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Operator spreading, often characterized by out-of-time-order correlators (OTOCs), is one of the central concepts in quantum many-body physics. However, measuring OTOCs is experimentally challenging due to the requirement of reversing the time evolution of systems. Here we apply Floquet engineering to investigate operator spreading in a superconducting 10-qubit chain. Floquet engineering provides an effective way to tune the coupling strength between nearby qubits, which is used to demonstrate quantum walks with tunable couplings, reversed time evolution, and the measurement of OTOCs. A clear light-cone-like operator propagation is observed in the system with multiple excitations, and has a nearly equal velocity as the single-particle quantum walk. For the butterfly operator that is nonlocal (local) under the Jordan-Wigner transformation, the OTOCs show distinct behaviors with (without) a signature of information scrambling in the near integrable system., Comment: 6+12 pages, 4 figures

Stimulated Raman adiabatic passage (STIRAP) offers significant advantages for coherent population transfer between un- or weakly-coupled states and has the potential of realizing efficient quantum gate, qubit entanglement, and quantum information transfer. Here we report on the realization of STIRAP in a superconducting phase qutrit - a ladder-type system in which the ground state population is coherently transferred to the second-excited state via the dark state subspace. The result agrees well with the numerical simulation of the master equation, which further demonstrates that with the state-of-the-art superconducting qutrits the transfer efficiency readily exceeds $99\%$ while keeping the population in the first-excited state below $1\%$. We show that population transfer via STIRAP is significantly more robust against variations of the experimental parameters compared to that via the conventional resonant $\pi$ pulse method. Our work opens up a new venue for exploring STIRAP for quantum information processing using the superconducting artificial atoms., Comment: 6 pages, 4 figures

Quantum phase diffusion in a small underdamped Nb/AlO$_x$/Nb junction ($\sim$ 0.4 $\mu$m$^2$) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant., Comment: 4 pages, 4 figures, 1 table

The properties of phase escape in a dc SQUID at 25 mK, which is well below quantum-to-classical crossover temperature $T_{cr}$, in the presence of strong resonant ac driving have been investigated. The SQUID contains two Nb/Al-AlO$_{x} $/Nb tunnel junctions with Josephson inductance much larger than the loop inductance so it can be viewed as a single junction having adjustable critical current. We find that with increasing microwave power $W$ and at certain frequencies $\nu $ and $\nu $/2, the single primary peak in the switching current distribution, \textrm{which is the result of macroscopic quantum tunneling of the phase across the junction}, first shifts toward lower bias current $I$ and then a resonant peak develops. These results are explained by quantum resonant phase escape involving single and two photons with microwave-suppressed potential barrier. As $W$ further increases, the primary peak gradually disappears and the resonant peak grows into a single one while shifting further to lower $I$. At certain $W$, a second resonant peak appears, which can locate at very low $I$ depending on the value of $\nu $. Analysis based on the classical equation of motion shows that such resonant peak can arise from the resonant escape of the phase particle with extremely large oscillation amplitude resulting from bifurcation of the nonlinear system. Our experimental result and theoretical analysis demonstrate that at $T\ll T_{cr}$, escape of the phase particle could be dominated by classical process, such as dynamical bifurcation of nonlinear systems under strong ac driving., Comment: 10 pages, 9 figures, 1 table

We have developed an adiabatic Abelian geometric quantum computation strategy based on the non-degenerate energy eigenstates in (but not limited to) superconducting phase-qubit systems. The fidelity of the designed quantum gate was evaluated in the presence of simulated thermal fluctuation in superconducting phase qubits circuit and was found to be rather robust against the random errors. In addition, it was elucidated that the Berry phase in the designed adiabatic evolution may be detected directly via the quantum state tomography developed for superconducting qubits., Comment: 14 pages, 6 figures

LaTiO$_{3+\delta/2}$, NdTiO$_{3+\delta/2}$, and Nd$_{1-x}$Sr$_{x}$TiO$_{3+\delta/2}$ thin films have been epitaxially grown on (100)SrTiO$_{3}$ and (100)LaAlO$_{3}$ single crystal substrates by using the pulsed laser deposition technique. The oxygen pressure during deposition has been carefully controlled to ensure that the films are lightly doped in the metal side near the metal-Mott-insulator boundary. A steep drop of the effective carrier number at low temperatures has been observed in some of the films, which may correspond to a gradually opening of the Spin Density Wave (SDW) gap due to the antiferromagnetic spin fluctuations. At elevated temperatures, a thermally induced decrease of the Hall coefficient can also be clearly observed. In spite of the fact that these films were prepared from different materials in varied deposition conditions, the temperature dependence of their resistance can all be almost perfectly fitted by a small-polaron coherent conduction model ($R_s(T)=R_s(0)+C\omega_\alpha/\sinh^2(T_\omega/T)$). Careful investigation on the fitting parameters implies that the frequency of the phonon coupled to the electrons may be partially related to the lattice distortion induced by the mismatch strain of the substrates., Comment: 6 pages, 6 figures

e have developed a simple process to obtain large magnetoresistance (MR) in perovskite manganite thin films by a combination of focused ion beam (FIB) milling and 120 keV H$_{2}^{+}$ ion implantation. Metal slits about 70 nm in width were printed by 30 kV focused Ga ion beam nanolithography on a 4 mm track, and the materials in these slits are then irradiated by the accelerated H$_{2}^{+}$ ions. Using this method, in a magnetic field of 5 T we can get a MR${>}$60% over a 230 K temperature scope, with a maximum value of 95% at around 70 K. This technique is very promising in terms of its simplicity and flexibility of fabrication and has potential for high-density integration., Comment: 3 pages, 5 figures

