Probing single electrons across 300 mm spin qubit wafers

Building a fault-tolerant quantum computer will require vast numbers of physical qubits. For qubit technologies based on solid state electronic devices, integrating millions of qubits in a single processor will require device fabrication to reach a scale comparable to that of the modern CMOS industry. Equally importantly, the scale of cryogenic device testing must keep pace to enable efficient device screening and to improve statistical metrics like qubit yield and process variation. Spin qubits have shown impressive control fidelities but have historically been challenged by yield and process variation. In this work, we present a testing process using a cryogenic 300 mm wafer prober to collect high-volume data on the performance of industry-manufactured spin qubit devices at 1.6 K. This testing method provides fast feedback to enable optimization of the CMOS-compatible fabrication process, leading to high yield and low process variation. Using this system, we automate measurements of the operating point of spin qubits and probe the transitions of single electrons across full wafers. We analyze the random variation in single-electron operating voltages and find that this fabrication process leads to low levels of disorder at the 300 mm scale. Together these results demonstrate the advances that can be achieved through the application of CMOS industry techniques to the fabrication and measurement of spin qubits.Comment: 15 pages, 4 figures, 7 extended data figure

Interplay between Ferroelasticity, Electron Pairing, and Magnetism in LaAlO3/SrTiO3 Nanostructures

The LaAlO3/SrTiO3 interface exhibits a wide range of rich physical properties including superconductivity, electron pairing without superconductivity, magnetism, and ferroelasticity. However, the microscopic origin of the low temperature transport properties is not well understood. A long-standing question involves the unknown interrelations between structural distortions, electron pairing, and low temperature magnetotransport properties at the LaAlO3/SrTiO3 interface. In this thesis, we delineate experimental signatures of interplay between ferroelastic domains, preformed electron pairs, magnetism and electronic nematicity in LaAlO3/SrTiO3 nanostructures. Low-temperature magnetotransport measurements on quasi-one dimensional (1D), cross-shaped electron waveguides, “nanocrosses” are reported. The nanocross devices exhibit quantized ballistic transport of electron and electron pairs and serves as a building block for understanding the 1D electron physics at the LaAlO3/SrTiO3 interface. First, a highly reproducible inhomogeneous energy landscape across the nanocrosses is reported, and ferroelastic domain models are provided to explain the observed inhomogeneity. Second, Hall measurements across the nanocross devices show an abrupt change in slope of the Hall resistance which is directly correlated to the depairing and subsequent spin polarization of preformed electron pairs. Angle-dependent measurements of nonlinear Hall effect also reveals an onset of electronic nematicity at high magnetic fields that again coincides with the electron pairing transition, strengthening the connection between the two phenomena. Potential correlations between ferroelastic domains and preformed electron pairs at the LaAlO3/SrTiO3 interface is further discussed. Finally, results indicating an unconventional electromechanical gating mechanism at the interface is presented. The results reported in this thesis highlights the significance of preformed pairs in the overall phase diagram of LaAlO3/SrTiO3 heterostructures. Functional interrelations between ferroelastic domains, electron pairing, and unusual transport signatures such as nonlinear Hall effect and electronic nematicity at the interface is explored. The given results provide a step forward in understanding the pairing mechanism and the rich correlated electron physics at the LaAlO3/SrTiO3 interface

Electron pairing and nematicity in LaAlO3/SrTiO3 nanostructures

Abstract Strongly correlated electronic systems exhibit a wealth of unconventional behavior stemming from strong electron-electron interactions. The LaAlO3/SrTiO3 (LAO/STO) heterostructure supports rich and varied low-temperature transport characteristics including low-density superconductivity, and electron pairing without superconductivity for which the microscopic origins is still not understood. LAO/STO also exhibits inexplicable signatures of electronic nematicity via nonlinear and anomalous Hall effects. Nanoscale control over the conductivity of the LAO/STO interface enables mesoscopic experiments that can probe these effects and address their microscopic origins. Here we report a direct correlation between electron pairing without superconductivity, anomalous Hall effect and electronic nematicity in quasi-1D ballistic nanoscale LAO/STO Hall crosses. The characteristic magnetic field at which the Hall coefficient changes directly coincides with the depairing of non-superconducting pairs showing a strong correlation between the two distinct phenomena. Angle-dependent Hall measurements further reveal an onset of electronic nematicity that again coincides with the electron pairing transition, unveiling a rotational symmetry breaking due to the transition from paired to unpaired phases at the interface. The results presented here highlights the influence of preformed electron pairs on the transport properties of LAO/STO and provide evidence of the elusive pairing “glue” that gives rise to electron pairing in SrTiO3-based systems