Quantum communication realizes information-theoretic security using photonic quantum states, for example, quantum secure direct communication (QSDC), which can achieve secure and reliable communication in a channel with both noise and eavesdroppers. However, QSDC suffers from large losses and short communication distances, thus being impractical for applications. Here, we have proposed a one-way quasi-QSDC protocol with single photons. This protocol enables the simultaneous transmission of information and key exchange using the same single photons and is robust against loss and error because it uses error correction and spectrum expansion techniques. In a proof-of-principle demonstration using weak coherent pulses, the system achieved a real-time secure transmission rate of 2.38 kilobits per second over a 104.8-kilometer standard telecommunication fiber, which set world records in both aspects. This system paved the way for the practical application of QSDC and offers a unique method to detect eavesdropping online, which is crucial in certain circumstances.

High-sensitivity gyroscope is widely used for rotation detection in several practical applications. Recently, exceptional points (EPs) have garnered considerable attention for enhancing the sensitivity of sensors based on optical cavities. Here, we propose an EP-enhanced optical gyroscope based on mechanical parity-time (PT) symmetry in a microcavity system. We demonstrate that by pumping the two optical modes with different colors, i.e., blue and red detuning, an effective mechanical PT-symmetric system can be obtained, and the system can be prepared at EP with appropriate parameters. The sensitivity of gyroscope at EP was enhanced by more than one order of magnitude in the weak perturbation regime as compared to that at diabolic point. This indicates that the sensitivity of gyroscope can be effectively enhanced by monitoring the mechanical modes rather than the optical modes. Overall, our work provides a promising approach to design high-sensitivity gyroscopes in optical microcavities and is potentially useful in a variety of research fields including fundamental physics and precision measurement.

The sensitivity of perturbation sensing can be effectively enhanced with higher-order exceptional points due to the nonlinear response to frequency splitting. However, experimental implementation is challenging since all the parameters need to be precisely prepared. The emergence of an exceptional surface (ES) improves the robustness of the system to the external environment, while maintaining the same sensitivity. Here, we propose, to our knowledge, the first scalable protocol for realizing a photonic high-order ES with passive resonators. By adding one or more additional passive resonators in the low-order ES photonic system, the three- or arbitrary N-order ES is constructed and proved to be easily realized in experiment. We show that the sensitivity is enhanced and the experimental demonstration is more resilient against fabrication errors. The additional phase-modulation effect is also investigated.

The highly efficient coupling of light from conventional optical components to optical mode volumes lies in the heart of chip-based micro-devices, which is determined by the mode-matching between propagation constants of fiber taper and the whispering-gallery-mode (WGM) of the resonator. Optical gyroscopes, typically realized as fiber-optic gyroscopes and ring-laser gyroscopes, have been the mainstay in diverse applications such as positioning and inertial sensing. Here, the mode-matching is theoretically analyzed and experimentally verified. We observe the Sagnac effect in a millimeter-scale wedged resonator gyroscope, which has attracted considerable attention and has been rapidly promoted in recent years. We demonstrate a bidirectional pump and probe scheme, which directly measures the frequency beat caused by the Sagnac effect. We establish the linear response between the detected beat frequency and the rotation velocity. The clockwise and counterclockwise rotation can also be distinguished according to the value of the frequency beat. The experimental results verify the feasibility of developing the gyroscope in a WGM resonator system and pave the way for future development.

The correctness of measurement-device-independent quantum key distribution (MDI-QKD) has been demonstrated in tremendous amount of researches. There is an interesting proposal which can collect all coincidences by transforming two pairs of orthogonal polarization states into the encoding states of MDI-QKD. The attempt is meaningful. Here, based on this proposal, we realize a proof-of-principle demonstration. Only one phase modulator is applied to encode the qubits. The phase-reference-frame is aligned by modulating the phase modulator of the phase modulation unit. We modulate the signals into three intensities, and a secure key rate of bits per pulse is successfully demonstrated. Our experimental system simplifies the encoding process and has the potential to implement the reference-frame-independent MDI-QKD protocol.