Controlling Josephson transport by manipulation of Andreev levels in ballistic mesoscopic junctions

We discuss how to control dc Josephson current by influencing the structure and nonequilibrium population of Andreev levels via external electrostatic gates, external current injection and electromagnetic radiation. In particular we will consider the "giant" Josephson current in "long" SIS tunnel junctions and the regular and anomalous nonequilibrium Josephson currents in three terminal SNS junctions. We will briefly discuss applications to the Josephson field effect transistor (JOFET) and to the newly invented Josephson interference transistor (JOINT).Comment: 10 pages, 3 figures; contribution to a special volume of Superlattices and Microstructures journal (ed. P.F. Bagwell

Toward bio-inspired information processing with networks of nano-scale switching elements

Unconventional computing explores multi-scale platforms connecting molecular-scale devices into networks for the development of scalable neuromorphic architectures, often based on new materials and components with new functionalities. We review some work investigating the functionalities of locally connected networks of different types of switching elements as computational substrates. In particular, we discuss reservoir computing with networks of nonlinear nanoscale components. In usual neuromorphic paradigms, the network synaptic weights are adjusted as a result of a training/learning process. In reservoir computing, the non-linear network acts as a dynamical system mixing and spreading the input signals over a large state space, and only a readout layer is trained. We illustrate the most important concepts with a few examples, featuring memristor networks with time-dependent and history dependent resistances

Superconducting Quantum Circuits, Qubits and Computing

This paper gives an introduction to the physics and principles of operation of quantized superconducting electrical circuits for quantum information processing.Comment: 59 pages 68 figures. Prepared for Handbook of Theoretical and Computational Nanotechnolog

Selective coupling of superconducting qubits via tunable stripline cavity

We theoretically investigate selective coupling of superconducting charge qubits mediated by a superconducting stripline cavity with a tunable resonance frequency. The frequency control is provided by a flux biased dc-SQUID attached to the cavity. Selective entanglement of the qubit states is achieved by sweeping the cavity frequency through the qubit-cavity resonances. The circuit is scalable, and allows to keep the qubits at their optimal points with respect to decoherence during the whole operation. We derive an effective quantum Hamiltonian for the basic, two-qubit-cavity system, and analyze appropriate circuit parameters. We present a protocol for performing Bell inequality measurements, and discuss a composite pulse sequence generating a universal control-phase gate

Scattering theory of superconductive tunneling in quantum junctions

We present a consistent theory of superconductive tunneling in single-mode junctions within a scattering formulation of Bogoliubov-de Gennes quantum mechanics. Both dc Josephson effect and dc quasiparticle transport in voltage biased junctions are considered. Elastic quasiparticle scattering by the junction determines equilibrium Josephson current. We discuss the origin of Andreev bound states in tunnel junctions and their role in equilibrium Josephson transport. In contrast, quasiparticle tunneling in voltage biased junctions is determined by inelastic scattering. We derive a general expression for inelastic scattering amplitudes and calculate the quasiparticle current at all voltages with emphasis on a discussion of the properties of subgap tunnel current and the nature of subharmonic gap structure.Comment: 47 pages, 9 figures, [preprint,eqsecnum,aps]{revtex

Interference effects in phtalocyanine controlled by H-H tautomerization: a potential two-terminal unimolecular electronic switch

We investigate the electrical transport properties of two hydrogen tautomer configurations of phthalocyanine (H2Pc) connected to cumulene and gold leads. Hydrogen tautomerization affects the electronic state of H2Pc by switching the character of molecular orbitals with the same symmetry close to the Fermi level. The near degeneracy between the HOMO and HOMO-1 leads to pronounced interference effects, causing a large change in current for the two tautomer configuratons, especially in the low-bias regime. Two types of planar junctions are considered: cumulene-H2Pc-cumulene and gold-H2Pc-gold. Both demonstrate prominent difference in molecular conductance between ON and OFF states. In addition, junctions with gold leads show pronounced negative differential resistance (NDR) at high bias voltage, as well as weak NDR at intermediate bias.Comment: 10 pages, 7 figures, accepted for publication in Physical Review

Readout methods and devices for Josephson-junction-based solid-state qubits

We discuss the current situation concerning measurement and readout of Josephson-junction based qubits. In particular we focus attention of dispersive low-dissipation techniques involving reflection of radiation from an oscillator circuit coupled to a qubit, allowing single-shot determination of the state of the qubit. In particular we develop a formalism describing a charge qubit read out by measuring its effective (quantum) capacitance. To exemplify, we also give explicit formulas for the readout time.Comment: 20 pages, 7 figures. To be published in J. Phys.: Condensed Matter, 18 (2006) Special issue: Quantum computin

Coherent multiple Andreev reflections and current resonances in SNS junctions

We study coherent multiple Andreev reflections in quantum SNS junctions of finite length and arbitrary transparency. The presence of superconducting bound states in these junctions gives rise to great enhancement of the subgap current. The effect is most pronounced in low-transparency junctions, $D\ll1$, and in the interval of applied voltage $\Delta/2<eV<\Delta$, where the amplitude of the current structures is proportional to the first power of the junction transparency $D$. The resonant current structures consist of steps and oscillations of the two-particle current and also of multiparticle resonance peaks. The positions of the two-particle current structures have pronounced temperature dependence which scales with $\Delta(T)$, while the positions of the multiparticle resonances have weak temperature dependence, being mostly determined by the junction geometry. Despite the large resonant two-particle current, the excess current at large voltage is small and proportional to $D^2$. Pacs: 74.50.+r, 74.80.Fp, 74.20.Fg, 73.23.AdComment: 23 pages, 16 figure