Development And Tuning Of Superconducting Josephson Junctions

Superconducting Josephson junctions are macroscopic quantum devices with characteristics such as Josephson effects,energy-level quantization,high nonlinearity,low dissipation,and quantum-limited sensitivity.Therefore,Josephson junctions have attracted considerable interest for their characteristics and a32plications.In traditional superconducting electronics,Josephson junctions are applied to superconducting quantum interference devices,millimeter wave/submillimeter wave generation and detection,Josephson voltage standard,etc.In modern superconducting electronics,they can be applied to quantum computing,quantum precision measurement,quantum information,etc.The parameters of Josephson junctions,such as critical current,normal state resistance,hysteresis parameter,etc.,determine the properties of the Josephson devices.For example,properties of superconducting quantum qubits,such as energy levels and relaxation of quantum states,are related to the superconducting properties of Josephson junctions.In the case of traditional Josephson junctions,once they are prepared,their physical dimensions and various superconducting properties cannot be easily tuned.Therefore,it is important to develop tunable Josephson junctions suitable for different research fields in superconducting quantum.Aiming at applications of modern superconducting electronics,we focus on the fabrication of high-quality Josephson junctions with controllable and tunable transport properties and sensitive microwave response.The in-situ tuning of the quantum properties of the novel Josephson junction is achieved by applying an electric field to adjust the Fermi level of the barrier.The main research results are as follows:1.Al/AlOx/Al Josephson junctions on Si substrates were successfully fabricated with a "bridge-free" method.Typical Rabi oscillation and energy relaxation were observed,indicating the junctions are ready for qubit.We employed a conducting polymer layer instead of a metallic layer to significantly reduce the forward scattering of the electron beam,improved the nanopattern quality,and fabricated high-quality Josephson junctions on sapphire substrates to reduce the quantum dissipation.We also systematically studied and used a cold-development technique to improve the reliability of fabrication processes and the consistency of device properties.2.We developed an in-situ micro-fabrication technique,and used black phosphorus(BP),a new semiconductor material,as the barrier to develop the Nb/BP/Nb Josephson junctions.Typical resistively and capacitively shunted characteristics were observed at liquid helium temperatures.By applying microwave radiation to the devices,a series of Shapiro steps were observed in the I-V curves.The realized method is suitable for the fabrication of devices based on air-sensitive materials such as BP,and can also be useful for Josephson junctions based on other two-dimensional materials.3.We designed a structure of Josephson junction-electric double layer FET,where an external electric field was used to adjust the BP Fermi level by ionic liquid.Without changing the physical structures of the device,we tuned the properties of the Nb/BP/Nb Josephson junction by an in-situ method.The superconducting current had a modulation depth of 61%,and the critical transition temperatures ranged from 5.1 K to 4.0 K.Therefore,the novel and modulable junctions can be used as the core building block to carry out basic and applied researches of superconducting quantum devices.