Publications (109)295.4 Total impact
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ABSTRACT: We present a new readout technique for a superconducting qubit dispersively coupled to a Josephson parametric oscillator. We perform degenerate parametric flux pumping of the Josephson inductance with a pump amplitude surpassing the threshold for parametric instability. We map the qubit states onto two distinct states of classical parametric oscillations: one oscillating state, with on average 180 photons in the resonator, and one with zero oscillation amplitude. We demonstrate singleshot readout performance, with a total state discrimination of 81.5%. When accounting for qubit errors, this gives a corrected fidelity of 98.7%, obviating the need for a following quantumlimited amplifier. An error budget indicates that the readout fidelity is currently limited by spurious switching events between two bistable states of the resonator. 
Article: Highgain weakly nonlinear fluxmodulated Josephson parametric amplifier using a SQUIDarray
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ABSTRACT: We have developed and measured a highgain quantumlimited microwave parametric amplifier based on a superconducting lumped LC resonator with the inductor L including an array of 8 superconducting quantum interference devices (SQUIDs). This amplifier is parametrically pumped by modulating the flux threading the SQUIDs at twice the resonator frequency. Around 5 GHz, a maximum gain of 31 dB, a product amplitudegain x bandwidth above 60 MHz, and a 1 dB compression point of 123 dBm at 20 dB gain are obtained in the nondegenerate mode of operation. Phase sensitive amplificationdeamplification is also measured in the degenerate mode and yields a maximum gain of 37 dB. The compression point obtained is 18 dB above what would be obtained with a single SQUID of the same inductance, due to the smaller nonlinearity of the SQUID array.Physical Review B 09/2014; 89(21). DOI:10.1103/PhysRevB.89.214517 · 3.74 Impact Factor  [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. DOI:10.1016/j.physc.2014.02.005 · 0.94 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.Physical Review B 11/2013; 89(21). DOI:10.1103/PhysRevB.89.214508 · 3.74 Impact Factor  [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). DOI:10.1088/13672630/15/10/105002 · 3.56 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. DOI:10.1021/nl4014265 · 13.59 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). DOI:10.1103/PhysRevB.87.184501 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
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.Physical review. B, Condensed matter 02/2012; 85(18). DOI:10.1103/PhysRevB.85.180506 · 3.66 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. DOI:10.1088/09534075/45/12/124019 · 1.98 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: BlochRedfield 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 selfconsistent Born approximation. This more accurate treatment reveals an exponential time dependent prefactor in the nonMarkovian contribution dominating the qubit longtime relaxation found in Phys. Rev. B 71, 035318 (2005). At the same time, the associated dephasing is not affected and is described by the BornMarkov approximation.EPL (Europhysics Letters) 10/2011; 96(4). DOI:10.1209/02955075/96/40003 · 2.10 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 = \Delta 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. DOI:10.1103/PhysRevB.83.184517 · 3.74 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. DOI:10.1103/PhysRevLett.106.097001 · 7.51 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. DOI:10.1103/PhysRevLett.105.233907 · 7.51 Impact Factor  [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 10/2010; 36(10):925932. DOI:10.1063/1.3516470 · 0.79 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a theoretical analysis of the transition from thermal activation (TA) regime to the macroscopic quantum tunneling (MQT) regime of the decay from a metastable persistent current state in grain boundary junctions of cuprate superconductors. This transition is conventionally characterized by a single crossover temperature determined by the potential profile and dissipative mechanisms. It is shown that due to the existence of low energy bound states (midgap states) for various relative orientations of the crystal axes, there exists a window of parameters where one finds, with lowering temperature, an inverse crossover from MQT to TA, followed by a subsequent reentrance of MQT. It is shown that these predictions are in reasonable agreement with recent experiments.Advances in Science and Technology 10/2010; 75:155160. DOI:10.4028/www.scientific.net/AST.75.155  [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 to nonlinear optical phenomena in media of twolevel atoms.Physical Review Letters 09/2010; 105(12):127001. DOI:10.1103/PhysRevLett.105.127001 · 7.51 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). DOI:10.1103/PhysRevB.82.104522 · 3.66 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.  [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). DOI:10.1103/PhysRevB.80.184510 · 3.66 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. DOI:10.1088/17426596/150/5/052159
Publication Stats
2k  Citations  
295.40  Total Impact Points  
Top Journals
Institutions

19932014

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


19851998

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