Research
Measuring penetration depths of nickelate superconductors
Superconductivity in nickelate materials has been sought for many years due to their structural and electronic similarities to cuprates, which suggest that comparing the two may give insight into the origins of high-temperature superconductivity. Superconductivity was finally discovered in infinite-layer nickelates in 2019. I have used mutual inductance measurements to measure their magnetic penetration depth, from which the superfluid density can be extracted. Measuring the superfluid density as a function of temperature gives evidence for the pairing symmetry of the material, which is an important piece of evidence in understanding how superconductivity is mediated in a material and can provide insight into the connection between nickelates and cuprates.
Investigating sources of dephasing in superconducting qubits with a materials approach
While tremendous progress has been made towards creating a quantum computer using superconducting qubits, there remain many important obstacles, including several arising from materials issues. My research focuses on optimizing material deposition and fabrication processes in order to improve coherence times in superconducting qubits. In addition, I am working to understand how different device designs alter the sensitivity to different noise sources. Using both materials characterization techniques and quantum measurements to study devices, I can pinpoint the sources of dephasing, identify pathways to reduce it, and broaden our understanding of two-level systems.
Image credit: Roth, T., Ma, R., Chew, W.C., 2022. IEEE Antennas and Propagation Magazine 2–14.
Measurement of noise affecting semiconductor spin qubits:
Spin qubits in semiconductor quantum dots show promise as quantum information processors due to their fast gate times and potential for scaling. However, interactions with their solid-state environment lead to decoherence, one of their major limiting factors. I used a Bayesian analysis of qubit measurements to study diffusion of the magnetic field arising from nearby nuclear spins. I was able to perform the measurements at a high enough rate that I could feedback on the qubit control to extend coherence times by over an order of magnitude. Such techniques may be used in the future for improved quantum sensing and control. I also worked on a project investigating the frequency dependence of charge noise in the material, finding that it had a 1/f dependence, and that spin qubits form highly sensitive electrometers.
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Coupling spin qubits using superconducting resonators
A second challenge facing spin qubits is how to improve the entangling gates that allow algorithms to be performed. I worked on methods to couple spin qubits using superconducting resonators, which would allow them to interact over far longer length scales than is currently possible, enabling scaling to larger systems. I developed a scheme inspired by the ‘circuit QED’ approaches that have been used with great success in superconducting qubits, but one which was adapted to the specific advantages and limitations of spin qubits. Because spin qubits have lower coupling to voltages compared to superconducting qubits, attaining fast gate speeds requires the use of high-impedance resonators. I worked to create a process for fabricating nanowires of niobium nitride, a high kinetic inductance material, adjacent to the quantum dots, which required careful optimization due to the nearly incompatible requirements for each process. Ultimately, we were able to demonstrate longitudinal coupling between a spin qubit and resonator for the first time.