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ABSTRACT: We consider a superconducting quantum point contact in a circuit quantum
electrodynamics setup. We study three different configurations, attainable with
current technology, where a quantum point contact is coupled galvanically to a
coplanar waveguide resonator. Furthermore, we demonstrate that the strong and
ultrastrong coupling regimes can be achieved with realistic parameters,
allowing the coherent exchange between a superconducting quantum point contact
and a quantized intracavity field.
02/2012;
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ABSTRACT: Bloch-Redfield equation is a common tool for studying evolution of qubit
systems weakly coupled to environment. We investigate the accuracy of the Born
approximation underlying this equation. We find that the high order terms in
the perturbative expansion contain accumulating divergences that make
straightforward Born approximation inappropriate. We develop diagrammatic
technique to formulate, and solve the improved self-consistent Born
approximation. This more accurate treatment reveals an exponential time
dependent prefactor in the non-Markovian contribution dominating the qubit
long-time relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time,
the associated dephasing is not affected and is described by the Born-Markov
approximation.
10/2011;
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ABSTRACT: We solve the coherent multiple Andreev reflection (MAR) problem and calculate current-voltage characteristics (IVCs) for Josephson SINIS junctions, where S are local-equilibrium superconducting reservoirs, I denotes tunnel barriers, and N is a short diffusive normal wire, the length of which is much smaller than the coherence length, and the resistance is much smaller than the resistance of the tunnel barriers. The charge transport regime in such junctions qualitatively depends on a characteristic value γ = τ d of relative phase shifts between the electrons and retroreflected holes accumulated during the dwell time τ d . In the limit of small electron-hole dephasing γ 1, our solution recovers a known formula for a short mesoscopic connector extended to the MAR regime. At large dephasing, the subharmonic gap structure in the IVC scales with γ −1 , which thus plays the role of an effective tunneling parameter. In this limit, the even gap subharmonics are resonantly enhanced, and the IVC exhibits portions with negative differential resistance.
Physical Review B 05/2011; 83(18):184517. · 3.69 Impact Factor
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ABSTRACT: We develop a theoretical description of nonadiabatic Josephson dynamics in superconducting junctions containing low energy quasiparticles. Within this approach we investigate the effects of midgap states in junctions of unconventional d-wave superconductors. We identify a reentrance effect in the transition between thermal activation and macroscopic quantum tunneling, and connect this phenomenon to the experimental observations in Phys. Rev. Lett. 94, 087003 (2005). It is also shown that nonlinear Josephson dynamics can be defined by resonant interaction with midgap states reminiscent of nonlinear optical phenomena in media of two-level atoms.
Physical Review Letters 09/2010; 105(12):127001. · 7.37 Impact Factor
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ABSTRACT: We investigate the performance of a three qubit error correcting code in the framework of superconducting qubit implementations. Such a code can recover a quantum state perfectly in the case of dephasing errors but only in situations where the dephasing rate is low. Numerical studies in previous work have however shown that the code does increase the fidelity of the encoded state even in the presence of high error probability, during both storage and processing. In this work we give analytical expressions for the fidelity of such a code. We consider two specific schemes for qubit-qubit interaction realizable in superconducting systems; one $\sigma_z\sigma_z$-coupling and one cavity mediated coupling. With these realizations in mind, and considering errors during storing as well as processing, we calculate the maximum operation time allowed in order to still benefit from the code. We show that this limit can be reached with current technology. Comment: 10 pages, 8 figures
03/2008;
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ABSTRACT: The influence of a noisy environment on coherent transport in Andreev states through a point contact between two superconductors is considered. The amount of dephasing is estimated for a microwave-activated quantum interferometer. Possibilities of experimentally investigating the coupling between a superconducting quantum point contact and its electromagnetic environment are discussed.
Physics-Uspekhi 10/2007; 44(10S):121. · 2.15 Impact Factor
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ABSTRACT: We investigate manipulation with Andreev bound states in Josephson quantum point contacts with magnetic scattering. Rabi oscillations in the two-level Andreev subsystems are excited by resonant driving the direction of magnetic moment of the scatterer, and by modulating the superconducting phase difference across the contact. The Andreev level dynamics is manifested by temporal oscillation of the Josephson current, accompanied, in the case of magnetic manipulation, also by oscillation of the Andreev states spin polarization. The interlevel transitions obey a selection rule that forbids manipulations in a certain region of external parameters, and results from specific properties of Andreev bound states in magnetic contacts: 4$\pi$-periodicity with respect to the superconducting phase, and strong spontaneous spin polarization.
10/2007;
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ABSTRACT: We formulate a theoretical framework to describe multiparticle current transport in planar superconducting tunnel junctions with diffusive electrodes. The approach is based on the direct solving of quasiclassical Keldysh–Green function equations for nonequilibrium superconductors, and consists of a combination of circuit theory analysis and improved perturbation expansion. The theory predicts a much greater scaling parameter for the subharmonic gap structure of the tunnel current in diffusive junctions compared to the one in ballistic junctions and mesoscopic constrictions with the same barrier transparency.
Superconductor Science and Technology 06/2007; 20(6):529-541. · 2.66 Impact Factor
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ABSTRACT: We formulate a theoretical framework to describe multiparticle current
transport in planar superconducting tunnel junctions with diffusive electrodes.
The approach is based on direct solving of quasiclassical Keldysh-Green
function equations for nonequilibrium superconductors, and consists of a
combination of a circuit theory analysis and improved perturbation expansion.
The theory predicts much greater scaling parameter for the subharmonic gap
structure of the tunnel current in diffusive junctions compared to the one in
ballistic junctions and mesoscopic constrictions with the same barrier
transparency.
