Electric double-layer field effect experiments were performed on ultrathin films of La0.325Pr0.3Ca0.375MnO3, which is noted for its micrometer-scale phase separation. A clear change of resistance up to 220% was observed and the characteristic metal-insulator transition temperature Tp was also shifted. The changes of both the resistance and Tp, suggest that the electric field induced not only tuning of the carrier density but also rebalancing of the phase separation states. The change of the charge-ordered insulating phase fraction was estimated to be temperature dependent, and a maximum of 16% was achieved in the phase separation regime. This tuning effect was partially irreversible, which might be due to an oxygen vacancy migration that is driven by the huge applied electric field.
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.
Micro-patterning is considered to be a promising way to analyze phase-separated manganites. We investigate resistance in micro-patterned La0.325Pr0.3Ca0.375MnO3 wires with width of 10 μm, which is comparable to the phase separation scale in this material. A reentrant of insulating state at the metal-insulator temperature Tp is observed and a giant resistance change of over 90% driven by electric field is achieved by suppression of this insulating state. This resistance change is mostly reversible, The I-V characteristics are measured in order to analyze the origin of the giant electroresistance and two possible explanations are proposed.
We study two flux qubits with a parameter coupling scenario. Under the rotating wave approximation, we truncate the 4-dimensional Hilbert space of a coupling flux qubits system to a 2-dimensional subspace spanned by two dressed states ｜01} and ｜10}. In this subspace, we illustrate how to generate an Aharnov Anandan phase, based on which, we can construct a NOT gate （as effective as a C-NOT gate） in this coupling flux qubits system. FinMly, the fidelity of the NOT gate is also calculated in the presence of the simulated classical noise.
Superconducting nanowire single photon detector （SNSPD）, as a new type of superconducting single photon detector （SPD）, has a broad application prospect in quantum communication and other fields. In order to prepare SNSPD with high performance, it is necessary to fabricate a large area of uniform meander nanowires, which is the core of the SNSPD. In this paper, we demonstrate a process of patterning ultra-thin NbN films into meander-type nanowires by using the nano- imprint technology. In this process, a combination of hot embossing nano-imprint lithography （HE-NIL） and ultraviolet nano-imprint lithography （UV-NIL） is used to transfer the meander nanowire structure from the NIL Si hard mold to the NbN film. We have successfully obtained a NbN nanowire device with uniform line width. The critical temperature （Tc） of the superconducting NbN meander nanowires is about 5 K and the critical current （lc） is about 3.5 μA at 2.5 K.
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.
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.
