Electromagnetic sensing below the Standard Quantum Limit: 3 kHz to 300 MHz

We present the use of Radio-frequency Quantum Upconverters (RQUs) for quantum metrology. RQUs utilize Josephson-junction interferometers to couple a low-frequency resonator to a microwave frequency resonator, upconverting radio-frequency signals into the microwave band. Thus, they are useful for applications in precision sensing of electromagnetic modes in LC resonators in the radio-frequency band (3kHz-300MHz), such as axion dark matter searches. The coupling between the low and high frequency resonators is described by the optomechanical Hamiltonian, and quantum techniques including backaction evasion can be applied to achieve noise levels below the standard quantum limit (SQL). Here, we describe the design of RQUs based on 1-, 3-, and 9 Josephson-junctions. The designs with 3 and 9 junctions allow for improved isolation, wider tunability, and improved noise performance compared to current single junction devices. We present a scheme for backaction evasion that will enable sub-SQL amplification in these next generation RQUs.