Over the last decade, microwave-optical (M-O) quantum frequency conversion (quantum transduction) has seen rapid progress. The efficient conversion of photons between these two frequencies provides a path for building quantum networks. Most of the effort in this space has been aimed at direct M-O transduction, which requires a relatively high amount of energy per converted bit of information -- limiting the data transfer rate and makes operation at millikelvin temperatures needed for quantum computing hardware difficult.
Recently, a direct connection of quantum processors using microwave rectangular waveguides has also been demonstrated. However, these microwave waveguides suffer from higher losses than optical fibers. Additionally, the complexity of the system increases greatly as it scales, as the entire system needs to be cooled down to millikelvin temperatures.
In this branch of the lab, we are investigating an alternative to the aforementioned approaches - a way to exchange information between microwave-frequency quantum machines by conversion to millimeter-waves (MMW). Millimeter-waves have a frequency from 30 GHz - 300 GHz so they sit in-between microwave and optical bands. Even though these photons have seen widespread utilization in chemistry, astronomy, medicine, and telecommunications, their use in the quantum information sciences has been minimal.
To open up the millimeter-wave band for quantum information science, we consider devices that take advantage of so-called kinetic inductors (KI). Kinetic inductors are superconducting circuit elements that possess a nonlinearity that enables frequency conversion (four-wave mixing). By carefully analyzing the details of the conversion process, we find that it achieves an appealing compromise between the M-O conversion and direct microwave approach: KI-based converters simultaneously require much less energy per converted bit and do not need to be cooled to millikelvin temperatures for operation.
In addition to the KI-based microwave-millimeter wave converter, we are also actively working to build a millimeter-wave to optical converter so that we can pursue a two-stage microwave-optical conversion.