Reconfigurable Cryogenic Microwave Devices Using Low Temperature Superconducting rf-SQUIDs

Abstract

In the physical implementation of modern superconducting quantum computing systems, microwaves play a significant role in the control and measurement of qubits. The advances towards the use of microwaves in the quantum signal processing regime have inspired research on a suite of superconducting radio frequency (RF) components such as phase shifters, beam splitters, circulators, isolators, Josephson parametric amplifiers, and kinetic inductance travelling wave amplifiers. Although many room-temperature microwave components are commercially available, but there is a need to develop cryogenic microwave components, considering the challenges involved in the successful delivery of room-temperature generated microwave signals to the qubit chip. For the furtherance of large-scale quantum systems, tremendous efforts are being made to integrate the microwave photon generation, modulation, and routing in the cryogenic conditions adjacent to the qubits. This has created an urgent demand for various superconducting RF components.

In order to tune the phase of on-chip coherent microwave sources in cryogenic environment, a compact, low-loss, fast-tunable, wideband phase shifter is needed. Superconducting phase shifters are also employed in the chip-to-chip quantum network communication with microwave photons and have recently found applications in the future secure communication using far-field microwave quantum technologies. This thesis reports the development of radio frequency superconducting quantum interference devices (rf-SQUIDs) based analog phase shifters for such quantum applications. Each rf-SQUID is a superconducting loop shunted by a Josephson junction (JJ) and a long array of rf-SQUIDs is coupled with a low temperature superconductor (LTS) niobium (Nb) microwave transmission line (TL). The inductance of rf-SQUIDs is dependent on the flux threading the loop, which can be precisely controlled by applying a dc current or RF power or a combination of the two. Since the variable inductance of an array of rf-SQUIDs is tightly coupled the TL, they change the inductance of the TL leading to a phase shift. The issue with this design is that the phase shift is achieved at the expense of changing characteristic impedance of the TL. In order to address this issue, a phase shifter using a reflective-type topology is developed. It utilizes a superconducting hybrid coupler monolithically integrated with two tunable reflective loads. An array of rf-SQUIDs is used to achieve inductive tuning in the reflective loads, resulting in a broadband true time delay phase shift. The inductance tuning using rf-SQUIDs is also demonstrated in a superconducting microwave resonator which offers ultra-wide tuning range of 1.24 GHz at the fundamental resonance at 5.6 GHz. Superconducting tunable resonators have vast potential applications in microwave tunable filters, tunable couplers, tunable parametric amplifiers, SQUID multiplexers, and astrophysical detectors.

This thesis reports first ever implementation of a power dependent cryogenic power limiter based on rf-SQUIDS. The objective is to develop a LTS power limiter which can provide protection against power levels above -15 dBm and can be monolithically integrated with other components on the superconducting chip. The output power increases linearly with the input power up to -15 dBm, and as the input power level increases beyond -15 dBm, the device offers an increasing attenuation to limit the output RF power to -15 dBm. Apart from the quantum measurement systems, cryogenic power limiters find applications in digital RF receivers. Such digital receivers are rapid single flux quantum (RSFQ) based, which cannot handle high power levels. When controlled by using dc current, this power limiter topology can be used as an analog variable attenuator as demonstrated in this thesis. Microwave circulators have found a prominent role in the qubit readout circuitry and are used in conjunction with Josephson parametric amplifiers. The scheme is based on the parametric modulation of three identical, strongly, and symmetrically coupled resonators.

The devices are realized using MIT Lincoln Laboratory SFQ5ee eight-layer niobium-based process which provides a solid technology platform for building superconducting circuits.