Optimizing Sensitivity of Quantum Interferometers: Mechanisms, Advances, and Future Challenges (Invited)

Significance A quantum interferometer is one of the core tools in the field of quantum precision measurement, combining quantum manipulation technology with interferometric measurement technology to achieve precision measurements beyond classical limits. Leveraging the advantages of quantum resources, this technology demonstrates significant application potential in quantum sensing fields such as displacement measurement, imaging, magnetic field measurement, angular velocity measurement, and electric field measurement. The typical workflow of a quantum interferometer includes quantum state preparation, phase modulation, interferometric measurement, and signal extraction. Based on interferometer structures, they are primarily classified into two types: linear interferometers and nonlinear interferometers. The core principle of linear quantum interferometers is to utilize the linear coherent superposition and interference effects of quantum states for phase measurement. This architecture combines the advantages of classical interferometers in terms of large phase-sensitive particle numbers with quantum advantages and high measurement precision, and has been successfully applied to quantum-enhanced detection of gravitational waves. Unlike linear quantum interferometers, nonlinear quantum interferometers utilize nonlinear interactions to control the interference process of quantum states. Their primary operational mechanisms include nonlinear phase-sensitive amplification and quantum correlation enhancement. As a result, they possess inherent advantages in suppressing nonlinear noise and amplifying signals, demonstrating greater adaptability in terms of environmental robustness. Optimization research on quantum interferometers will break through classical limits, laying the technological foundation for practical applications in the field of quantum sensing. Progress In the field of quantum interferometers, researchers have proposed and realized two configurations: linear interference (Fig. 1) and nonlinear interference (Fig. 2). Additionally, they have innovatively proposed a hybrid interferometer combining light and atoms on the physical carrier of the interference arm (Fig. 3). Due to the inherent fragility and complexity of quantum resources, the challenge of how to fully leverage quantum advantages in lossy environments to enhance the phase sensitivity of quantum interferometers has become a key obstacle to advancing their practical applications. In linear quantum interferometers, researchers have proposed optimal quantum resource allocation schemes (Fig. 5) to address internal losses and optimized quantum amplification compensation schemes (Fig. 6) to address external losses. Both types of schemes can reduce the impact of losses on quantum resources. In nonlinear quantum interferometers, researchers have proposed various optimization schemes, including: an asymmetric gain optimization scheme applicable to both external (Fig. 7) and internal (Fig. 8) losses; coherent feedback optimization (Fig. 9); and photon-level operations (Fig. 10). In the application field of quantum interferometers, typical technological breakthroughs include: LIGO achieving a 3 dB quantum noise advantage in low-frequency (around 50 Hz) gravitational wave detection (Fig. 11); approximately 1.2 dB electric field quantum enhancement measurement being achieved using common-path interference (Fig. 12); achieving a 5.5 dB quantum enhancement measurement of magnetic fields using entangled beams generated via four-wave mixing, with sensitivity reaching 18 fT/(cm ⋅ Hz) at 20 Hz (Fig. 13); achieving approximately 0.95 (°)/h angular rate bias stability by combining entangled photon pairs with a fiber-optic gyroscope (Fig. 14); quantum interference effects being used to suppress four-wave mixing noise in quantum memory (Fig. 15); a photon number quantum non-destructive measurement scheme based on light-atom hybrid interference being proposed (Fig. 16). Additionally, to maintain long-term stability of the quantum state, a multi-parameter dynamic optimization model constructed using deep neural networks is employed to achieve stable quantum state output for 2 h (Fig. 17). Conclusions and Prospects Quantum interferometers surpass the standard quantum limit (SQL) by leveraging quantum state resources, yet their phase sensitivity advantage is highly susceptible to degradation from photon loss and limited detection efficiency. Maintaining this quantum advantage presents a core challenge: achieving stable, high-sensitivity quantum manipulation. This paper systematically reviews research progress in this field. Theoretically, new mechanisms like optimal resource allocation and gain compensation have been developed. Technically, key breakthroughs include high-loss-tolerant designs and noise suppression techniques. Although existing loss mitigation schemes show significant promise, the fundamental problem of absolute sensitivity decaying with loss remains unresolved. Notably, multi-parameter control techniques based on artificial intelligence have effectively enhanced quantum light source stability, offering new pathways to suppress environmental disturbances. Developing intelligent control methods to simultaneously address quantum decoherence and phase instability will be critical for realizing practical quantum precision measurement systems.