Publications (148)441.19 Total impact
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ABSTRACT: It has been proposed that adding disorder to a topologically trivial mercury telluride/cadmium telluride (HgTe/CdTe) quantum well can induce a transition to a topologically nontrivial state. The resulting state was termed topological Anderson insulator and was found in computer simulations of the BernevigHughesZhang model. Here, we show that the topological Anderson insulator is a more universal phenomenon and also appears in the KaneMele model of topological insulators on a honeycomb lattice. We numerically investigate the interplay of the relevant parameters, and establish the parameter range in which the topological Anderson insulator exists. A staggered sublattice potential turns out to be a necessary condition for the transition to the topological Anderson insulator. For weak enough disorder, a calculation based on the lowestorder Born approximation reproduces quantitatively the numerical data. Our results thus considerably increase the number of candidate materials for the topological Anderson insulator phase.  [Show abstract] [Hide abstract]
ABSTRACT: Snake states are open trajectories for charged particles propagating in two dimensions under the influence of a spatially varying perpendicular magnetic field. In the quantum limit they are protected edge modes that separate topologically inequivalent ground states and can also occur when the particle density rather than the field is made nonuniform. We examine the correspondence of snake trajectories in singlelayer graphene in the quantum limit for two families of domain walls: (a) a uniform doped carrier density in an antisymmetric field profile and (b) antisymmetric carrier distribution in a uniform field. These families support different internal symmetries but the same pattern of boundary and interface currents. We demonstrate that these physically different situations are gauge equivalent when rewritten in a Nambu doubled formulation of the two limiting problems. Using gauge transformations in particlehole space to connect these problems, we map the protected interfacial modes to the Bogoliubov quasiparticles of an interfacial onedimensional pwave paired state. A variational model is introduced to interpret the interfacial solutions of both domain wall problems.  [Show abstract] [Hide abstract]
ABSTRACT: We study the full counting statistics of interferometers for chiral Majorana fermions with two incoming and two outgoing Dirac fermion channels. In the absence of interactions, the FCS can be obtained from the $4\times4$ scattering matrix $S$ that relates the outgoing Dirac fermions to the incoming Dirac fermions. After presenting explicit expressions for the higherorder current correlations for a modified Hanbury BrownTwiss interferometer, we note that the cumulantgenerating function can be interpreted such that unitcharge transfer processes correspond to two independent halfcharge transfer processes, or alternatively, to two independent electronhole conversion processes. By a combination of analytical and numerical approaches, we verify that this factorization property holds for a general $SO(4)$ scattering matrix, i.e. for a general interferometer geometry.  [Show abstract] [Hide abstract]
ABSTRACT: We study synchronization of two dissipatively coupled Van der Pol oscillators in the quantum regime. Due to quantum noise strict frequency locking is absent and is replaced by a crossover from weak to strong frequency entrainment. We discuss the differences to the behavior of one quantum Van der Pol oscillator subject to an external drive. Moreover, we describe a possible experimental realization of two coupled quantum van der Pol oscillators in an optomechanical setting. 
Article: Detection of weak forces based on noiseactivated switching in bistable optomechanical systems
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ABSTRACT: We propose to use cavity optomechanical systems in the regime of optical bistability for the detection of weak harmonic forces. Due to the optomechanical coupling an external force on the mechanical oscillator modulates the resonance frequency of the cavity and consequently the switching rates between the two bistable branches. A large difference in the cavity output fields then leads to a strongly amplified homodyne signal. We determine the switching rates as a function of the cavity detuning from extensive numerical simulations of the stochastic master equation as appropriate for continuous homodyne detection. We develop a twostate rate equation model that quantitatively describes the slow switching dynamics. This model is solved analytically in the presence of a weak harmonic force to obtain approximate expressions for the power gain and signaltonoise ratio that we then compare to force detection with an optomechanical system in the linear regime.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate Josephson junctions on the surface of a threedimensional topological insulator in planar, step, and edge geometries. The elliptical nature of the Dirac cone representing the side surface states of the topological insulator results in a scaling factor in the Josephson current in a step junction as compared to the planar junction. In edge junctions, the contribution of the Andreev bound states to the Josephson current vanishes due to spinmomentum locking of the surface states. Furthermore, we consider a junction with a ferromagnetic insulator between the superconducting regions. In these ferromagnetic junctions, we find an anomalous finite Josephson current at zero phase difference if the magnetization is pointing along the junction (and perpendicular to the Josephson current). An outofplane magnetization with respect to the central region of the junction opens up an exchange gap and leads to a nonmonotonic behavior of the critical Josephson current for sufficiently large magnetization as the chemical potential increases.  [Show abstract] [Hide abstract]
