Ultrafast helicity control of surface currents in topological insulators with nearunity fidelity
Nature Communications
, Volume 6, page: 6617
2015
Abstract: In recent years, a class of solid state materials, called threedimensional topological insulators, has emerged. In the bulk, a topological insulator behaves like an ordinary insulator with a band gap. At the surface, conducting gapless states exist showing remarkable properties such as helical Dirac dispersion and suppression of backscattering of spinpolarized charge carriers. The characterization and control of the surface states via transport experiments is often hindered by residual bulk contributions yet at cryogenic temperatures. Here, we show that surface currents in Bi2Se3 can be controlled by circularly polarized light on a picosecond time scale with a fidelity near unity even at room temperature. We reveal the temporal separation of such ultrafast helicitydependent surface currents from photoinduced thermoelectric and drift currents in the bulk. Our results uncover the functionality of ultrafast optoelectronic devices based on surface currents in topological insulators. DOI: 10.1038/ncomms7617

Positivity of linear maps under tensor powers
2015
Abstract: We investigate linear maps between matrix algebras that remain positive under tensor powers, i.e., under tensoring with n copies of themselves. Completely positive and completely copositive maps are trivial examples of this kind. We show that for every n∈\mathbbN there exist nontrivial maps with this property and that for twodimensional Hilbert spaces there is no nontrivial map for which this holds for all n. For higher dimensions we reduce the existence question of such nontrivial "tensorstable positive maps" to a oneparameter family of maps and show that an affirmative answer would imply the existence of NPPT bound entanglement. As an application we show that any tensorstable positive map that is not completely positive yields an upper bound on the quantum channel capacity, which for the transposition map gives the wellknown cbnorm bound. We furthermore show that the latter is an upper bound even for the LOCCassisted quantum capacity, and that moreover it is a strong converse rate for this task. 
Linear and Nonlinear Response of Lithographically Defined Plasmonic Nanoantennas
Proceedings of SPIE 9371, 93711D (2015)
2015
Abstract: We present numerical studies, nanofabrication and optical characterization of bowtie nanoantennas demonstrating their superior performance with respect to the electric field enhancement as compared to other Au nanoparticle shapes. For optimized parameters, we found mean intensity enhancement factors >2300x in the feedgap of the antenna, decreasing to 1300x when introducing a 5nm titanium adhesion layer. Using electron beam lithography we fabricated gold bowties on various substrates with feedgaps and tip radii as small as 10nm. In polarization resolved measurement we experimentally observed a blue shift of the surface plasmon resonance from 1.72eV to 1.35eV combined with a strong modification of the electric field enhancement in the feedgap. Under excitation with a 100fs pulsed laser source, we observed nonlinear light emission arising from twophoton photoluminescence and second harmonic generation from the gold. The bowtie nanoantenna shows a high potential for outstanding conversion efficiencies and the enhancement of other optical effects which could be exploited in future nanophotonic devices. DOI: 10.1117/12.2079104

Observation of manybody localization of interacting fermions in a quasirandom optical lattice
2015
Abstract: We experimentally observe manybody localization of interacting fermions in a onedimensional quasirandom optical lattice. We identify the manybody localization transition through the relaxation dynamics of an initiallyprepared charge density wave. For sufficiently weak disorder the time evolution appears ergodic and thermalizing, erasing all remnants of the initial order. In contrast, above a critical disorder strength a significant portion of the initial ordering persists, thereby serving as an effective order parameter for localization. The stationary density wave order and the critical disorder value show a distinctive dependence on the interaction strength, in agreement with numerical simulations. We connect this dependence to the ubiquitous logarithmic growth of entanglement entropy characterizing the generic manybody localized phase. 
Thermofieldbased chain mapping approach for open quantum systems
2015
Abstract: We consider a thermofield approach to analyze the evolution of an open quantum system coupled to an environment at finite temperature. In this approach, the finite temperature environment is exactly mapped onto two virtual environments at zero temperature. These two environments are then unitarily transformed into two different chains of oscillators, leading to a one dimensional structure that can be numerically studied using tensor network techniques. 
Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas
Phys. Rev. Lett.
, Volume 112,, page: 143003
April
2014
Abstract: The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on geometry of the Fermi surface. Superradiant light generation and selforganization of the atoms can be achieved at low pumping threshold due to resonant atomphoton Umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands, that, despite strong atomlight coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particlehole continuum and the suppression of inhomogeneous broadening and thermal fluctuations in the collisionless Fermi gas. 
