Publications (224)929.81 Total impact
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ABSTRACT: We investigate the twophoton scattering properties of a JaynesCummings (JC) nonlinearity consisting of a twolevel system (qubit) interacting with a single mode cavity, which is coupled to two waveguides, each containing a single incident photon wave packet initially. In this scattering setup, we study the interplay between the HongOuMandel effect arising due to quantum interference and effective photonphoton interactions induced by the presence of the qubit. We calculate the twophoton scattering matrix of this system analytically and identify signatures of interference and interaction in the second order auto and crosscorrelation functions of the scattered photons. In the dispersive regime, when qubit and cavity are far detuned from each other, we find that the JC nonlinearity can be used as an almost linear, insitu tunable beam splitter giving rise to ideal HongOuMandel interference, generating a highly pathentangled twophoton NOON state of the scattered photons. The latter manifests itself in strongly suppressed waveguide crosscorrelations and Poissonian photon number statistics in each waveguide. If the twolevel system and the cavity are on resonance, the JC nonlinearity strongly modifies the ideal HOM conditions leading to a smaller degree of path entanglement and subpoissonian photon number statistics. In the latter regime, we find that photon blockade is associated with bunched autocorrelations in both waveguides, while a twopolariton resonance can lead to bunched as well as antibunched correlations.12/2014;  [Show abstract] [Hide abstract]
ABSTRACT: We study the charge dynamics of a quantum dot as measured by a nearby quantum point contact probing the dot via individual singleparticle wave packets. We contrast the two limiting cases of weak and strong systemdetector coupling exerting vanishing and strong backaction on the system and analyze the resulting differences in the chargecharge correlator. Extending the study to multiple projective measurements modelling a continuous strong measurement, we identify a transition from a charge dynamics dominated by the system's properties to a universal dynamics governed by the measurement.Physical Review B 06/2014; 90(7). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In a coupled system of one classical and one quantum mechanical degree of freedom, the quantum degree of freedom can facilitate the escape of the whole system. Such unusual escape characteristics have been theoretically predicted as "M\"unchhausen effect". We implement such a system by shunting one of the two junctions of a dcSQUID with an additional capacitance. In our experiments, we detect a crossover between quantum and classical escape processes related to the direction of escape. We find that, under varying external magnetic flux, macroscopic quantum tunneling periodically alternates with thermally activated escape, a hallmark of the "M\"unchhausen effect".Physical review letters. 04/2014; 113(24).  [Show abstract] [Hide abstract]
ABSTRACT: We study the magnetic response of a superconducting double strip, i.e., two parallel coplanar thin strips of width $2w$, thickness $d \ll w$ and of infinite length, separated by a gap of width $2s$ and subject to a perpendicular magnetic field $H$. The magnetic properties of this system are governed by the presence of a geometric energy barrier for vortex penetration which we investigate as a function of applied field $H$ and gap parameter $s$. The new results deal with the case of a narrow gap $s \ll w$, where the field penetration from the inner edges is facilitated by large flux focusing. Upon reducing the gap width $2s$, we observe a considerable rearrangement of the screening currents, leading to a strong reduction of the penetration field and the overall magnetization loop, with a suppression factor reaching $\sim (d/w)^{1/2}$ as the gap drops below the sample thickness, $2s < d$. We compare our results with similar systems of different shapes (elliptic, rectangular platelet) and include effects of surface barriers as well. Furthermore, we verify that corrections arising from the magnetic response of the Shubnikov phase in the penetrated state are small and can be omitted. Extending the analysis to multiple strips, we determine the specific sequence of flux penetrations into the different strips. Our studies are relevant for the understanding of platelet shaped samples with cracks or the penetration into layered superconductors at oblique magnetic fields.Physical Review B 02/2014; 89(10). