Quantum computers have potential to speed up information processing through the use of quantum algorithms and to enable better understanding of quantum systems through quantum simulation. The work in this thesis addresses experimental challenges in realizing quantum computers based on electron spins bound to donors in silicon or floating on the surface of superfluid helium, both among the most promising platforms for building scalable quantum computers.
Typical operation of an electron spin as a quantum bit (qubit) requires the application of static magnetic fields. Quantum gates can then be implemented using microwaves. One of the challenges that arises in the operation of electron spin qubits is the presence of spurious fluctuations in the static 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.
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. Signals cannot be averaged due to their randomized phases that arise from the different snapshots of magnetic field fluctuations sampled by each experiment. One solution is the use of superconducting microwave resonators that enhance the detection sensitivity of traditional electron spin resonance experiments. 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.
One approach to using electrons floating on superfluid helium as qubits 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.