full width at half maximum (FWHM) bandwidth of ~0.18 nm, a free spectral range (FSR) of 11.23 nm and a peak transmittance of over 80%.
The large-photon-number quantum state is a fundamental but nonresolved request for practical quantum information applications. We propose an N-photon state generation scheme that is feasible and scalable, using lithium niobate on insulator circuits. Such a scheme is based on the integration of a common building block called photon-number doubling unit (PDU) for deterministic single-photon parametric downconversion and upconversion. The PDU relies on a 107-optical-quality-factor resonator and mW-level on-chip power, which is within the current fabrication and experimental limits. N-photon state generation schemes, with cluster and Greenberger–Horne–Zeilinger state as examples, are shown for different quantum tasks.
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