Objective   Optical interferometry metrology techniques and devices are pillars of modern precision metrology. With the development of laser and light field shaping technologies, their performance has achieved significant improvements across multiple orders of magnitude. However, they are still limited by the wavelength of the light source. Due to the easy absorption and difficult manipulation of extremely short-wavelength optical fields, the resolution of interferometers cannot be infinitely improved by simply reducing the wavelength. "Phase superresolution" refers to the technological means to overcome the limitation imposed by the light source wavelength. Currently, research on phase superresolution mainly focuses on manipulating N-photon entangled states to achieve this goal. However, the extremely high difficulty in preparing and controlling N-photon entangled states, as well as the low efficiency of coincidence counting, renders this approach impractical for actual measurements. Therefore, it is necessary to realize real-time phase superresolution measurements to meet practical application requirements.

  Methods   To overcome these aforementioned cutting-edge challenges, the collaborative team has taken a novel approach of utilizing the modal structure evolution of orbital angular momentum (OAM) coherent states during parametric conversion processes to simulate the behavior of N00N states in SU(2). Consequently, they have achieved a more efficient means of actively preparing multi-photon amplitude signals carrying interferometer arm phase information.

  Results and Discussions   Phase superresolution signal carried by coherent states with an N-fold enhancement (N=4) has been achieved in a single artificial metamaterial crystal using multiple quasi-phase matching in quasi-periodic optical superlattice (Fig.1(a)). By cascading parametric conversions of the superresolution signal, a phase superresolution interference signal with enhanced resolution of up to N=12 has been realized. Remarkably, the signal intensity remains visible to the naked eye, and real-time recording can be achieved with low-cost photodetectors. In the near future, using this scheme with appropriate technical improvement (Geometric phase elements) with N>100, corresponding to an extreme-ultrviolet de Broglie wavelength, is expected to be an attainable goal.

  Conclusions   The preparation of N-photon entangled states typically involves the use of spontaneous parametric down-conversion (SPDC) process to convert the short-wavelength pump light into a photon stream with extremely low efficiency. The target signal is then selected using inefficient photon coincidence counting systems. As a result, the performance of such systems is significantly lower compared to interferometers that directly utilize the pump light source for sensing. In the approach proposed by the collaborative team, spatial modes are employed to encode phase information into the pump light field. By actively constructing multiphoton amplitudes through a strong stimulated parametric process, the signal power loss incurred in cascaded nonlinearities can be regained through phase-sensitive amplification, leading to a significant improvement in system performance. Therefore, their result paves a promising way for the development of practical phase superresolution interferometry techniques and instruments for metrology.