01/2007;
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ABSTRACT: Non-equilibrium current noise of a short quasi–one-dimensional constriction between two superconductors is considered. We derive a general expression for the frequency-dependent current-current correlation function valid for arbitrary temperatures, transparencies, and bias voltages. This formalism is then applied to a single current-carrying quantum mode with perfect transparency, and at zero frequency and temperature. Contrary to a transparent channel separating two normal conductors, a weak link between two superconductors exhibits a finite level of noise. The source of noise is the fractional Andreev scattering of quasi-particles with energies |E| greater than the half-width Δ of the superconducting gap. For high bias voltages, V >> Δ/e, the zero-frequency limit of the noise spectrum, S(0), as well as the excess current Iexc, are twice as large than for a normal-superconductor junction, S(0) = (2/5)|e|Iexc.
EPL (Europhysics Letters) 01/2007; 34(1):49. · 2.17 Impact Factor
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ABSTRACT: We discuss the implementation of an iterative quantum phase estimation algorithm, with a single ancillary qubit. We suggest using this algorithm as a benchmark for multi-qubit implementations. Furthermore we describe in detail the smallest possible realization, using only two qubits, and exemplify with a superconducting circuit. We discuss the robustness of the algorithm in the presence of gate errors, and show that 7 bits of precision is obtainable, even with very limited gate accuracies. Comment: Improved error model. Dephasing is considered
10/2006;
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ABSTRACT: 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.
08/2006;
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ABSTRACT: We calculate the subgap current in planar superconducting tunnel junctions with thin-film diffusive leads. It is found that the subharmonic gap structure of the tunnel current scales with an effective tunneling transpar-ency which may exceed the junction transparency by up to two orders of magnitude depending on the junction geometry and the ratio between the coherence length and the elastic scattering length. These results provide an alternative explanation of enhanced values of the subgap current in tunneling experiments often ascribed to imperfection of the insulating layer. We also discuss the effect of finite lifetime of quasiparticles as the possible origin of additional enhancement of multiparticle tunnel currents.
Physical Review B 06/2006; 73(22):220506. · 3.69 Impact Factor
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ABSTRACT: 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 computing
02/2006;
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ABSTRACT: We calculate analytically the full counting statistics for a short normally con-ducting diffusive wire connecting a normal reservoir and a short superconductor-normal metal-superconductor junction, at arbitrary applied voltages and temper-atures. The cumulant-generating function oscillates with the phase difference φ across the junction and approaches the normal-state value at φ = π. At T = 0 and at applied voltage much smaller than the proximity gap ∆ φ , the current noise P I doubles and the third current cumulant C 3 is 4 times larger compared to the normal state; at eV ≫ ∆ φ they acquire large excess components. At the gap edge, eV = ∆ φ , the differential shot noise dP I /dV exhibits sharp peak, while the dif-ferential Fano factor dP I /dI turns to zero along with the differential resistance, which reflects the transmission resonance associated with the singularity of the density of states. At nonzero temperature, C 3 shows a non-monotonous voltage dependence with a dip near eV = ∆ φ ; the zero-bias slope of C 3 (V) is much larger (up to 5 times) than at the zero temperature.
Theory of Quantum Transport in Metallic and Hybrid Nanostructures edited by A.Glatz et al., 01/2006; Springer.
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ABSTRACT: This paper gives an introduction to the physics and principles of operation of quantized superconducting electrical circuits for quantum information processing.
09/2005;
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ABSTRACT: We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbour coupling. The qubit coupling is achieved by placing large Josephson junctions (JJs) at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional JJs included in the qubit loops for qubit readout can also be employed for qubit coupling.
New Journal of Physics 08/2005; 7(1):178. · 4.18 Impact Factor
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ABSTRACT: We study nonequilibrium effects in current transport through voltage biased tunnel junction with long diffusive superconducting leads at low applied voltage eV 2, and finite temperatures. Due to a small value of the Josephson frequency, the quasiparticle spectrum adiabatically follows the time evolution of the super-conducting phase difference, which results in the formation of oscillating bound states in the vicinity of the tunnel junction Andreev band. The quasiparticles trapped by the Andreev band generate higher even harmon-ics of the Josephson ac current, and also, in the presence of inelastic scattering, a nonequilibrium dc current, which may considerably exceed the dc quasiparticle current given by the tunnel model. The distribution of traveling quasiparticles also deviates from the equilibrium due to the spectrum oscillations, which results in an additional contribution to the dc current, proportional to V.
Physical Review B 07/2005; 72(1):014501. · 3.69 Impact Factor
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ABSTRACT: We investigate the design and functionality of a network of loop-shaped charge qubits with switchable nearest-neighbor coupling. The qubit coupling is achieved by placing large Josephson junctions at the intersections of the qubit loops and selectively applying bias currents. The network is scalable and makes it possible to perform a universal set of quantum gates. The coupling scheme allows gate operation at the charge degeneracy point of each qubit, and also applies to charge-phase qubits. Additional Josephson junctions included in the qubit loops for qubit readout can also be employed for qubit coupling. Comment: 23 pages, 4 figures
04/2005;
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ABSTRACT: We present an experimental and theoretical investigation of coherent current transport in wide ballistic superconductor-two dimensional electron gas-superconductor junctions. It is found experimentally that upon increasing the junction length, the subharmonic gap structure in the current-voltage characteristics is shifted to lower voltages, and the excess current at voltages much larger than the superconducting gap decreases. Ap-plying a theory of coherent multiple Andreev reflection, we show that these observations can be explained in terms of transport through Andreev resonances.
Physical Review B 12/2004; 70(21):212505. · 3.69 Impact Factor