Unambiguous determination of spin dephasing times in ZnO
Phys. Status Solidi B
, Volume 251,(9), page: 1861
April
2014
Abstract: Timeresolved magnetooptics is a wellestablished optical pump probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T_2^* can easily be determined if their values are comparable to the available pumpprobedelays. If T_2^* exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T_2^*. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T_2^* and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature dependent T_2^* of a single spin ensemble, while the two spin species have T_2^* values which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T_2^* values extracted from delay line scans, it results in seemingly shorter T_2^* values in RSA. 
Resonances and Partial Delocalization on the Complete Graph
2014
Abstract: Random operators may acquire extended states formed from a multitude of mutually resonating local quasimodes. This mechanics is explored here in the context of the random Schrödinger operator on the complete graph. The operators exhibits local quasi modes mixed through a single channel. While most of its spectrum consists of localized eigenfunctions, under appropriate conditions it includes also bands of states which are delocalized in the \ell^1though not in \ell^2sense, where the eigenvalues have the statistics of \vSeba spectra. The analysis proceeds through some general observations on the scaling limits of random functions in the HerglotzPick class. The results are in agreement with a heuristic condition for the emergence of resonant delocalization, which is stated in terms of the tunneling amplitude among quasimodes. 
Decoherence of an entangled state of a stronglycorrelated double quantum dot structure through tunneling processes
2014
Abstract: We consider two quantum dots described by the Andersonimpurity model with one electron per dot. The goal of our work is to study the decay of a maximally entangled state between the two electrons localized in the dots. We prepare the system in a perfect singlet and then tunnelcouple one of the dots to leads, which induces the nonequilibrium dynamics. We identify two cases: if the leads are subject to a sufficiently large voltage and thus a finite current, then direct tunneling processes cause decoherence and the entanglement as well as spin correlations decay exponentially fast. At zero voltage or small voltages and beyond the mixedvalence regime, virtual tunneling processes dominate and lead to a slower loss of coherence. We analyze this problem by studying the realtime dynamics of the spin correlations and the concurrence using two techniques, namely the timedependent density matrix renormalization group method and a masterequation method. The results from these two approaches are in excellent agreement in the directtunneling regime for the case in which the dot is weakly tunnelcoupled to the leads. We present a quantitative analysis of the decay rates of the spin correlations and the concurrence as a function of tunneling rate, interaction strength, and voltage. 
Meanfield phase diagram of the BoseFermi Hubbard model
Phys. Rev. B
, Volume 89, page: 094502
2014
Abstract: We analyze the groundstate properties of mixtures consisting of scalar bosons and spin12 fermions using a meanfield treatment of the local bosonfermion interaction on a simple cubic lattice. In the deep superfluid limit of the boson sector and the BCS regime of the fermion sector, we derive BCStype equations to determine the phase diagram of the system. We find a competition between a charge density wave and a superconducting phase. In the opposite limit, we study the Mottinsulatortosuperfluid transition of the boson sector in the presence of a staggered densityinduced alternating potential provided by the fermions, and determine the meanfield transition line. In the twosuperfluids phase of the mixture, we restrict to nearestneighborinduced interactions between the fermions and consider the extended Hubbard model. We perform a meanfield analysis of the critical temperature for the formation of bosonassisted s, extended s−, d, and pwave pairs at fermionic halffilling. We compare our results with a recent dynamical meanfield study [P. Anders et al., Phys. Rev. Lett. 109, 206401 (2012)]. 
Universal Conductivity in a TwoDimensional SuperfluidtoInsulator Quantum Critical System
Phys. Rev. Lett.
, Volume 112, page: 030402
2014
Abstract: We compute the universal conductivity of the (2+1)dimensional XY universality class, which is realized for a superfluidtoMott insulator quantum phase transition at constant density. Based on largescale Monte Carlo simulations of the classical (2+1)dimensional Jcurrent model and the twodimensional BoseHubbard model, we can precisely determine the conductivity on the quantum critical plateau, σ(∞)=0.359(4)σQ with σQ the conductivity quantum. The universal conductivity curve is the standard example with the lowest number of components where the bottomsup AdS/CFT correspondence from string theory can be tested and made to use [R. C. Myers, S. Sachdev, and A. Singh, Phys. Rev. D 83, 066017 (2011)]. For the first time, the shape of the σ(iωn)−σ(∞) function in the Matsubara representation is accurate enough for a conclusive comparison and establishes the particlelike nature of charge transport. We find that the holographic gaugegravity duality theory for transport properties can be made compatible with the data if temperature of the horizon of the black brane is different from the temperature of the conformal field theory. The requirements for measuring the universal conductivity in a cold gas experiment are also determined by our calculation. 
Dipoles in Graphene Have Infinitely Many Bound States
2014
Abstract: We show that in graphene charge distributions with nonvanishing dipole moment have infinitely many bound states. The corresponding eigenvalues accumulate at the edges of the gap faster than any power. 
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