Publications (102)274.48 Total impact
 [Show abstract] [Hide abstract]
ABSTRACT: We develop a theory for the currentvoltage characteristics of diffusive superconductornormal metalsuperconductor Josephson junctions with resistive interfaces and the distance between the electrodes smaller than the superconducting coherence length. The theory allows for a quantitative analytical and numerical analysis in the whole range of the interface transparencies and asymmetry. We focus on the regime of large interface resistance compared to the resistance of the normal region, when the electronhole dephasing in the normal region is significant and the finite length of the junction plays a role. In the limit of strong asymmetry we find pronounced current structures at the combination subharmonics of $\Delta+\Delta_g$, where $\Delta_g$ is the proximity minigap in the normal region, in addition to the subharmonics of the energy gap $2\Delta$ in the electrodes. In the limit of rather transparent interfaces, our theory recovers a known formula for the current in a short mesoscopic connector  a convolution of the current through a singlechannel point contact with the transparency distribution for an asymmetric doublebarrier potential.Physica C Superconductivity 02/2014; 499:1523. · 0.72 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricated and investigated devices with suspended gate controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal state resistances. We analyze the main features of the currentvoltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field and gate voltage, and compare them with theory. The Josephson critical current for a short length device, $L=30$ nm, exhibits a record high magnitude of $800$\,nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows large number of structures, some of them are identified as subharmonic gap structures generated by Multiple Andreev Reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either superconducting gap or length of the wire. By varying the gate voltage in suspended devices we are able to observe a cross over from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current.11/2013;  [Show abstract] [Hide abstract]
ABSTRACT: We experimentally study the behavior of a parametrically pumped nonlinear oscillator, which is based on a superconducting \lambda /4 resonator, and is terminated by a fluxtunable SQUID. We extract parameters for two devices. In particular, we study the effect of the nonlinearities in the system and compare to theory. The Duffing nonlinearity, \alpha, is determined from the probepower dependent frequency shift of the oscillator, and the nonlinearity, \beta, related to the parametric flux pumping, is determined from the pump amplitude for the onset of parametric oscillations. Both nonlinearities depend on the parameters of the device and can be tuned insitu by the applied dc flux. We also suggest how to cancel the effect of \beta by adding a small dc flux and a pump tone at twice the pump frequency.New Journal of Physics 10/2013; 15(10). · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report conductance and supercurrent of InAs nanowires coupled to Alsuperconducting electrodes with short channel lengths and good Ohmic contacts. The nanowires are suspended 15\,nm above a local gate electrode. The charge density in the nanowires can be controlled by a small change in the gate voltage. For large negative gate voltages, the number of conducting channels is reduced gradually and we observe a stepwise decrease of both conductance and critical current before the conductance vanishes completely.Nano Letters 07/2013; 13(8):3614–3617. · 13.03 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We develop a theory of parametric resonance in tunable superconducting cavities. The nonlinearity introduced by the SQUID attached to the cavity, and damping due to connection of the cavity to a transmission line are taken into consideration. We study in detail the nonlinear classical dynamics of the cavity field below and above the parametric threshold for the degenerate parametric resonance, featuring regimes of multistability and parametric radiation. We investigate the phasesensitive amplification of external signals on resonance, as well as amplification of detuned signals, and relate the amplifier performance to that of linear parametric amplifiers. We also discuss applications of the device for dispersive qubit readout. Beyond the classical response of the cavity, we investigate small quantum fluctuations around the amplified classical signals. We evaluate the noise power spectrum both for the internal field in the cavity and the output field. Other quantum statistical properties of the noise are addressed such as squeezing spectra, second order coherence, and twomode entanglement.Physical review. B, Condensed matter 02/2013; 87(18). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A quantum coherent interface between optical and microwave photons can be used as a basic building block within a future quantum information network. The interface is envisioned as an ensemble of rareearth ions coupled to a superconducting resonator, allowing for coherent transfer between optical and microwave photons. Towards this end, we have realized a hybrid device coupling a Er 3+ doped Y2SiO5crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewidth of down to 75 MHz. a Er$^{3+}$ doped Y$_2$SiO$_5$ crystal in a superconducting coplanar waveguide cavity. We observe a collective spin coupling of 4 MHz and a spin linewdith of down to 75 MHz.Journal of Physics B Atomic Molecular and Optical Physics 01/2012; 45. · 2.03 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We solve the coherent multiple Andreev reflection (MAR) problem and calculate currentvoltage characteristics (IVCs) for Josephson SINIS junctions, where S are localequilibrium 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 electronhole 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. