Precision Metrology with Radiofrequency Quantum Upconverters

While much effort has gone into developing quantum metrology techniques for precision measurement of microwave signals, comparatively little work has focused on signals in the very high frequency (VHF) band between 30-300 MHz and below. This is primarily due to the fact that even at dilution refrigerator temperatures of ~ 10mK, a VHF mode is subject to high thermal excitations (kT >> hf). However, in resonator-based QCD axion searches, thermal noise is suppressed outside of the response bandwidth, so that quantum acceleration increases the search bandwidth. In this talk, we present the Radio-frequency Quantum Upconverter (RQU). The RQU is a quantum sensor capable of phase-sensitive gain, allowing the implementation of quantum metrology techniques in the electromagnetic VHF band. We will discuss the physics principles behind the RQU, i.e. photon upconversion, and describe implementations using Josephson junctions and superconducting circuit elements. We will present initial experimental results of RQU performance, including the demonstration of 30dB of gain contrast in phase sensitive mode.