SQUID-Based Broadband Microwave Isolator
As superconducting quantum processors grow in size and complexity, so must the peripheral hardware required for the control and readout of such processors. One singular piece of hardware common to superconducting quantum processors setups is the microwave isolator. Current microwave isolator technology can be generally understood in the context of breaking time-reversal symmetry. In general, to achieve this symmetry breaking, a ferrite material is utilized. While generally exhibiting wide (> GHz) bandwidths and large (>20 dB) directionality, these ferrite-based devices are physically large with volumes exceeding 15 cm^3. Relying on ferrite materials, these devices can also introduce uncontrolled magnetic fields at or near the quantum processor resulting in deleterious effects such as frequency shifts, excess flux noise, or flux vortex formation. At the scale required of quantum processors to achieve quantum advantage, a replacement must be found. As an alternative we describe a circuit level broadband microwave isolator utilizing DC-SQUIDs. Employing a bandpass filter topology, we replace a portion of the shunt inductance of every filter pole with an equivalent DC-SQUID. With appropriate application of DC and microwave flux drives to each of the DC-SQUIDs, power at the signal frequency travelling in one direction remains at the signal frequency while power flowing in the opposite direction is dispersed amongst one or more sidebands stemming from 3-wave mixing processes. To demonstrate the feasibility of such devices in replacing current isolator technology, we will present measured data on a variety of nanofabricated devices The data shows excellent model-hardware correlation with directionality greater than 15 dB for bandwidths approaching 700 MHz with minimal insertion loss. We will also describe how such devices can also be used for directional amplification.