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the singlephoton transport properties of a onedimensional coupled cavity array (CCA) containing a single qubit in its central site by coupling the CCA to two transmission lines supporting propagating bosonic modes with linear dispersion. We find that even in the nominally weak lightmatter coupling regime, the transmission through a long array exhibits two ultranarrow resonances corresponding to longlived selfprotected polaritonic states localized around the site containing the qubit. The lifetime of these states is found to increase exponentially with the number of array sites in sharp distinction to the polaritonic Bloch modes of the cavity array.Physical Review A 09/2013; 89(2). · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the random directed polymer problem—the shortscale behavior of an elastic string (or polymer) in one transverse dimension subject to a disorder potential and finite temperature fluctuations. We are interested in the polymer shortscale wandering expressed through the displacement correlator <[δ u( X)]2>, with δ u( X) being the difference in the displacements at two points separated by a distance X. While this object can be calculated at short scales using the perturbation theory in higher dimensions d > 2, this approach becomes illdefined and the problem turns out to be nonperturbative in the lower dimensions and for an infinitelength polymer. In order to make progress, we redefine the task and analyze the wandering of a string of a finite length L. At zero temperature, we find that the displacement fluctuations <[δ u( X)]2> ∝ LX 2 depend on L and scale with the square of the segment length X, which differs from a straightforward Larkintype scaling. The result is best understood in terms of a typical squared angle <α2> ∝ L, where α = ∂ x u, from which the displacement scaling for the segment X follows naturally, <[δ u( X)]2> ∝ <α2> X 2. At high temperatures, thermal fluctuations smear the disorder potential and the lowestorder results for disorderinduced fluctuations in both the displacement field and the angle vanish in the thermodynamic limit L → ∞. The calculation up to the second order allows us to identify the regime of validity of the perturbative approach and provides a finite expression for the displacement correlator, albeit depending on the boundary conditions and the location relative to the boundaries.Journal of Experimental and Theoretical Physics 09/2013; 117(3):570578. · 0.93 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We discuss the JaynesCummingsHubbard model (JCHM) describing the superfluidMott insulator transition of polaritons (i.e., dressed photonqubit states) in coupled qubitcavity arrays in the crossover from strong to weak correlations. In the strongly correlated regime the phase diagram and the elementary excitations of lattice polaritons near the Mott lobes are calculated analytically using a slave boson theory (SBT). The opposite regime of weakly interacting polariton superfluids is described by a weakcoupling meanfield theory (MFT) for a generalised multimode Dicke model. We show that a remarkable relation between the two theories exists in the limit of large photon bandwidth and large negative detuning, i.e., when the nature of polariton quasiparticles becomes qubitlike. In this regime, the weak coupling theory predicts the existence of a single Mott lobe with a change of the universality class of the phase transition at the tip of the lobe, in perfect agreement with the slaveboson theory. Moreover, the spectra of low energy excitations, i.e., the sound velocity of the Goldstone mode and the gap of the amplitude mode match exactly as calculated from both theories.Journal of Physics B Atomic Molecular and Optical Physics 07/2013; · 1.92 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We propose a quantumenhanced iterative (with $K$ steps) measurement scheme based on an ensemble of $N$ twolevel probes which asymptotically approaches the Heisenberg limit $\delta_K \propto R^{K/(K+1)}$, $R$ the number of quantum resources. The protocol is inspired by Kitaev's phase estimation algorithm and involves only collective manipulation and measurement of the ensemble. The iterative procedure takes the shotnoise limited primary measurement with precision $\delta_1\propto N^{1/2}$ to increasingly precise results $\delta_K\propto N^{K/2}$. A straightforward implementation of the algorithm makes use of a twocomponent atomic cloud of Bosons in the precision measurement of a magnetic field.Physical Review A 04/2013; · 2.99 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We propose and analyze a mesoscopic device producing ondemand entangled pairs of electrons. The system consists of two capacitively coupled MachZehnder interferometers implemented in a quantum Hall structure. A pair of electron wavepackets is injected into the chiral edge states of two (of the four) incoming arms; scattering on the incoming interferometers splits the wavepackets into four components of which two interact. The resulting interaction phase associated with this component leads to the entanglement of the state; the latter is scattered at the outgoing beam splitter and analyzed in a Bell violation test measuring the presence of particles