Josephson junctions with high critical current density grown by molecular beam epitaxy

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Future error-corrected quantum computers will need ~106 qubits per chip. The current world-leading solid-state qubit technology is based on resonator-coupled Josephson junctions. To fit 106 qubits inside a dilution fridge their linear dimensions will need to be an order of magnitude smaller than current technology. This in turn requires the qubit to operate at higher frequencies, which in turn requires the Josephson junctions to have an order of magnitude higher critical current density than at present. Current Josephson device technology is limited to current densities in the 0.1 to 10 kAcm-2 range since higher values necessitate thinner barriers (less than 3 nm) which are susceptible to pinholes. By using molecular-beam epitaxy (MBE) and plasma-assisted in situ oxidation we will be able to make ultra-thin oxide barriers, enabling critical current densities up to 100 kAcm-2. This will open up applications in the short term for Josephson junctions in (classical) high-speed digital logic and in the longer term for future-generations of high-packing-density qubit circuits. We are looking for a motivated experimentalist to fabricate Zn-ZnO-Zn trilayers using the oxide-plasma-assisted MBE. In-situ electron diffraction and optical interferometry will be used to develop ultra-thin oxide barriers with atomically-sharp interfaces. These will be characterised using transmission electron microscopy (TEM). The trilayers will be fabricated into Josephson devices at the LCN cleanroom. The critical current density will be determined using low-temperature transport measurements at 300 mK, with device uniformity characterised by measurements of the dependence of the critical current on an in-plane magnetic field. The absolute magnitude and the process variability of the critical current density will be correlated with the barrier properties as determined by TEM.