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

未来的纠错量子计算机将需要每个芯片约106个量子比特。目前世界领先的固态量子比特技术是基于谐振耦合约瑟夫森结。为了将106个量子位放入稀释冰箱中，它们的线性尺寸需要比现有技术小一个数量级。这反过来又要求量子比特在更高的频率下工作，这反过来又要求约瑟夫森结具有比目前高一个数量级的临界电流密度。目前的约瑟夫森器件技术限于0.1至10 kAcm-2范围内的电流密度，因为较高的值需要较薄的势垒（小于3 nm），这对针孔敏感。通过使用分子束外延（MBE）和等离子体辅助原位氧化，我们将能够制作超薄氧化物屏障，使临界电流密度高达100 kAcm-2。这将在短期内为（经典）高速数字逻辑中的约瑟夫森结开辟应用，并在长期内为未来几代高封装密度量子位电路开辟应用。我们正在寻找一个积极的实验家，利用氧化物等离子体辅助分子束外延制备Zn-ZnO-Zn三层膜。原位电子衍射和光学干涉将用于开发超薄氧化物屏障与原子级尖锐的界面。这些将使用透射电子显微镜（TEM）进行表征。三层将在LCN洁净室制作成约瑟夫森器件。临界电流密度将使用在300 mK下的低温传输测量来确定，其中器件均匀性通过测量临界电流对面内磁场的依赖性来表征。临界电流密度的绝对大小和工艺可变性将与通过TEM确定的阻挡特性相关。

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.