Developing Singlet-Triplet Qubits in Gallium Arsenide as a Platform for Quantum Computing

Abstract

There is a huge, worldwide effort to build a quantum computer due to its potential to perform chemical and materials simulations and to break current cryptography schemes. While there has been incredible progress in recent years, each potential platform still faces meaningful obstacles. The singlet-triplet qubit in gallium arsenide, which is based on electric spin, boasts several advantages, including fast single-gate times and readout, long lifetimes, and a clear path to large-scale integration using current semiconductor manufacturing techniques. However, it also suffers from electric and magnetic field noise that limits coherence time and weak two-qubit interactions, which have prevented the construction of multi-qubit systems so far. In this thesis, I will discuss my research measuring and working to improve noise and entangling gates in singlet-triplet qubits and will examine the viability of the singlet-triplet qubit as the building block for a quantum computer. By measuring the dynamics of the electric and magnetic field noise that the singlet-triplet experiences, we can develop an understanding of their origins and how to reduce their impact. The singlet-triplet can be operated rapidly enough to use it as a sensor and directly measure the fluctuating magnetic field arising from the nuclear spins in the semiconductor heterostructure surrounding it. We use Hamiltonian parameter estimation, a technique that maximizes the rate at which we learn information, given the strange properties of quantum measurement, to measure magnetic noise to within 15 μT at a rate of 2 kHz, and we use these measurements to provide real-time feedback to system controls, increasing the coherence time by a factor of 30. Charge noise has much larger high-frequency components than magnetic noise, and thus requires use of dynamical decoupling methods borrowed from NMR to measure it. We measure its power spectrum up to 1 MHz and find that it has a 1/f spectrum. We perform these measurements for several different devices and have begun to determine how our growth and fabrication processes affect dephasing. Placing a high-impedance resonator between two singlet-triplet qubits to mediate their interaction shows promise as way to create long-distance entanglement and to increase the speed of the entangling gate. We analyze this gate theoretically, finding that we expect the fidelity to be over 95%, with a straightforward path to increasing it to above 99% with improved resonator parameters. However, because the singlet-triplet qubit is extremely sensitive to changes in its electrostatic environment, which can be affected both by the resonator’s fabrication process and the presence of the proximal resonator gate, implementing this gate experimentally is challenging. We have developed processes for fabricating narrow, thin, NbN resonators directly adjacent to the double quantum dots that compose singlet-triplet qubits. These processes yield quantum dots that can be tuned into low-noise, stable singlet-triplet qubits, and superconducting resonators with impedances over 1 kOhm.