Here we report a breakthrough in the fabrication of a long lifetime transmon qubit. We use tantalum films as the base superconductor. By using a dry etching process, we obtained transmon qubits with a best T1 lifetime of 503 μs. As a comparison, we also fabricated transmon qubits with other popular materials, including niobium and aluminum, under the same design and fabrication processes. After characterizing their coherence properties, we found that qubits prepared with tantalum films have the best performance. Since the dry etching process is stable and highly anisotropic, it is much more suitable for fabricating complex scalable quantum circuits, when compared to wet etching. As a result, the current breakthrough indicates that the dry etching process of tantalum film is a promising approach to fabricate medium- or large-scale superconducting quantum circuits with a much longer lifetime, meeting the requirements for building practical quantum computers.
A dynamical quantum phase transition can occur during time evolution of sudden quenched quantum systems across a phase transition. It corresponds to the nonanalytic behavior at a critical time of the rate function of the quantum state return amplitude, analogous to nonanalyticity of the free energy density at the critical temperature in macroscopic systems. A variety of many-body systems can be represented in momentum space as a spin-1/2 state evolving on the Bloch sphere, where each momentum mode is decoupled and thus can be simulated independently by a single qubit. Here, we report the observation of a dynamical quantum phase transition in a superconducting qubit simulation of the quantum quench dynamics of many-body systems. We take the Ising model with a transverse field as an example for demonstration. In our experiment, the spin state, which is initially polarized longitudinally, evolves based on a Hamiltonian with adjustable parameters depending on the momentum and strength of the transverse magnetic field. The time evolving quantum state is read out by state tomography. Evidence of dynamical quantum phase transitions, such as paths of time evolution states on the Bloch sphere, non-analytic behavior of the dynamical free energy and the emergence of Skyrmion lattice in momentum-time space, is observed. The experimental data agrees well with theoretical and numerical calculations. The experiment demonstrates for the first time explicitly the topological invariant, both topologically trivial and non-trivial, for dynamical quantum phase transitions. Our results show that the quantum phase transitions of this class of many-body systems can be simulated successfully with a single qubit by varying certain control parameters over the corresponding momentum range. * schen@iphy.ac.cn † dzheng@iphy.ac.cn ‡ hfan@iphy.ac.cn curring at critical temperature. The DQPT is intimately related to quantum phase transitions in many-body systems [20][21][22][24][25][26][27][28][29][30][31][32][33][34][35][36].Recently, experimental explorations of DQPT have been performed in ion-trap systems [3,5] and cold atom systems [4,6] with dozens of individual addressable qubits or a cloud of fermionic atoms. Our experiment follows the DQPT simulation approach by emulating a corresponding two-band model separately for each momentum mode with a single qubit. By ranging over the Brillouin zone of momentum space, the results are equivalent to that of simulating manybody systems in space. The finite size effect can be observed for a finite number of momenta implemented experimentally. Our experimental system consists of superconducting Xmon qubits, which is one of the most promising platforms for quantum simulation and quantum computation [37][38][39][40][41]. We provide concrete evidence that the DQPT is successfully simulated. In particular, we demonstrate experimentally the topological invariant in DQPT, which was studied recently in Refs. [24,33], and have obtained quantitatively the dynamical free energy and Skyrmion lattice.
Nanoimprint lithography (NIL) is an attractive nonconventional lithographic technique in the fabrication of superconducting nanowires for superconducting nanowire single-photon detectors (SNSPDs) with large effective detection areas or multi-element devices consisting of hundreds of SNSPDs, due to its low cost and high throughput. In this work, NIL was used to pattern superconducting nanowires with meander-type structures based on ultra-thin (~4 nm) Nb films deposited by DC-magnetron sputtering at room temperature. A combination of thermal-NIL and UV-NIL was exploited to transfer the meander pattern from the imprint hard mold to Nb films. The hard mold based on Si wafer was defined by e-beam lithography (EBL), which was almost nonexpendable due to the application of IPS ® as a soft mold to transfer the pattern to the imprint resist in the NIL process.The specimens fabricated by NIL keep good superconducting properties which are comparable to that by conventional EBL process.
The superconducting transition temperature and grain size of dc sputtered Nb films are systematically investigated. The results show that the superconductivity is closely related to the grain size, rather than to the scattering strength of electrons or the surface layer proximity effect of the films. dc sputtering, superconductivity, Nb film, grain size effect, localization effect, proximity effect It almost becomes a common sense that the superconductivity of a thin film is progressively destroyed when the thickness of the film reduces to well below the coherence length, yet the underlying mechanism of the phenomenon is still a standing problem, in spite of the continuous experimental efforts on different systems [1] . In the present work, a series of dc sputtered high quality Nb films are investigated. The results show a close correlation between the superconducting transition and the grain size of the films.Nb films are easily contaminated by reactive gases, especially oxygen, during the depositing process. Two special measures are taken to ensure the high quality of the samples. First, the magnetron sputtering system is prevacuumed to better than 2×10 −7 Pa. Then the pump is switched off and a built-in Non-Evaporable Getter (NEG) 1) is activated. Thanks to the very low leakage of the system and the usefulness of NEG in selective absorption of oxygen, the system can be kept in a nearly "oxygen free" situation in the whole sputtering process. Secondly, a "static" Ar atmosphere is established for sputtering. In contrast to the usual way where a flowing gas is used, the present method is proved to be useful in avoiding contamination by flowing gas and getting a stable sputtering rate. Experiments and resultsAn Nb target with a purity of 99.999% is used. The film substrates are high-polished Si covered by a SiO 2 layer of about 500 nm. The substrates are precleaned in an ultrasonic cleaner by sequentially using acetone, alcohol and distilled water. Before sputtering, the substrates are further baked in-situ to 150℃ for about 7-8 h to degas the surfaces.Superconducting transition temperature, normal state resistivity and its temperature dependence, and XRD are measured for Nb films of thicknesses from 740 to 2.5 nm. The high quality of the samples is demonstrated by comparing their resistivities with MBE films [2] and sputtering samples in UHV base vacuum [3,4] (Figure 1). We see that the resistivities of all these samples can roughly be normalized to follow a single thickness dependence, except for the data of the e-beam films reported in ref. [5]. The superconducting transition width ΔT c gives the information of how the homogeneity of the film is. The results are plotted in Figure 2. In most cases, ΔT c of our samples are narrower than that of the MBE samples [2] .
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