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In highly resistive superconducting tunnel junctions, excess subgap current is usually observed and is often attributed to microscopic pinholes in the tunnel barrier. We have studied the subgap current in superconductorinsulatorsuperconductor (SIS) and superconductorinsulatornormalmetal (SIN) junctions. In Al/AlO(x)/Al junctions, we observed a decrease of 2 orders of magnitude in the current upon the transition from the SIS to the SIN regime, where it then matched theory. In Al/AlO(x)/Cu junctions, we also observed generic features of coherent diffusive Andreev transport in a junction with a homogenous barrier. We use the quasiclassical KeldyshGreen function theory to quantify single and twoparticle tunneling and find good agreement with experiment over 2 orders of magnitude in transparency. We argue that our observations rule out pinholes as the origin of the excess current.Physical Review Letters 03/2011; 106(9):097001. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report the observation of photon generation in a microwave cavity with a timedependent boundary condition. Our system is a microfabricated quarterwave coplanar waveguide cavity. The electrical length of the cavity is varied by using the tunable inductance of a superconducting quantum interference device. It is measured at a temperature significantly less than the resonance frequency. When the length is modulated at approximately twice the static resonance frequency, spontaneous parametric oscillations of the cavity field are observed. Timeresolved measurements of the dynamical state of the cavity show multiple stable states. The behavior is well described by theory. Our results may be considered a preliminary step towards demonstrating the dynamical Casimir effect.Physical Review Letters 12/2010; 105(23):233907. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
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 dwave 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 twolevel atoms.Physical Review Letters 09/2010; 105(12):127001. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: This paper presents a theoretical analysis of the recently realized tuneable coupler for superconducting phase qubits (R. C. Bialczak et al., Ref.\ \protect\onlinecite{Bialczak}). The coupling can be turned off by compensating a negative mutual inductance with a tuneable Josephson inductance. The main coupling in this system is of the $XX$ type and can be zeroed exactly, while there is also a small undesired contribution of the $ZZ$ type. We calculate both couplings as functions of the tuning parameter (bias current) and focus on the residual coupling in the OFF regime. In particular, we show that for typical experimental parameters the coupling OFF/ON ratio is few times $10^{3}$, and it may be zeroed by proper choice of parameters. The remaining errors due to physical presence of the coupler are on the order of $10^{6}$.Physical review. B, Condensed matter 06/2010; 82(10). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We theoretically analyze a scheme for tunable coupling of two phase qubits, which has been recently realized experimentally. In this scheme, two inductors create a direct magnetic interaction between the qubits via mutual inductance, and an additional Josephson junction creates an indirect interaction which may be tuned via the bias current of the junction. These two contributions to sigmaxsigmax coupling of qubits have opposite signs and for some value of the bias current cancel each other, thus producing zero coupling. However, a small sigmazsigmaz coupling, which originates due to qubit anharmonicity, gets cancelled at a slightly different bias current, that leads to a small residual coupling. We calculate the residual coupling and the corresponding ON/OFF ratio analytically and numerically. We also discuss a minor modification of the scheme, for which the residual coupling may be zeroed.03/2010;  [Show abstract] [Hide abstract]
ABSTRACT: Conventional models of Josephson junction dynamics rely on the absence of lowenergy quasiparticle states owing to a large superconducting gap. With this assumption the quasiparticle degrees of freedom are and the phase difference becomes the only free variable, acting as a fictitious particle in a temporally localized Josephson potential related to the adiabatic and nondissipative supercurrent across the junction. In this article we develop a general framework to incorporate the effects of lowenergy quasiparticles interacting nonadiabatically with the phase degree of freedom. These quasiparticle states typically exist in constriction type junctions with high transparency channels or resonant states, as well as in junctions of unconventional superconductors. Recent experiments have also revealed the existence of spurious lowenergy ingap states in tunnel junctions of conventional superconductorsa system for which the adiabatic assumption is typically assumed to be valid. We show that a resonant interaction with these lowenergy states, rather than the Josephson potential, determines the nonlinear Josephson dynamics at small amplitudes.Low Temperature Physics 01/2010; 36:925932. · 0.82 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the outofequilibrium current through a quantum dot which is placed between two superconducting leads held at fixed voltage bias, considering both cases of the absence and the presence of an additional normal lead connected to the dot. Using the nonequilibrium Keldysh technique, we focus on the subgap bias region, where multiple Andreev reflections are responsible for charge transfer through the dot. Attention is put on the dc current and on the first harmonics of the supercurrent. Varying the position and/or the width of the dot level, we first investigate the crossover between a quantumdot and quantum pointcontact regimes in the absence of a normal lead. We then study the effect of the normal electrode connected to the dot, which is understood to lead to dephasing, or alternatively to induce reverse proximity effect. By increasing the dot coupling to the normal probe, we show the full crossover from zero dephasing to the incoherent case. We also compute the Josephson current in the presence of the normal lead and find it in excellent agreement with the values of the nonequlibrium current extrapolated at zero voltage.Physical review. B, Condensed matter 08/2009; 80(18). