Continuously Expanding the Response Frequency of Rydberg Atom-Based Microwave Sensor by Using Quantum Mixer

Abstract: Microwave electric (MW) field measurements utilizing Rydberg atoms have witnessed significant advancements, achieving remarkable sensitivity, albeit limited to discrete MW frequencies resonant with Rydberg states. Recently, various continuous-frequency measurement schemes have emerged. However, when the MW detuning surpasses 1 GHz, the sensitivity degrades by over an order of magnitude compared to resonant measurements. In this paper, we successfully extend the response frequency range by harnessing a controlled driving field in conjunction with a quantum mixer and heterodyne technology, theoretically enabling infinite scalability. Notably, second-order effects stemming from quantum mixing necessitate careful consideration to ensure accurate electric field measurements. In addition, compared to resonant measurements, the sensitivity decline for far-detuned MW fields exceeding 1 GHz is less than twice, representing a significant improvement of several orders of magnitude over alternative schemes. Furthermore, the sensitivity of far-detuned MW fields can be efficiently enhanced by augmenting the intensity and frequency of the controlled field. For detunings ranging from 100 MHz to 2 GHz, we present optimal sensitivity values and the corresponding methods to achieve them. Our findings pave the way for Rydberg atom-based MW receivers characterized by both high sensitivity and an exceptionally broad bandwidth.