ABSTRACT: Synchronization is a universal phenomenon that is important both in fundamental studies and in technical applications. Here we investigate synchronization in the simplest quantummechanical scenario possible, i.e., a quantummechanical selfsustained oscillator coupled to an external harmonic drive. Using the power spectrum we analyze synchronization in terms of frequency entrainment and frequency locking in close analogy to the classical case. We show that there is a steplike crossover to a synchronized state as a function of the driving strength. In contrast to the classical case, there is a finite threshold value in driving. Quantum noise reduces the synchronized region and leads to a deviation from strict frequency locking.  [Show abstract] [Hide abstract]
ABSTRACT: Chiral Majoranafermion modes are shown to emerge as edge excitations in a superconductortopologicalinsulator hybrid structure that is subject to a magnetic field. The velocity of this mode is tunable by changing the magneticfield magnitude and/or the superconductor's chemical potential. We discuss how quantumtransport measurements can yield experimental signatures of these modes. A normal lead coupled to the Majoranafermion edge state through electron tunneling induces resonant Andreev reflections from the lead to the grounded superconductor, resulting in a distinctive pattern of differentialconductance peaks. 
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ABSTRACT: We study the lowenergy edge states of a superconductor  3D topologicalinsulator hybrid structure (NS junction) in the presence of a perpendicular magnetic field. The hybridization of electronlike and holelike Landau levels due to Andreev reflection gives rise to chiral edge states within each Landau level. We show that by changing the chemical potential of the superconductor, this junction can be placed in a regime where the sign of the effective charge of the edge state within the zeroth Landau level changes more than once resulting in neutral edge modes with a finite value of the guidingcenter coordinate. We find that the appearance of these neutral edge modes is related to the level repulsion between the zeroth and the first Landau levels in the spectra. We also find that these neutral edge modes come in pairs, one in the zeroth Landau level and its corresponding pair in the first.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate transport properties of a double quantum dot based Cooper pair splitter, where the superconducting lead consists of Sr$_2$RuO$_4$. The proposed device can be used to explore the symmetry of the superconducting order parameter in Sr$_2$RuO$_4$ by testing the presence of gapless chiral edge states, which are predicted to exist if the bulk superconductor is described by a chiral $p$wave state. The odd orbital symmetry of the bulk order parameter ensures that we can realize a regime where the electrons tunneling into the double dot system come from the chiral edge states and thereby leave their signature in the conductance. The proposed Cooper pair splitter has the potential to probe order parameters in unconventional superconductors.  [Show abstract] [Hide abstract]
ABSTRACT: A longstanding problem in quantum mesoscopic physics is which operator order corresponds to noise expressions like ⟨I(ω)I(ω)⟩, where I(ω) is the measured current at frequency ω. Symmetrized order describes a classical measurement while nonsymmetrized order corresponds to a quantum detector, e.g., one sensitive to either emission or absorption of photons. We show that both order schemes can be embedded in quantum weakmeasurement theory taking into account measurements with memory, characterized by a memory function which is independent of a particular experimental detection scheme. We discuss the resulting quasiprobabilities for different detector temperatures and how their negativity can be tested on the level of secondorder correlation functions already. Experimentally, this negativity can be related to the squeezing of the manybody state of the transported electrons in an acdriven tunnel junction.  [Show abstract] [Hide abstract]
ABSTRACT: We study the optical bistability of an optomechanical system in which the position of a mechanical oscillator modulates the cavity frequency. The steadystate meanfield equation of the optical mode is identical to the one for a Kerr medium, and thus we expect it to have the same characteristic behavior with a lower, a middle, and an upper branch. However, the presence of position fluctuations of the mechanical resonator leads to a new feature: the upper branch will become unstable at sufficiently strong driving in certain parameter regimes. We identify the appropriate parameter regime for the upper branch to be stable, and we confirm, by numerical investigation of the quantum steady state, that the mechanical mode indeed acts as a Kerr nonlinearity for the optical mode in the lowtemperature limit. This equivalence of the optomechanical system and the Kerr medium will be important for future applications of cavity optomechanics in quantum nonlinear optics and quantum information science.  [Show abstract] [Hide abstract]
ABSTRACT: We show that the interplay of cyclotron motion and Andreev reflection experienced by masslessDiraclike charge carriers in topologicalinsulator surface states generates a Majoranaparticle excitation. On the basis of an envelopefunction description of the DiracAndreev edge states, we discuss the kinematic properties of the Majorana mode and find them to be tunable by changing the superconductor's chemical potential and/or the magnitude of the perpendicular magnetic field. Our proposal opens up new possibilities for studying Majorana fermions in a controllable setup.  [Show abstract] [Hide abstract]
