Publications

Controlling several atoms in a cavity
Keyl, Michael, Zeier, Robert and Schulte Herbrüggen, Thomas
New Journal of Physics , Volume 16(6), page: 065010
2014

Abstract: We treat control of several two-level atoms interacting with one mode of the electromagnetic field in a cavity. This provides a useful model to study pertinent aspects of quantum control in infinite dimensions via the emergence of infinite-dimensional system algebras. Hence we address problems arising with infinite-dimensional Lie algebras and those of unbounded operators. For the models considered, these problems can be solved by splitting the set of control Hamiltonians into two subsets: the first obeys an Abelian symmetry and can be treated in terms of infinite-dimensional Lie algebras and strongly closed subgroups of the unitary group of the system Hilbert space. The second breaks this symmetry, and its discussion introduces new arguments. Yet, full controllability can be achieved in a strong sense: e.g., in a time dependent Jaynes Cummings model we show that, by tuning coupling constants appropriately, every unitary of the coupled system (atoms and cavity) can be approximated with arbitrarily small error.

Ac-conductivity and electromagnetic energy absorption for the Anderson model in linear response theory
Klein, Abel and Müller, Peter
2014

Abstract: We continue our study of the ac-conductivity in linear response theory for the Anderson model using the conductivity measure. We establish further properties of the conductivity measure, including nontriviality at nonzero temperature, the high temperature limit, and asymptotics with respect to the disorder. We also calculate the electromagnetic energy absorption in linear response theory in terms of the conductivity measure.

Few-cycle, Broadband, Mid-infrared Optical Parametric Oscillator Pumped by a 20-fs Ti:sapphire Laser
Chaitanya Kumar, Suddapalli, Esteban-Martin, Adolfo, Ideguchi, Takuro, Yan, Ming, Holzner, Simon, W. Haensch, Theodor, Picque, Nathalie and Ebrahim-Zadeh, Majid
2014

Abstract: We report a few-cycle, broadband, singly-resonant optical parametric oscillator (OPO) for the mid-infrared based on MgO-doped periodically-poled LiNbO3 (MgO:PPLN), synchronously pumped by a 20-fs Ti:sapphire laser. By using crystal interaction lengths as short as 250 um, and careful dispersion management of input pump pulses and the OPO resonator, near-transform-limited, few-cycle idler pulses tunable across the mid-infrared have been generated, with as few as 3.7 optical cycles at 2682 nm. The OPO can be continuously tuned over 2179-3732 nm by cavity delay tuning, providing up to 33 mW of output power at 3723 nm. The idler spectra exhibit stable broadband profiles with bandwidths spaning over 422 nm (FWHM) recorded at 3732 nm. We investigate the effect of crystal length on spectral bandwidth and pulse duration at a fixed wavelength, confirming near-transform-limited idler pulses for all grating interaction lengths. By locking the repetition frequency of the pump laser to a radio-frequency reference, and without active stabilization of the OPO cavity length, an idler power stability better than 1.6% rms over >2.75 hours is obtained when operating at maximum output power, in excellent spatial beam quality with TEM00 mode profile.

Simplex valence-bond crystal in the spin-1 kagome Heisenberg antiferromagnet
Liu, Tao, Li, Wei, Weichselbaum, Andreas, von Delft, Jan and Su, Gang
2014

Abstract: We investigate the ground state properties of a spin-1 kagome antiferromagnetic Heisenberg (KAH) model using tensor-network methods. We find a trimerized ground state, with energy per site e_0\simeq-1.409 obtained by accurate calculations directly in the thermodynamic limit. The symmetry between left and right triangles is spontaneously broken, with a relative energy difference of δ ≈ 20%. The spin-spin, dimer-dimer, and chiral correlation functions are found to decay exponentially with a rather short correlation length, showing that the ground state is gapped. Based on this unambiguous numerical evidence, we identify the ground state of the spin-1 KAH model to be a simplex valence-bond crystal (SVBC). Besides the KAH model, we also discuss the spin-1 bilinear-biquadratic Heisenberg model on a kagome lattice, and determine its ground state phase diagram. In particular, we find a quantum phase transition between the SVBC and ferro-quadrupolar nematic states.

Algorithms for finite Projected Entangled Pair States
Lubasch, Michael, Ignacio Cirac, J. and Banuls, Mari-Carmen
Phys. Rev. B , Volume 90,, page: 064425
2014

Abstract: Projected Entangled Pair States (PEPS) are a promising ansatz for the study of strongly correlated quantum many-body systems in two dimensions. But due to their high computational cost, developing and improving PEPS algorithms is necessary to make the ansatz widely usable in practice. Here we analyze several algorithmic aspects of the method. On the one hand, we quantify the connection between the correlation length of the PEPS and the accuracy of its approximate contraction, and discuss how purifications can be used in the latter. On the other, we present algorithmic improvements for the update of the tensor that introduce drastic gains in the numerical conditioning and the efficiency of the algorithms. Finally, the state-of-the-art general PEPS code is benchmarked with the Heisenberg and quantum Ising models on lattices of up to 21 × 21 sites.

