Making use of acousto-optic Bragg diffraction effect, Acousto-optic modulators (AOMs) are versatile active optical devices that can efficiently change both the frequency and the propagating direction of an optical field in real time, AOMs are widely used in optics and laser technology. In the experiments related with quantum optics and quantum information technology, AOMs have been used in various applications, including the controlling of probe light in quantum memory, observation the beating signal from single photons, generation of a phase locking reference in squeezed vacuum state generation experiment and in photon subtraction based Non-Gaussian state generation experiment.

Currently, the maximum diffraction efficiency of commercially available AOMs is around 85%, and there are also demonstrations of high diffraction efficiency acousto-optic modulators at GHz frequency in laboratory. However, in the design and implementation of acousto-optic modulators, the three parameters of diffraction efficiency, optical (through and collection) efficiency, and driving frequency (bandwidth) are often mutually restricted by factors such as incident beam size, RF driving power and thermal effect. However, in quantum-related technology, any form of loss will eventually be manifested as a decrease in the purity of the quantum state and the performance of the quantum system. In order to solve the above problems, The research group led by Prof. Xiaoying Li from Tianjin University designed and implemented an active phase-control bi-frequency interferometer based on acousto-optic modulators (as shown in Figure1), and proposed the application of the "interferometric enhanced Bragg diffraction effect" that can be achieved by the interferometer in the field of quantum optical experiment and optical quantum information technology. A specific plan has been made, and a preliminary experimental verification has been made. Relevant results are published in Chinese Optics Letters, Volume 22, Issue 2, 2024. Wenqi Li, Qiqi Deng, Xueshi Guo, Xiaoying Li. Acousto-optic modulator-based bi-frequency interferometer for quantum technology[J]. Chinese Optics Letters, 2024, 22(2): 022703.

In this work, two acousto-optic modulators are used as beam splitter and beam combiner respectively. Through reasonable optical path design, the overall optical efficiency of the interferometer device is as high as 95% ±1%, and the interference visibility of beating signal is as high as 99.5% ±0.2%. The phase of the interferometer is locked by the active feedback system, and the arbitrary phase locking can be stably realized in chopped locking mode. In the implementation of the phase locking scheme, the thesis innovatively proposes an introduction method of optical phase modulation: that is to modulate the RF driving signal (with the frequency of 80 MHz) of the acousto-optic modulator used as the beam splitter, the frequency of modulation signal is 200 kHz. On this basis, the thesis further discusses and preliminarily tests the application of the bi-frequency interferometer in quantum optical experiments, including the use of the device to achieve bi-frequency coherent combination, quantum state optical frequency tuning, and quantum state optical switching.

Figure1. Schematic diagram of phased-locked bi-frequency interferometer based on acousto-optic modulators and its application of being a quantum state frequency tuning device

The bi-frequency interferometer can be used in a variety of quantum optical experiments and optical quantum information technology schemes. Its near-perfect interference visibility shows that the overall quantum efficiency based on the device is only limited by the transmittance of the optical device. In view of the increasing maturity of ultra-low-loss optical coating technology, we have reason to believe that the overall system efficiency of this scheme can eventually reach about 98% to 99%. At present, the team has carried out follow-up work based on the device and achieved preliminary results, achieving " Frequency tuning of a squeezed vacuum state using interferometric enhanced Bragg diffraction effect [arXiv:2401.05619 (2024)]".