We propose a experimentally feasible scheme to demonstrate the geometric phase in flux qubits by means of detuning coherent microwave pulse techniques. Through measuring the probability of the persistent current state in flux qubits, one can detect the Berry phase that is acquired with system's Hamiltonian adiabatical circular evolution in the parameter space. Furthermore, we show that one should choose an appropriate amplitude of pulses in an experiment to obtain high readout resolution when detuning frequency of pulses is fixed and controlled phase shift gates can be implemented based on the geometric phases by inductance coupling two flux qubits., Comment: 4pages,3figures

Pr$_{1-x}$Ca$_x$Ba$_2$Cu$_3$O$_{7-\delta}$($x\approx$0.5) thin films have been grown on SrTiO$_3$ and YSZ substrates by the pulsed laser ablation. The substrate temperature dependence of orientation and superconducting poperties were systematically studied. Good quality \textit{c} and {a}-axis orientated films can be obtained on SrTiO$_3$ via changing the substrate temperature solely. On YSZ, films with good \textit{c}-axis orientation can be grown, while it is hard to grow films with good \emph{a}-axis orientation by changing substrate temperature alone. The highest $T_{C0}$ is about 37K, which is found in the films grown on YSZ with a good \emph{c}-axis orientation. For the films grown on STO, however, the highest $T_{C0}$ is about 35.6K with a mixed orientation of \emph{c}-axis and \emph{a}-axis. In most of the superconducting films, the weak temperature dependence of the normal state resistivity, as characterized by small $R(290K)/R(50K)\leq2$ ratios, together with a weak localization behavior just above $T_C$ could be attributed to the essential scattering due to the localized electronic states. The superconducting transitions in a field up to 10 T along \textit{c}-axis have been measured on a \textit{c}-axis oriented film grown on SrTiO$_3$. The zero-temperature in-plane upper critical field $B^{ab}_{C2}$(0) is estimated from the resistivity transition data., Comment: 12 pages, 5 figures. To be appear in Supercond. Sci. Technol. 17 (2004)

La2/3Ca1/3MnO3 thin films have been grown on yttria-stabilized zirconia (YSZ) buffered silicon-on-insulator (SOI) substrate by the pulsed laser deposition technique. While full cube-on-cube epitaxy was achieved for the YSZ layer, the top manganite layer was multi-domain-oriented, with a coexistence of cube-on-cube and cube-on-diagonal epitaxy. Due to a combined influence from the magnetocrystalline anisotropy and the magnetoelastic anisotropy, in zero field the local spin orientation varies across the twin boundaries. As a result, a quite large low-field magnetoresistance (LFMR) based on spin-dependent tunnelling was observed. The film shows a resistance change of ~20% in a magnetic field <1000 Oe at 50 K, which is promising for real applications., Comment: 3 pages, 4 figures

Epitaxial Ba0.5Sr0.5TiO3 thin films grown on the (001) LaAlO3 substrates with the ferroelectric transition of about 250K have been investigated by TEM and off-axis electron holography. Cross-sectional TEM observations show that the 350nm-thick Ba0.5Sr0.5TiO3 film has a sharp interface with notable misfit dislocations. Off-axis electron holographic measurements reveal that, at low temperatures, the ferroelectric polarization results in systematic accumulations of negative charges on the interface and positive charges on the film surface, and, at room temperature, certain charges could only accumulate at the interfacial dislocations and other defective areas., Comment: 14 pages, 4figures, submitted to APL

The superconducting and structural properties of a series of Mg1-xAlxB2 samples have been investigated. X-ray diffraction results confirmed the existence of a structural transition associated with the significant change in inter-boron layer distance as reported previously by Slusky et al. Moreover,transmission-electron-microscopy observations revealed the existence of a superstructure with doubled lattice constant along the c-axis direction. We propose that this superstructure is essentially related to the structural transition. The modifications of superconducting transition temperature Tc, the normal state resistivity, and the upper critical field Bc2 by Al doping are discussed in terms of Al-substitution induced changes in the electronic structure at the Fermi energy., Comment: 15 pages, 7 figures

Magnetic measurements on heavily overdoped $La_{2-x}Sr_xCuO_4$, $Tl_2Ba_2CuO_6$, $Bi_2Sr_2CuO_6$ and $Bi_2Sr_2CaCu_2O_8$ single crystals reveal a new type magnetization hysteresis loops characterized by the vanishing of usual central peak near zero field. Since this effect has been observed in various systems with very different structural details, it reflects probably a generic behavior for all high temperature superconductors. This easy penetration of magnetic flux can be understood in the picture of percolative superconductivity due to the inhomogeneous electronic state in heavily overdoped regime., Comment: 4 pages, 5 figures