Oscar W. Kennedy

Superconducting quantum computers leverage superconducting qubits, systems with well-defined energy levels, the spacing of which is determined by the properties of embedded Josephson junctions. Indeterminacy of Josephson junction manufacturing limits large-scale superconducting circuits; however, junction-by-junction tuning affords the possibility of an order of magnitude improvement of determinacy—although the origin of this tuning effect is uncertain. Understanding how tuning techniques may physically modify the tunnel barrier of a Josephson junction is important and will enable these techniques to be optimized. We develop a model of junction tuning based on depinning theory to interpret a phase diagram of the tuning rate. We extract the dependence on temperature, time-varying voltages, and oscillation frequency. Comparing the effect of junction tuning on room-temperature I − V ’s, electrical breakdown, and frequency of higher-energy levels of transmon qubits indicates that the tuning process leaves the barrier thickness unchanged but modifies its composition.

Josephson junctions manufactured to tight tolerances are necessary components for superconducting quantum computing. Developing precise manufacturing techniques for Josephson junctions requires an understanding of their make-up and robust feedback metrics against which to optimize. Here, we consider complementary techniques and assess what conclusions they allow us to draw about the barriers in junctions. Monte Carlo simulations of barriers show that standard deviations of 15–20% of the total barrier thickness are compatible with our experimental data. Electrical breakdown allows us to probe the weakest points in barriers. Narrowing the distribution of this breakdown provides a promising feedback mechanism for barrier optimisation. Grouping junctions by breakdown voltage allows us to identify sub-ensembles of junctions with different median resistance. Transmission electron microscopy can be used to find average barrier thickness, although we highlight challenges in forming robust conclusions on the distribution of thicknesses in a barrier from these experiments.

As in conventional computing, memories for quantum information benefit from high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four weak, coherent microwave pulses in distinct memory modes and retrieving them on demand up to 2 ms later. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds, while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.

We investigated the use of dielectric layers produced by atomic layer deposition (ALD) as an approach to strain mitigation in composite silicon/superconductor devices operating at cryogenic temperatures. We show that the addition of an ALD layer acts to reduce the strain of spins closest to silicon/superconductor interface where strain is highest. We show that appropriately biasing our devices at the hyperfine clock transition of bismuth donors in silicon, we can remove strain broadening and that the addition of ALD layers left T ₂ (or temporal inhomogeneities) unchanged in these natural silicon devices.

We have used a neon focused-ion-beam to fabricate both nanoscale Nb Dayem bridges and NbN phase-slip nanowires located at the short-circuited end of quarter-wavelength coplanar waveguide resonators. The Dayem bridge devices show flux-tunability and intrinsic quality factor exceeding 10,000 at 300 mK up to local fields of at least 60 mT. The NbN nanowires show signatures of incoherent quantum tunnelling of flux at 300 mK.

ZnO nanobelts may grow with their polar axis perpendicular to growth direction. Heterostructured nanobelts therefore contain hetero-interfaces along the polar axis of ZnO where polarisation mismatch may induce electron confinement. These interfaces run along the length of the nanobelts. Such heterostructure nanobelts are grown by molecular beam epitaxy and TEM images confirm the core–shell structure. The effects of shell-growth temperature on nano-heterostructures is investigated using photoluminescence and secondary ion mass spectrometry in a focussed ion-beam microscope with Ne⁺ as the primary ion beam. We perform low temperature photoluminescence on ensembles of such heterostructures and single nanostructures. We show how single nanobelts have photoluminescence spectra rich in features and attribute these to band misalignment at ZnO/ZnMgO interfaces embedded within nano-heterostructures.

Nanowire heterostructures, combining multiple phases within a single nanowire, modify functional properties and offer a platform for novel device development. Here, ZnO/ZnMgO core–shell nanowires are grown by molecular beam epitaxy. At growth temperatures above 750 °C, Mg diffuses into ZnO making heterostructure growth impossible; at lower shell-growth temperatures (500 °C), the core–shell structure is retained. Even very thin ZnMgO shells show increased intensity photoluminescence (PL) across the ZnO band-gap and a suppression in defect-related PL intensity, relative to plain ZnO nanowires. EDX measurements on shell thickness show a correlation between shell thickness and core diameter which is explained by a simple growth model.