Fabrication of rhenium Josephson junctions

Rhenium is a superconductor with a relatively weak tendency to oxidize, which is advantageous in superconducting quantum circuit and qubit applications. In this work, Re/A1-A1Ox/Re Josephson tunnel junctions were fabricated using a selective film-etching process similar to that developed in Nb trilayer technology. The Re films had a superconducting transition temperature of 4.8 K and a transition width of 0.2 K. The junctions were found to be highly reproducible using the fabrication process and their characteristics had good quality with a low leakage current and showed a superconducting gap of 0.55 meV.     

Measuring Loschmidt echo via Floquet engineering in superconducting circuits

The Loschmidt echo is a useful diagnostic for the perfection of quantum time-reversal process and the sensitivity of quantum evolution to small perturbations. The main challenge for measuring the Loschmidt echo is the time reversal of a quantum evolution. In this work, we demonstrate the measurement of the Loschmidt echo in a superconducting 10-qubit system using Floquet engineering and discuss the imperfection of an initial Bell-state recovery arising from the next-nearest-neighbor (NNN) coupling present in the qubit device. Our results show that the Loschmidt echo is very sensitive to small perturbations during quantum-state evolution, in contrast to the quantities like qubit population that is often considered in the time-reversal experiment. These properties may be employed for the investigation of multiqubit system concerning many-body decoherence and entanglement, etc., especially when devices with reduced or vanishing NNN coupling are used.     

Increasing energy relaxation time of superconducting qubits with nonmagnetic infrared filter and shield

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.     

Fabrication of Al/AlO_x/Al junctions using pre-exposure technique at 30-keV e-beam voltage

We fabricate high-quality A1/A1O_x/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.     

High quality factor superconducting coplanar waveguide fabricated with TiN

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.     

Fabrication of Al/AlOx/Al Josephson junctions and superconducting quantum circuits by shadow evaporation and a dynamic oxidation process

Besides serving as promising candidates for realizing quantum computing,superconducting quantum circuits are one of a few macroscopic physical systems in which fundamental quantum phenomena can be directly demonstrated and tested,giving rise to a vast field of intensive research work both theoretically and experimentally.In this paper we report our work on the fabrication of superconducting quantum circuits,starting from its building blocks:Al/AlOx /Al Josephson junctions.By using electron beam lithography patterning and shadow evaporation,we have fabricated aluminum Josephson junctions with a controllable critical current density(jc) and wide range of junction sizes from 0.01 μm2 up to 1 μm2.We have carried out systematical studies on the oxidation process in fabricating Al/AlOx/Al Josephson junctions suitable for superconducting flux qubits.Furthermore,we have also fabricated superconducting quantum circuits such as superconducting flux qubits and charge-flux qubits.     

弓张式超磁致伸缩材料换能器的设计与分析

为了提高超磁致伸缩换能器的作动行程,以获得足够大的发音强度,并使之满足发音装置体积小、装配零件少的要求,结合超磁致伸缩材料（GMM）的优良特性,提出一种基于三角放大原理的弓张式GMM换能器。该换能器以GMM棒作为驱动元件,通过固定弓张结构的一端,将双向输出转变为单向输出,同时利用柔性铰链结构,进一步增大换能器的位移输出。通过分析换能器的工作原理,计算得到其理论放大倍数为2.73,与所建立的有限元仿真模型计算得到的放大倍数2.8相近。制作了试验样机并搭建了相应的试验系统,得到在1 kHz范围内换能器最大输出位移为15.5 m,与仿真结果14.058 m相近。提出的弓张结构实现了换能器的位移放大,相应的分析方法也较好地反映了换能器的输出特性。     

Spectroscopy and coherent manipulation of single and coupled flux qubits

Measurements of three-junction flux qubits, both single flux qubits and coupled flux qubits, using a coupled direct current superconducting quantum interference device （dc-SQUID） for readout are reported. The measurement procedure is described in detail. We performed spectroscopy measurements and coherent manipulations of the qubit states on a single flux qubit, demonstrating quantum energy levels and Rabi oscillations, with Rabi oscillation decay time TRabi =- 78 ns and energy relaxation time T~ = 315 ns. We found that the value of TRabi depends strongly on the mutual inductance between the qubit and the magnetic coil. We also performed spectroscopy measurements on inductively coupled flux qubits.     

Flux qubit with a large loop size and tunable Josephson junctions

We present the design of a superconducting flux qubit with a large loop inductance. The large loop inductance is desirable for coupling between qubits. The loop is configured into a gradiometer form that could reduce the interference from environmental magnetic noise. A combined Josephson junction, i.e., a DC-SQUID is used to replace the small Josephson junction in the usual 3-JJ (Josephaon junction) flux qubit, leading to a tunable energy gap by using an independent external flux line. We perform numerical calculations to investigate the dependence of the energy gap on qubit parameters such as junction capacitance, critical current, loop inductance, and the ratio of junction energy between small and large junctions in the flux qubit. We suggest a range of values for the parameters.