in the four outgoing leads. We study the twoparticle case and determine the conditions to reach and observe full entanglement. We extend our twoparticle analysis to include the underlying Fermi seas in the quantum Hall device; the change in shape of the wavefunction, the generation of electronhole pairs in the interaction regime, and a time delay between the pulses all reduce the degree of visible entanglement and the violation of the Bell inequality, effects which we analyze quantitatively. We determine the device settings optimizing the entanglement and the Bell test and find that violation is still possible in the presence of the Fermi seas, with a maximal Bell parameter reaching ${\cal B} = 2.18 > 2$ in our setup.Physical review. B, Condensed matter 12/2012; 87(16). · 3.66 Impact Factor 
Article: Transition from slow Abrikosov to fast moving Josephson vortices in iron pnictide superconductors.
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ABSTRACT: Iron pnictides are layered high T(c) superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosovlike to Josephsonlike in the pnictide superconductor SmFeAs(O,F) with T(c)~4850 K on cooling below a temperature T(*)~4142 K, despite its moderate electronic anisotropy γ~46. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the fluxflow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T(*) coincides well with the temperature where the coherence length ξ(c) perpendicular to the layers matches half of the FeAslayer spacing. For fields slightly outofplane (> 0.1° 0.15°) the vortices are completely immobilized as wellpinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from wellpinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.Nature Material 11/2012; · 36.43 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the modification in mesoscopic electronic transport due to electronelectron interactions making use of scattering states. We demonstrate that for a specific (finite range) interaction kernel, the knowledge of the scattering matrix is sufficient to take interaction effects into account. We calculate perturbatively the corrections to the current and currentcurrent correlator; in agreement with previous work, we find that, in linear response, interaction effects can be accounted for by an effective (renormalized) transmission probability. Beyond linear response, simple renormalization of scattering coefficients is not sufficient to describe the currentcurrent correlator, as additional corrections arise due to irreducible twoparticle processes. Furthermore, we find that the correlations between oppositespin currents induced by interaction are enhanced for an asymmetric scatterer, generating a nonzero result already to lowest order in the interaction.Physical review. B, Condensed matter 07/2012; 86(12). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the coherence and fluorescence properties of the coherently pumped and dissipative JaynesCummingsHubbard model describing polaritons in a coupledcavity array. At weak hopping we find strong signatures of photon blockade similar to singlecavity systems. At strong hopping the state of the photons in the array depends on its size. While the photon blockade persists in a dimer consisting of two coupled cavities, a coherent state forms on an extended lattice, which can be described in terms of a semiclassical model.Physical Review Letters 06/2012; 108(23):233603. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We determine the currentvoltage characteristic of typeII superconductors in the presence of strong pinning centers. Focusing on a small density of defects, we derive a generic form for the characteristic with a linear fluxflow branch shifted by the critical current (excesscurrent characteristic). The details near onset, a hysteretic jump (for κ≫1) or a smooth velocity turnon (κ→1), depend on the Labusch parameter κ characterizing the pinning centers. Pushing the singlepin analysis into the weak pinning domain, we reproduce the collective pinning results for the critical current.Physical Review Letters 05/2012; 108(21). · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent Nernst and interlayer transport experiments in Bi2Sr2CaCu2O8+y (BSCCO) high temperature superconductors report hugely different limiting magnetic fields. We demonstrate that both fields convert to the same pseudogap energy scale T* upon transformation as orbital and Zeeman critical fields, respectively. We suggest a consistent interpretation of this finding based on separation of spin and charge degrees of freedom residing in different regions of a truncated Fermi surface.International Journal of Modern Physics B 01/2012; 19(01n03). · 0.46 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study an Andreev quantum dot, that is a quantum dot inserted in a superconducting ring, with several levels or conducting channels. We analyze the degeneracy of the ground state as a function of the phase difference and of the gate voltage and find its dependence on the Coulomb interaction within and between channels. We compute a (non integer) charge of the dot region and Josephson current. The chargetophase and currenttogate voltage sensitivities are studied. We find that, even in the presence of Coulomb interaction between the channels, the sensitivity increases with the number of channels, although it does not scale linearly as in the case with no interactions. The Andreev quantum dot may therefore be used as a sensitive detector of magnetic flux or as a Josephson transistor.Physical review. B, Condensed matter 01/2012; 85(12). