Abstract A high‐dimensional system, so‐called quNit (‐level quantum system with ), provides several unique advantages, and high‐precision manipulation of arbitrary single‐quNit is demonstrated. In this study, a program to construct arbitrary qubit‐quNit quantum computation up to a global phase, and an efficient construction that requires single‐particle and generalized controlled (GCX) gates in the worst case are proposed. Moreover, by encoding the computing basis in polarization and spatial degrees of freedom of a single‐photon state, the optical architectures to implement the elementary‐gate library GCX, single‐partite rotation in a high‐dimension system are designed using linear optics. The necessary single‐photon polarization state and flexibility of linear optical elements make the quantum circuit feasible with current experimental technologies.

Abstract Nuclear Magnetic Resonance (NMR) sensors, with their potential advantages of miniaturization and high precision, have become one of the best choices for angular velocity measurement sensors in future Inertial Navigation Systems (INS). Under the limitations of magnetic field control technology and external magnetic field disturbances, current technology is difficult to satisfy the magnetic field precision requirements of NMR sensors to attain navigation‐level measurements. Existing methods typically reduce the sensor's dependence on the axial magnetic field accuracy by closed‐loop control of the carrier frequency. However, because of the limitations of the model and control accuracy, the effect of main magnetic field variations on the sensor performance cannot be overlooked. To address this problem, this paper proposes an innovative a method for modeling and compensation the axial magnetic field error of NMR sensors. First, the mechanism influencing of the axial magnetic field change on the system demodulation process is examined, and a physical model describing the axial magnetic field variation and sensor error is established using the Bloch equation; then, the model is simplified based on the principle of ignoring small quantities and function approximation; finally, a compensation experiment of the axial magnetic field error is carried out. Experimental results show that compared with the existing methods, the proposed method achieves a 19.5% improvement in frequency error stability under the condition of axial magnetic field changes. This work provides effective guidance for future sensor magnetic error compensation.

Abstract A novel configuration of dual‐polarization second‐order topological photonic crystal (SOTPC) is presented, which is constructed by drilling 8‐shaped air holes on the dielectric background, and propose a scheme to achieve a wide common bandgap within this SOTPC for both transverse‐magnetic (TM) and transverse‐electric (TE) polarizations. The dual‐polarization topological transport of edge states along three different straight waveguides is demonstrated. Then proposed, for the first time, a T‐shaped SOTPC‐based topological polarization beam splitter (TPBS) and validate the ability of this TPBS to effectively separate TM and TE modes over a broad operating bandwidth. Ultrahigh extinction ratios of 108 dB for TM polarization and 112 dB for TE polarization are simultaneously achieved at the wavelength of 1550 nm. Moreover, it is demonstrated that the polarization beam splitting effect, along with the corresponding performance of the TPBS, exhibits significant robustness against structural defects. Given the excellent features of the proposed TPBS, such as very compact footprint, ultrahigh extinction ratios and wide operating bandwidth, its potential is anticipated as a promising platform for applications in polarization multiplexing photonic integrated circuits and optical communication systems.

Abstract Microwave quantum illumination with entangled pairs of microwave signal and optical idler modes can achieve sub‐optimal performance with joint measurement of the signal and idler modes. Here, a testbed for microwave quantum illumination is proposed with an optical memory simulated with a delay line in the idler mode. It provides the amount of input two‐mode squeezing necessary to compensate for the loss of optical memory while maintaining a quantum advantage over a coherent state. When the memory is lossy, the input two‐mode squeezing has to be higher through high cooperativity in the optical mode. Under the testbed, a single‐mode phase conjugate receiver is proposed consisting of a low‐reflectivity beam splitter, an electro‐optomechanical phase conjugator, and a photon number‐resolving detector. The performance of the newly proposed receiver approaches the sub‐optimal quantum advantage of 3 dB. Furthermore, the receiver achieves the quantum advantage even with an on‐off detection while being robust against the loss of the memory.

The authors declare no conflict of interest.

Abstract This work proposes a novel class of parameterized entanglement measures that are named as ‐concurrence () (), and demonstrates comprehensively that they satisfy all the necessary axiomatic conditions required for an entanglement measure. Furthermore, an analytical formula relating to concurrence for the range of within two‐qubit systems is derived. Additionally, a new polygamy relation of multiqubit quantum entanglement in terms of ‐concurrence of assistance () is proved. However, it fails to obey the monogamy relation, but it is demonstrated that the squared ‐concurrence () does obey a general monogamy relation in an arbitrary ‐qubit mixed state. Utilizing the monogamy properties of , the corresponding multipartite entanglement indicators can be constructed, which can detect all genuine multiqubit entangled states even in the case of ‐tangle vanishes. In addition, for multipartite higher‐dimensional systems, it is illustrated that still has the applicability of the monogamy relation.

Abstract Quantum communication in the terahertz (THz) band is an important technology for next‐generation high‐capacity wireless networks, and inter‐satellite quantum key distribution in low‐earth‐orbit (LEO) is one of the critical research directions. A terahertz continuous‐variable measurement‐device‐independent quantum key distribution (CV‐MDI‐QKD) with photon subtraction for secure inter‐satellite link communications, derive the security bounds for the protocol, is proposed and the performance of the inter‐satellite link in the asymptotic limit, in addition to obtaining a tighter protocol bound by considering finite size effects, is evaluated. Through simulation calculations, the method can maintain high secret key rates and robust security, which will provide a more effective way to build future quantum networks.

Abstract A protocol is proposed for the preparation of high‐fidelity entangled cat states with composite pulses. The physical model contains two Kerr‐nonlinear resonators and a cavity. By properly designing the parameters, each Kerr‐nonlinear resonator is confined in the cat‐state subspace and the entangled cat states can be generated efficiently. Composite two‐photon drives are introduced with multiple amplitudes and frequencies to improve the fidelity of the entangled cat states in the presence of parameter errors. The performance of the protocol is estimated by taking into account the parametric errors and decoherence. Numerical simulation results show that, the protocol is robustness to timing error, detuning error, and decoherence. It is hoped that the protocol may provide a method for preparing stable entangled cat states.