304 research outputs found
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
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, ,
and in the interval of applied voltage , where the
amplitude of the current structures is proportional to the first power of the
junction transparency . 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 , 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
. Pacs: 74.50.+r, 74.80.Fp, 74.20.Fg, 73.23.AdComment: 23 pages, 16 figure
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
Energy Spectrum and Exact Cover in an Extended Quantum Ising Model
We investigate an extended version of the quantum Ising model which includes
beyond-nearest neighbour interactions and an additional site-dependent
longitudinal magnetic field. Treating the interaction exactly and using
perturbation theory in the longitudinal field, we calculate the energy spectrum
and find that the presence of beyond-nearest-neighbour interactions enhances
the minimal gap between the ground state and the first excited state,
irrespective of the nature of decay of these interactions along the chain. The
longitudinal field adds a correction to this gap that is independent of the
number of qubits. We discuss the application of our model to implementing
specific instances of 3-satisfiability problems (Exact Cover) and make a
connection to a chain of flux qubits.Comment: 9 pages, 3 figures, published versio
Cumulene Molecular Wire Conductance from First Principles
We present first principles calculations of current-voltage characteristics
(IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire
junctions with realistic contacts. The transport properties are calculated
using full self-consistent ab initio NEGF-DFT methods under external bias. The
conductance of the cumulene wires shows oscillatory behavior depending on the
number of carbon atoms (double bonds). Among all conjugated oligomers, we find
that cumulene wires with odd number of carbon atoms yield the highest
conductance with metallic-like ballistic transport behavior. The reason is the
high density of states in broad LUMO levels spanning the Fermi level of the
electrodes. The transmission spectrum and the conductance depend only weakly on
applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V.
Cumulene wires are therefore potential candidates for metallic connections in
nanoelectronic applications.Comment: Accepted in Phys. Rev. B; 5 pages and 6 figure
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