Voltage-controlled phase slips for superconducting quantum devices

Quantum computers promise to execute certain tasks exponentially faster than classical computers. A fundamental challenge for upscaling quantum computers is the volatility of their building blocks—namely, qubits. For example, superconducting qubits require an external magnetic flux bias to tune their frequency, making them prone to dephasing via low-frequency flux noise. This thesis proposes an alternative platform for superconducting qubits that can be tuned electronically based on voltage-biased superconducting loops.

Today, superconducting qubits are based on Josephson junctions. Interrupting a superconducting loop with such a junction couples its discrete flux states and yields a flux qubit. Similarly, interrupting the loop with a nanowire weak link couples its flux states. This phenomenon is known as coherent quantum phase slips and has been harnessed to build phase-slip flux qubits. Nonetheless, specifying the frequency of such qubits entails complete control over the size and properties of the nanowire, which renders their fabrication challenging.

This thesis examines inducing the weak link electronically in uninterrupted superconducting rings to alleviate the strict fabrication requirements while enabling electronic tunability of the coupling of the flux states of the ring. Specifically, I evaluate the effect of the bias voltage on deterministic and quantum phase slips. Solving the time-dependent Ginzburg-Landau equations, I show that the bias voltage controls the free-energy barrier governing the dynamics of these phase slips. I accordingly propose two novel devices. First, I present a scalable superconducting memory whose state is stored and retrieved via picosecond voltage pulses. Superconducting memories are an essential ingredient for quantum computers owing to their compatibility with cryogenic working temperatures. Second, I propose a phase-slip flux qubit that is tunable by bias voltage and immune to fluctuations smaller than the coherence length of the superconductor. This design is therefore a promising candidate for scalable phase-slip flux qubits. As with other weak links, the work presented in this thesis suggests a route towards new superconducting quantum devices.