On the Classical Model for Microwave Induced Escape from a Josephson Washboard Potential

We revisit the interpretation of earlier low temperature experiments on Josephson junctions under the influence of applied microwaves. It was claimed that these experiments unambiguously established a quantum phenomenology with discrete levels in shallow wells of the washboard potential, and macroscopic quantum tunneling. We here apply the previously developed classical theory to a direct comparison with the original experimental observations, and we show that the experimental data can be accurately represented classically. Thus, our analysis questions the necessity of the earlier quantum mechanical interpretation.Comment: 4 pages, one table, three figures. Submitted for publication on December 14, 200

The dynamically asymmetric SQUID: M\"unchhausen effect

We report on a complex zero-temperature decay channel of a classical object in a metastable state coupled to a quantum degree of freedom. This setting can be realized in a dc-SQUID where both Josephson-junctions have identical critical currents I_c but feature strongly asymmetric dynamical parameters; more precisely, selecting both parameters C and 1/R adequately large for one and small for the other junction makes the first junction behave essentially classical but lets quantum effects be present for the second one. The decay process is initiated by the tunneling of the quantum junction, which distorts the trapping potential of the classical junction; the metastable state of the latter then becomes unstable if the distortion is large enough. We present the dynamical phase diagram of this system providing the dependence of this decay channel on the external bias current I and on the coupling strength between the two junctions, determined by the loop inductance L.Comment: 4 pages, 3 figures, to appear in the VORTEX V proceedings in Physica

Subharmonic Gap Structure in Superconductor/Ferromagnet/Superconductor Junctions

The behavior of dc subgap current in magnetic quantum point contact is discussed for the case of low-transparency junction with different tunnel probabilities for spin-up ($D_\uparrow$) and spin-down ($D_\downarrow$) electrons. Due to the presence of Andreev bound states $\pm \epsilon_0$ in the system the positions of subgap electric current steps $eV_n = (\Delta \pm \epsilon_0)/n$ are split at temperature $T \neq 0$ with respect to the nonmagnetic result $eV_n=2\Delta/n$. It is found that under the condition $D_\uparrow \neq D_\downarrow$ the spin current also manifests subgap structure, but only for odd values of $n$. The split steps corresponding to $n=1,2$ in subgap electric and spin currents are analytically calculated and the following steps are described qualitatively.Comment: 4 pages, 1 figure, minor stylistic changes, journal-ref adde

Superconducting Quantum Interference Device Amplifiers with over 27 GHz of Gain-Bandwidth Product Operated in the 4 GHz--8 GHz Frequency Range

We describe the performance of amplifiers in the 4 GHz--8 GHz range using Direct Current Superconducting Quantum Interference Devices(DC SQUIDs) in a lumped element configuration. We have used external impedance transformers to couple power into and out of the DC SQUIDs. By choosing appropriate values for coupling capacitors, resonator lengths and output component values, we have demonstrated useful gains in several frequency ranges with different bandwidths, showing over 27 GHz of power gain-bandwidth product. In this work, we describe our design for the 4 GHz--8 GHz range and present data demonstrating gain, bandwidth, dynamic range, and drift characteristics.Comment: four pages, 5 figure

On Quantum Tunneling in Real Time

A detailed real time description of quantum tunneling in the semiclassical limit is given, using complex classical trajectories. This picture connects naturally with the ideas of post-selection and weak measurement introduced by Aharonov and collaborators. I show that one can precisely identify the {\it complex} classical trajectory which a post-selected tunneling particle has followed, and which dominates the path integral in the limit as Planck's constant $\hbar$ tends to zero. Detailed analytical calculations are presented for tunneling in cubic and quartic potentials. For a long post-selected tunneling time, the imaginary part of the tunneling coordinate is found to achieve very large values just before the particle tunnels. I discuss how the real and imaginary parts of the particle's coordinate may, in principle, be independently measured using weak measurements. It would be very interesting to observe this effect, which would demonstrate the essential role of complex numbers in our closest possible classical description of reality. Extensions to quantum field theory and general relativity are briefly discussed.Comment: 13 pages, 3 figures. Accepted and to appear in New Journal of Physic

Gap soliton dynamics in an optical lattice as a parametrically driven pendulum

A long wavelength optical lattice is generated in a two-level medium by low-frequency contrapropagating beams. Then a short wave length gap soliton generated by evanescent boundary instability (supratransmission) undergoes a dynamics shown to obey the Newton equation of the parametrically driven pendulum, hence presenting extremely rich, possibly chaotic, dynamical behavior. The theory is sustained by numerical simulations and provides an efficient tool to study soliton trajectories

Superconducting Supercomputers and Quantum Computation

Information technologies have been developing at a formidable pace. While miniaturization has been the driver for this in the past decades (Moore\u27s law), the attention is now focusing to the energy consumption. Already a considerable fraction of the worlds\u27 energy use is in information technologies. Also, the on-chip energy dissipation and concomittant high temperatures form a bottleneck in further speeding up processors. For these reasons a great interest exist in the exploration of new computing paradigms. In my presentation, I will introduce two of such paradigms and discuss their current progress and prospects, namely superconducting \u27RSQF\u27 circuitry and quantum-computation. Both technologies require (ultra)-low temperatures, providing interesting challenges for cryogenic engineering

Coherent Quantum Network of Superconducting Qubits as a Highly Sensitive Detector of Microwave Photons for Searching of Galactic Axions

We propose a novel approach to detect a low power microwave signal with a frequency of the order of several GHz based on a coherent collective response of quantum states occurring in a superconducting qubits network (SQN). An SQN composes of a large number of superconducting qubits embedded in a low-dissipative superconducting resonator. Our theory predicts that an SQN interacting with the off-resonance microwave radiation, demonstrates the collective alternating current Stark effect that can be measured even in the limit of single photon counting. A design of the layout of three terminals SQN detectors containing 10 flux qubits weakly coupled to a low-dissipative R-resonator and T-transmission line was developed. The samples were fabricated by Al-based technology with Nb resonator. The SQN detector was tested in terms of microwave measurements of scattering parameters and two-tone spectroscopy. A substantial shift of the frequency position of the transmission coefficient drop induced by a second tone pump signal was observed, and this effect clearly manifests a nonlinear multiphoton interaction between the second-tone microwave pump signal and an array of qubits