Superconducting kinetic inductance devices for pulsed electron spin resonance and transport of electrons on shallow superfluid helium

Abstract

Quantum computers have potential to speed up information processing through the use of quantum algorithms and to enable better understanding of quantum mechanical systems through quantum simulation. One of the most natural physical systems that is capable of realizing this potential is the spin of an electron, which is a naturally occurring two-level system. Quantum computers based on electron spins bound to donors in silicon or floating on the surface of superfluid helium constitute two promising platforms for building scalable quantum computers. This thesis addresses experimental challenges in both platforms.

Typical operation of an electron spin qubit requires the application of DC magnetic fields that result in a Zeeman splitting between the two levels. Quantum gates can then be implemented using microwave electric or magnetic fields. One of the challenges that arises in the operation of electron spin qubits is the presence of spurious fluctuations in the Zeeman magnetic field. These small magnetic field fluctuations (few parts-per-billion) result in a loss of quantum control of the electron spin qubit when they accumulate for hundreds of microseconds. This thesis presents experimental methods that can be used to track and compensate for magnetic field fluctuations to enable quantum control of electron spin qubits for timescales beyond few hundred microseconds. In particular, we demonstrate a promising method for creating microwaves that are locked to the magnetic field environment of the electron spin qubit, and design novel superconducting devices that are capable of sensing small magnetic field fluctuations within large background magnetic fields by taking advantage of the kinetic inductance of the superconductor NbTiN.

Another challenge that is presented by magnetic field fluctuations is the inability to do repeatable electron spin resonance experiments at timescales beyond few hundred microseconds. In the absence of single-shot sensitivity, signals cannot be averaged due to their randomized phases that arise from the different snapshots of magnetic field fluctuations that are sampled by each experiment. This difficulty can be overcome by the use of superconducting microwave resonators that enhance the detection sensitivity of traditional electron spin resonance experiments with a small mode volume. This thesis presents the first superconducting microwave resonators that can be used to address two spin species with different transition frequencies by tuning the resonator’s frequency dynamically during the experiments.

In order to use the spins of electrons floating on superfluid helium as qubits, a fundamental challenge is to fabricate quantum dots underneath the helium that can be used to isolate individual electrons. This is experimentally challenging because it requires the patterning of extremely smooth metallic electrodes beneath a shallow layer of superfluid helium that enables high mobility transport of the electrons. This thesis presents a solution to this experimental challenge by using thin films of amorphous metals which exhibit smooth surfaces and homogenous work functions for patterning the gate electrodes.