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Experiments on critical current and magnetic properties of ceramic YBa2Cu3O7−δ are discussed and analyzed in terms of an adapted critical state model.Modern Physics Letters B 11/2011; 03(05). · 0.69 Impact Factor 
Article: Dynamic Aspects of Strong Pinning
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ABSTRACT: We determine the currentvoltage characteristic of type II superconductors in the presence of strong pinning centers. Focusing on a small density of defects, we derive a generic form for the characteristic with a linear fluxflow branch shifted by the critical current (excesscurrent characteristic). The details near onset, a hysteretic jump (for $\kappa \gg 1$) or a smooth velocity turnon ($\kappa \to 1$), depend on the Labusch parameter $\kappa$ characterising the pinning centers. Pushing the singlepin analysis into the weak pinning domain, we reproduce the collective pinning results for the critical current.10/2011;  [Show abstract] [Hide abstract]
ABSTRACT: We analyze properties of excitations due to voltage pulses applied to a 1D noninteracting electron gas, assuming that the integral of the voltage over time is equal to the unit of flux. We show that the average charge transfer due to such pulses does not depend on the pulse shape. For pulses with a Lorentzian profile, we prove the singleparticle nature of the electron and the hole excitations.10/2011;  [Show abstract] [Hide abstract]
ABSTRACT: We study a singleelectron pulse injected into the chiral edge state of a quantum Hall device and subject it to a capacitive Coulomb interaction. We find that the scattered multiparticle state remains unentangled and hence can be created itself by a suitable classical voltage pulse. The application of an appropriate inverse pulse corrects for the shakeup due to the interaction and resurrects the original injected wave packet. We suggest an experiment with an asymmetric MachZehnder interferometer where the application of such pulses manifests itself in an improved visibility.Physical Review Letters 08/2011; 107(7):076803. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We generalize the binary quantum counting algorithm of Lesovik, Suslov, and Blatter [Phys. Rev. A 82, 012316 (2010)] to higher counting bases. The algorithm makes use of qubits, qutrits, and qudits to count numbers in a base 2, base 3, or base d representation. In operating the algorithm, the number n < N = d^K is read into a Kqudit register through its interaction with a stream of n particles passing in a nearby wire; this step corresponds to a quantum Fourier transformation from the Hilbert space of particles to the Hilbert space of qudit states. An inverse quantum Fourier transformation provides the number n in the base d representation; the inverse transformation is fully quantum at the level of individual qudits, while a simpler semiclassical version can be used on the level of qudit registers. Combining registers of qubits, qutrits, and qudits, where d is a prime number, with a simpler singleshot measurement allows to find the powers of 2, 3, and other primes d in the number n. We show, that the counting task naturally leads to the shift operation and an algorithm based on the quantum Fourier transformation. We discuss possible implementations of the algorithm using quantum spind systems, dwell systems, and their emulation with spin1/2 or doublewell systems. We establish the analogy between our counting algorithm and the phase estimation algorithm and make use of the latter's performance analysis in stabilizing our scheme. Applications embrace a quantum metrological scheme to measure a voltage (analog to digital converter) and a simple procedure to entangle multiparticle states.Physical Review A 11/2010; · 2.99 Impact Factor
Publication Stats
8k  Citations  
929.81  Total Impact Points  
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Institutions

1970–2014

ETH Zurich
 Institute for Theoretical Physics
Zürich, Zurich, Switzerland


2002–2004

Rutgers, The State University of New Jersey
 Department Physics and Astronomy
New Brunswick, NJ, United States


1999

Hochschule für Technik Zürich
Zürich, Zurich, Switzerland


1993

Weizmann Institute of Science
 Department of Physics of Condensed Matter
Tel Aviv, Tel Aviv, Israel
