Spin qubits
Spin qubits in semiconductors are formed by confining small numbers of electrons or holes into nanoscale quantum dots, at which point they effectively act as isolated spins and their quantum state can be controlled by applying magnetic and electric fields. By fabricating multiple dots close enough together that charges can rapidly tunnel between them, we can entangle qubits together or realize more complex qubits containing multiple spins with distinct control techniques and sensitivities to noise.
Spin qubits’ advantage in the quantum computing race comes from their close connection to the transistors used in classical computers, creating the potential for them to be highly scalable to large-scale quantum computers.
Our research centers on investigating the noise arising from the solid state environment spin qubits exist in. One important example is charge noise: electrical fluctuations that slightly shift the qubit energy over time, leading to decoherence. We’ve adapted our cryogenic laser-scanning system to reduce the beam size and improve alignment accuracy, enabling local illumination that excites free charges, making it possible to controllably modify the local charge environment.
Coulomb blockade diamonds in a quantum dot, plotted here as the differential conductance through the dot. Under finite source-drain bias, electrons can only pass through the dot when its electrochemical potential, tuned by the plunger gate voltage, passes into the window determined by the voltage difference between the source and drain.
Questions about noise impacting spin qubits are interesting to us on several levels. First, they can provide actionable guidance on how to mitigate the effects of charge noise, whether by reducing its level through materials engineering or finding techniques to make the qubit less sensitive to it. Second, charge noise is a widespread issue that affects many quantum platforms as well as conventional electronic devices. Spin qubits provide a sensitive probe that can provide insight to how such noise can arise from ensembles of two-level-systems. Finally, studying decoherence naturally connects to fundamental questions in open quantum systems and the interplay between classical and quantum degrees of freedom.
Images of a two-qubit, 6-dot exchange only device, showing a TEM cross section and SEM birds-eye view of the nanoscale gates. Image adapted from: Ha et al., Nano Lett. 22, 1443 (2022)
We also have strong interest in technologies that improve the functionality and scalability for spin qubits, including improving cryogenic readout hardware for rapid measurement and speeding up the tune up and characterization of qubits through automation.
We work with devices from HRL and Intel as part of the Quantum Computing Foundry.