  We present the RF Quantum Upconverter (RQU) and describe its application to
quantum metrology of electromagnetic modes between dc and the Very High
Frequency band (VHF) ($\lesssim$300MHz). The RQU uses a Josephson
interferometer made up of superconducting loops and Josephson junctions to
implement a parametric interaction between a low-frequency electromagnetic mode
(between dc and VHF) and a mode in the microwave C Band ($\sim$ 5GHz),
analogous to the radiation pressure interaction between electromagnetic and
mechanical modes in cavity optomechanics. We analyze RQU performance with
quantum amplifier theory, and show that the RQU can operate as a
quantum-limited op-amp in this frequency range. It can also use non-classical
measurement protocols equivalent to those used in cavity optomechanics,
including back-action evading (BAE) measurements, sideband cooling, and
two-mode squeezing. These protocols enable experiments using dc--VHF
electromagnetic modes as quantum sensors with sensitivity better than the
Standard Quantum Limit (SQL). We demonstrate signal upconversion from low
frequencies to microwave C band using an RQU and show a phase-sensitive gain
(extinction ratio) of $46.9$\;dB, which is a necessary step towards the
realization of full BAE.
