We fabricate different-sized Al/AlO_x/Al Josephson junctions by using a simple bridge-free technique, in which only single-layer E-beam resist polymethyl methacrylate（PMMA） is exposed at low accelerate voltage（below 30 kV） and the size of junction can be varied in a large range. Compared with the bridge technique, this fabrication process is very robust because it can avoid collapsing the bridge during fabrication. This makes the bridge-free technique more popular to meet different requirements for Josephson junction devices especially for superconducting quantum bits.
We implemented the superadiabatic population transfer within the nonadiabatic regime in a two-level superconducting qubit system. To realize the superadiabatic procedure, we added an additional term in the Hamiltonian, introducing an auxiliary counter-diabatic field to cancel the nonadiabatic contribution in the evolution. Based on the superadiabatic procedure, we further demonstrated quantum Phase and NOT gates. These operations, which possess both of the fast and robust features, are promising for quantum information processing.
We fabricate high-quality A1/A1Ox/A1 junctions using improved bridge and bridge-free techniques at 30-keV e-beam voltage,in which the length of undercut and the size of junction can be well controlled by the pre-exposure technique.The dose window is 5 times as large as that used in the usual Dolan bridge technique,making this technique much more robust.Similar results,comparable with those achieved using a 100-keV e-beam writer,are obtained,which indicate that the 30-keV e-beam writer could be an economic choice for the superconducting qubit fabrication.
One of the primary origins of the energy relaxation in superconducting qubits is the quasiparticle loss. The quasiparticles can be excited remarkably by infrared radiation. In order to minimize the density of quasiparticle and increase the qubit relaxation time, we design and fabricate the infrared filter and shield for superconducting qubits. In comparison with previous filters and shields, a nonmagnetic dielectric is used as the infrared absorbing material, greatly suppressing the background magnetic fluctuations. The filters can be made to impedance-match with other microwave devices. Using the as-fabricated infrared filter and shield, we increased the relaxation time of a transmon qubit from 519 ns to 1125 ns.
Superconducting coplanar waveguide（CPW） can be widely used as two-dimensional（2 D） resonator, transmission line or feedline, providing an important component for superconducting quantum circuit which is a promising candidate for quantum information processing. Due to the discontinuities and asymmetries in the ground planes, CPW usually exhibits the spurious resonance, which is a common source of decoherence in circuit quantum electrodynamics experiments. To mitigate the spurious resonance, we fabricated superconducting aluminum air-bridges on Nb CPW. The fabricated airbridges are approximately 3 m high and up to 120 m long. Compared with other methods, the fabrication procedures of our air-bridges are simpler, and the air-bridge can withstand strong ultrasound.
We fabricated TiN coplanar waveguides using standard lithography techniques followed by ICP etch. In order to achieve high quality factor, we investigated the film growth by choosing different deposition conditions for various substrates. Quality factors of waveguide resonators were measured at 20 mK in both high and low microwave power limits. An inner quality factor of several million was achieved at high power limit for a predominantly（200）-oriented TiN film which was grown on HF cleaned silicon wafer. A quality factor of larger than one million was achieved at high power limit for TiN film grown on sapphire.