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study quantum phase fluctuations in planar junctions between highTc superconductors for a range of relative orientations of the ab crystal axes. Specifically, attention is directed towards the fermionic subspace of low energy Andreev bound states, midgap states (MGS). This subspace, being responsible for most of the Josephson current, strongly couples to the phase degree of freedom. Low frequency phase fluctuations may introduce intrinsic dynamics in this subspace, which would lead to a strong dynamical modulation of the Josephson current, and thus affect the phase quantum dynamics. In this article it is found that transitions between pairs of MGS induced by the phase fluctuations are forbidden due to specific symmetry properties of the MGS, and the macroscopic quantum Hamiltonian remains essentially adiabatic within the assumption of a specular interface.Journal of Physics Conference Series 03/2009; 150(5):052159.  [Show abstract] [Hide abstract]
ABSTRACT: We have fabricated and characterized tunable superconducting transmission line resonators. To change the resonance frequency, we modify the boundary condition at one end of the resonator through the tunable Josephson inductance of a SQUID. We demonstrate a large tuning range, high quality factors and that we can change the frequency of a fewphoton field on a time scale orders of magnitude faster than the photon lifetime. When parametrically pumped at twice their resonance frequency, the devices can act as parametric amplifiers. When pumped strongly, a threshold is crossed where the resonators oscillate spontaneously. Within this regime of parametric oscillations, the devices can exist in a variety of dynamical states. We observe a rich pattern in the dynamics of switching between these states. We study the possibility of using this dynamical bifurcation for qubit readout. Finally, recent theoretical work has suggested that it may be easier to observe dynamical tunneling in this system than in the Duffing oscillator.03/2009;  [Show abstract] [Hide abstract]
ABSTRACT: Photons are fundamental excitations of the electromagnetic field and can be captured in cavities. For a given cavity with a certain size, the fundamental mode has a fixed frequency {\it f} which gives the photons a specific "color". The cavity also has a typical lifetime $\tau$, which results in a finite linewidth $\delta${\it f}. If the size of the cavity is changed fast compared to $\tau$, and so that the frequency change $\Delta${\it f} $\gg \delta${\it f}, then it is possible to change the "color" of the captured photons. Here we demonstrate superconducting microwave cavities, with tunable effective lengths. The tuning is obtained by varying a Josephson inductance at one end of the cavity. We show data on four different samples and demonstrate tuning by several hundred linewidths in a time $\Delta t \ll \tau$. Working in the few photon limit, we show that photons stored in the cavity at one frequency will leak out from the cavity with the new frequency after the detuning. The characteristics of the measured devices make them suitable for different applications such as dynamic coupling of qubits and parametric amplification. Comment: 2nd International Workshop on SolidState Quantum Computing, June 2008, Taipei, TaiwanAIP Conference Proceedings 11/2008;  [Show abstract] [Hide abstract]
ABSTRACT: Nondissipative Josephson current through nanoscale superconducting constrictions is carried by spectroscopically sharp energy states, the socalled Andreev states. Although theoretically predicted almost 40 years ago, no direct spectroscopic evidence of these Andreev bound states exists to date. We propose a novel type of spectroscopy based on embedding a superconducting constriction, formed by a singlelevel molecule junction, in a microwave QED cavity environment. In the electrondressed cavity spectrum we find a polariton excitation at twice the Andreev bound state energy, and a superconductingphasedependent ac Stark shift of the cavity frequency. Dispersive measurement of this frequency shift can be used for Andreev bound state spectroscopy.Physical Review Letters 09/2008; 101(8):087002. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have fabricated and characterized tunable superconducting transmission line resonators. To change the resonance frequency, we modify the boundary condition at one end of the resonator through the tunable Josephson inductance of a superconducting quantum interference device. We demonstrate a large tuning range (several hundred megahertz), high quality factors (104), and that we can change the frequency of a fewphoton field on a time scale orders of magnitude faster than the photon lifetime of the resonator. This demonstration has implications in a variety of applications.Applied Physics Letters 05/2008; 92:203501. · 3.79 Impact Factor 
Article: Implementation of the threequbit phaseflip error correction code with superconducting qubits
[Show abstract] [Hide abstract]
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 qubitqubit 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 figuresPhysical review. B, Condensed matter 03/2008; · 3.77 Impact Factor
Publication Stats
1k  Citations  
274.48  Total Impact Points  
Top Journals
Institutions

1994–2014

Chalmers University of Technology
 • Department of Microtechnology and Nanoscience
 • Applied Quantum Physics Laboratory
 • Department of Applied Physics
Goeteborg, Västra Götaland, Sweden


2007

Moscow State Forest University
Mytishi, Moskovskaya, Russia


1985–2001

B.Verkin Institute for Low Temperature Physics and Engineering
Charkow, Kharkivs’ka Oblast’, Ukraine
