Środowiskowe Seminarium z Informacji i Technologii Kwantowych

Recently, there has been an increased interest in platforms that coherently convert photons from the microwave to the optical domain. This has been mainly motivated by the advent of quantum information technologies because room-temperature photonic links could transfer quantum states between cryogenic superconducting microwave circuits over long distances. While several approaches are under investigation to this end, there is a second potential application of efficient photonic upconverters that is often overlooked: Low-noise detection of weak signals at microwave, millimeter-wave, and THz frequencies by using photodetectors with an upconverter as an intermediary. Even at room temperature, this technique could one day surpass the noise performance of cryogenic low-noise amplifiers (LNAs) and mixers at microwave/THz frequencies, enabling applications in areas such as radio astronomy, space instrumentation, earth observation, etc.

In this talk we will briefly introduce electro-optic modulators based on high-Q lithium niobate whispering-gallery mode resonators serving as upconverters in both lab setups, and photonic integrated circuit platforms. Then, we will discuss coherent Rydberg-atom electrometers exploiting electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting effects to optically detect microwave signals. By acknowledging that most practical microwave engineering applications would require coupling of these atomic receivers to a microwave port rather than free-space, we engineered high-Q field-enhancing microwave cavities. However, a trade-off is found between field enhancement and thermal noise introduced by the cavity due to its physical temperature. This leads to an optimal coupling strength or impedance mismatch that minimizes total noise, analogous to the optimal noise reflection coefficient of the first-stage transistor in an LNA. We will overview the theory behind these findings as well as the preliminary experimental results we have obtained along with a few unexpected challenges. Finally, perspectives will be given on other possible RF and microwave engineering applications currently getting the attention of funding agencies.