ABSTRACT: Majorana bound states have been proposed as building blocks for qubits on which certain operations can be performed in a topologically protected way using braiding. However, the set of these protected operations is not sufficient to realize universal quantum computing. We show that the electric field in a microwave cavity can induce Rabi oscillations between adjacent Majorana bound states. These oscillations can be used to implement an additional singlequbit gate. Supplemented with one braiding operation, this gate allows us to perform arbitrary singlequbit operations.  [Show abstract] [Hide abstract]
ABSTRACT: Despite wide interest in organic molecular crystals and the recognition that electronphonon (eph) coupling strength crucially determines the nature of charge carriers in these materials, abinitio studies of eph coupling elements in these materials are still lacking. In this work [1], we calculated the eph coupling elements throughout the whole Brillouin zone in crystalline naphthalene using density functional perturbation theory within the generalized gradient approximation. FourierWannier interpolation scheme [2] was then used to obtain the eph coupling constants on a fine kpoint grid necessary for accurate evaluation of physical properties. Using the obtained eph coupling elements, we evaluated the quasiparticle residues for electrons and holes, obtaining the values of 0.74 and 0.78, respectively. These values suggest that eph coupling strength is insufficient for formation of small polarons in crystalline naphthalene and other oligoacene semiconductors. [1] N. Vukmirovic, C. Bruder, and V. M. Stojanovic, Phys. Rev. Lett. 109, 126407 (2012). [2] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007).  [Show abstract] [Hide abstract]
ABSTRACT: We propose to use weak measurements away from the weakvalue amplification regime to carry out precision measurements of time delays of light. Our scheme is robust to several sources of noise that are shown to only limit the relative precision of the measurement. Thus, they do not set a limit on the smallest measurable phase shift, contrary to standard interferometry and weakvaluebased measurement techniques. Our idea is not restricted to phaseshift measurements and could be used to measure other small effects using a similar protocol.  [Show abstract] [Hide abstract]
ABSTRACT: We propose microwavecontrolled rotations for qubits realized as Majorana bound states. To this end we study an inhomogeneous Kitaev chain in a microwave cavity. The chain consists of two topologically nontrivial regions separated by a topologically trivial, gapped region. The Majorana bound states at the interfaces between the left (right) regions and the central region are coupled, and their energies are split by virtual cotunneling processes. The amplitude for these cotunneling processes decreases exponentially in the number of sites of the gapped region, and the decay length diverges as the gap of the topologically trivial region closes. We demonstrate that microwave radiation can exponentially enhance the coupling between the Majorana bound states, both for classical and quantized electric fields. By solving the appropriate Liouville equation numerically we show that microwaves can drive Rabi oscillations in the Majorana sector. Our model emerges as an effective description for a topological semiconductor nanowire in a microwave cavity. Thus, our proposal provides an experimentally feasible way to obtain full singlequbit control necessary for universal quantum computation with Majorana qubits.  [Show abstract] [Hide abstract]
ABSTRACT: We present a method for implementing stabilizerbased codes with encoding schemes of the operator quantum error correction paradigm, e.g., the "standard" fivequbit and CSS codes, on solidstate qubits with Ising or XYtype interactions. Using pulse sequences, we show how to induce the effective dynamics of the stabilizer Hamiltonian, the sum of an appropriate set of stabilizer operators for a given code. Within this approach, the encoded states (ground states of the stabilizer Hamiltonian) can be prepared without measurements and preserved against both the time evolution governed by the original qubit Hamiltonian, and energynonconserving errors caused by the environment.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we study cavity optomechanical systems in which the position of a mechanical oscillator modulates both the resonance frequency (dispersive coupling) and the linewidth (dissipative coupling) of a cavity mode. Using a quantum noise approach we calculate the optical damping and the opticallyinduced frequency shift. We find that dissipatively coupled systems feature two parameter regions providing amplification and two parameter regions providing cooling. To investigate the strongcoupling regime, we solve the linearized equations of motion exactly and calculate the mechanical and optical spectra. In addition to signatures of normalmode splitting that are similar to the case of purely dispersive coupling, the spectra contain a striking feature that we trace back to the Fano line shape of the force spectrum. Finally, we show that purely dissipative coupling can lead to optomechanicallyinduced transparency which will provide an experimentally convenient way to observe normalmode splitting.
Publication Stats
6k  Citations  
441.19  Total Impact Points  
Top Journals
 Physical review. B, Condensed matter (25)
 Physical Review A (17)
 Physical Review B (16)
 Physical Review Letters (14)
 Physical Review Letters (10)
Institutions

19982015

Universität Basel
 Department of Physics
Bâle, BaselCity, Switzerland 
Antioch University, Santa Barbara
Santa Barbara, California, United States


2003

Russian Academy of Sciences
 L. D. Landau Institute for Theoretical Physics
Moskva, Moscow, Russia


1997

University of Bayreuth
Bayreuth, Bavaria, Germany
