Stanford

“From Fundamentals to Applications”

ABSTRACT

The superconducting Radio-frequency Quantum Upconverter (RQU) leverages the flux-dependent inductance of a three-junction interferometer to upconvert low-frequency signals (<300 MHz) to microwave frequencies (4 – 8 GHz), enabling sensitive quantum measurement protocols such as going beyond the Standard Quantum Limit and evading backaction noise. The RQU design incorporates a microwave resonator terminated by a Josephson junction interferometer, which is biased via inductively coupled flux loops. However, these flux bias ports can introduce loss and decoherence through parasitic coupling between the low-frequency flux bias and high-frequency flux signal lines, as well as coupling to parasitic resonance modes. I present a design to isolate the low-frequency bias lines from the flux signal lines and the high-frequency RF circuit. The analysis includes simulation of RQUs in Sonnet with an on-chip low-pass filter circuit, showing effective mitigation of losses to preserve the resonator’s coherence and quality factor. Applications include probing fundamental physics through the detection of axions, a leading candidate for dark matter.