Variational matrix product operators for the steady state of dissipative quantum systems
2015
Abstract: We present a new variational method, based on the matrix product operator (MPO) ansatz, for finding the steady state of dissipative quantum chains governed by master equations of the Lindblad form. Instead of requiring an accurate representation of the system evolution until the stationary state is attained, the algorithm directly targets the final state, thus allowing for a faster convergence when the steady state is a MPO with small bond dimension. Our numerical simulations for several dissipative spin models over a wide range of parameters illustrate the performance of the method and show that indeed the stationary state is often well described by a MPO of very moderate dimensions. 
Optical control of internal electric fields in bandgap graded InGaN nanowires
Nano Lett.
, Volume 15(1), page: 332–338
2015
Abstract: InGaN nanowires are suitable building blocks for many future optoelectronic devices. We show that a linear grading of the indium content along the nanowire axis from GaN to InN introduces an internal electric field evoking a photocurrent. Consistent with quantitative band structure simulations we observe a sign change in the measured photocurrent as a function of photon flux. This negative differential photocurrent opens the path to a new type of nanowirebased photodetector. We demonstrate that the photocurrent response of the nanowires is as fast as 1.5 ps. 
Ultrafast photocurrents and THz generation in single InAsnanowires
2015
Abstract: To clarify the ultrafast temporal interplay of the different photocurrent mechanisms occurring in single InAsnanowirebased circuits, an onchip photocurrent pumpprobe spectroscopy based on coplanar striplines was utilized. The data are interpreted in terms of a photothermoelectric current and the transport of photogenerated holes to the electrodes as the dominating ultrafast photocurrent contributions. Moreover, it is shown that THz radiation is generated in the optically excited InAsnanowires, which is interpreted in terms of a dominating photoDember effect. The results are relevant for nanowirebased optoelectronic and photovoltaic applications as well as for the design of nanowirebased THz sources. 
Towards onchip generation, routing and detection of nonclassical light
Proc. SPIE 9373
2015
Abstract: We fabricate an integrated photonic circuit with emitter, waveguide and detector on one chip, based on a hybrid superconductorsemiconductor system. We detect photoluminescence from selfassembled InGaAs quantum dots onchip using NbN superconducting nanowire single photon detectors. Using the fast temporal response of these detectors we perform timeresolved studies of nonresonantly excited quantum dots. By introducing a temporal ?ltering to the signal, we are able to resonantly excite the quantum dot and detect its resonance uorescence onchip with the integrated superconducting single photon detector. 
Quantum GrossPitaevskii Equation
2015
Abstract: We introduce a noncommutative generalization of the GrossPitaevskii equation for onedimensional quantum field theories. This generalization is obtained by applying the DiracFrenkel timedependent variational principle to the variational manifold of continuous matrix product states. This allows for a full quantum description of the many body system including entanglement and correlations and thus extends significantly beyond the usual meanfield description of the GrossPitaevskii equation, which is known to fail for onedimensional systems. 
Thermodynamics of the BoseHubbard model in a Bogoliubov+U theory
2015
Abstract: We derive the Bogoliubov+U formalism to study the thermodynamical properties of the BoseHubbard model. The framework can be viewed as the zerofrequency limit of bosonic dynamical meanfield theory (BDMFT), but equally well as an extension of the meanfield decoupling approximation in which pair creation and annihilation of depleted particles is taken into account. The selfenergy on the impurity site is treated variationally, minimizing the grand potential. The theory containing just 3 parameters that are determined selfconsistently reproduces the T=0 phase diagrams of the 3d and 2d BoseHubbard model with an accuracy of 1 % or better. The superfluid to normal transition at finite temperature is also reproduced well and only slightly less accurately than in BDMFT. 
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. 
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. 
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