A carrier relaxation bottleneck probed in single InGaAs quantum dots using integrated superconducting single photon detectors
Reithmaier, G., Flassig, F., Hasch, P., Lichtmannecker, S., Müller, K., Vučković, J., Gross, R., Kaniber, M. and Finley, J. J.
Applied Physics Letters , Volume 105(8), page: -
2014

Abstract: Using integrated superconducting single photon detectors, we probe ultra-slow exciton capture and relaxation dynamics in single self-assembled InGaAs quantum dots embedded in a GaAs ridge waveguide. Time-resolved luminescence measurements performed with on- and off-chip detection reveal a continuous decrease in the carrier relaxation time from 1.22 ± 0.07 ns to 0.10 ± 0.07 ns upon increasing the number of non-resonantly injected carriers. By comparing off-chip time-resolved spectroscopy with spectrally integrated on-chip measurements, we identify the observed dynamics in the rise time (τr ) as arising from a relaxation bottleneck at low excitation levels. From the comparison with the temporal dynamics of the single exciton transition with the on-chip emission signal, we conclude that the relaxation bottleneck is circumvented by the presence of charge carriers occupying states in the bulk material and the two-dimensional wetting layer continuum. A characteristic τr ∝ P −2∕3 power law dependence is observed suggesting Auger-type scattering between carriers trapped in the quantum dot and the two-dimensional wetting layer continuum which circumvents the phonon relaxation bottleneck.

Polarization dependent, surface plasmon induced photoconductance in gold nanorod arrays
Diefenbach, S., Erhard, N., Schopka, J., Martin, A., Karnetzky, C., Iacopino, D. and Holleitner, A. W.
physica status solidi (RRL) – Rapid Research Letters , Volume 8(3), page: 264--268
2014
ISSN: 1862-6270

Abstract: We report on the photoconductance in two-dimensional arrays of gold nanorods. The arrays are formed by a combination of droplet deposition and stamping methods. We find that the plasmon induced photoconductance is sensitive to the linear polarization of the exciting photons consistent with the excitation of the longitudinal surface plasmon resonance of the nanorods.

Keywords: surface plasmons, optical sensors, nanorods, photoconductance, gold

Optical Thermometry of an Electron Reservoir Coupled to a Single Quantum Dot in the Millikelvin Range
Seilmeier, F., Hauck, M., Schubert, E., Schinner, G. J., Beavan, S. E. and Högele, A.
Phys. Rev. Applied , Volume 2, page: 024002
2014

Abstract: We show how resonant laser spectroscopy of the trion optical transitions in a self-assembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field, the spin-state occupation of the Zeeman-split quantum-dot electron ground states is governed by thermalization with the electron reservoir via cotunneling. With resonant spectroscopy of the corresponding excited trion states, we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the subkelvin temperature range where it is most sensitive and where the electron temperature is not necessarily given by the cryostat base temperature.

Sub-Kelvin optical thermometry of an electron reservoir coupled to a self-assembled InGaAs quantum dot
Seilmeier, F., Hauck, M., Schubert, E., J. Schinner, G., E. Beavan, S. and H, A.
Phys. Rev. Applied , Volume 2,, page: 024002
2014

Abstract: We show how resonant laser spectroscopy of the trion optical transitions in a self-assembled quantum dot can be used to determine the temperature of a nearby electron reservoir. At finite magnetic field the spin-state occupation of the Zeeman-split quantum dot electron ground states is governed by thermalization with the electron reservoir via co-tunneling. With resonant spectroscopy of the corresponding excited trion states we map out the spin occupation as a function of magnetic field to establish optical thermometry for the electron reservoir. We demonstrate the implementation of the technique in the sub-Kelvin temperature range where it is most sensitive, and where the electron temperature is not necessarily given by the cryostat base temperature.

Locating environmental charge impurities with confluent laser spectroscopy of multiple quantum dots
Hauck, M., Seilmeier, F., Beavan, S. E., Badolato, A., Petroff, P. M. and Högele, A.
Phys. Rev. B , Volume 90, page: 235306
2014
Exponential vanishing of the ground-state gap of the QREM via adiabatic quantum computing
Adame, Juan and Warzel, Simone
2014

Abstract: In this note we compile and slightly generalise ideas of Farhi, Goldstone, Gosset, Gutmann, Nagaj and Shor by discussing a lower bound on the run time of their quantum adiabatic search algorithm and its use for an upper bound on the energy gap above the ground-state of the generators of this algorithm. We illustrate these ideas by applying them to the quantum random energy model (QREM). Our main result is a simple proof of the conjectured exponential vanishing of the energy gap of the QREM.

Two-bath spin-boson model: Phase diagram and critical properties
Bruognolo, Benedikt, Weichselbaum, Andreas, Guo, Cheng, von Delft, Jan, Schneider, Imke and Vojta, Matthias
Phys. Rev. B , Volume 90,, page: 245130
2014

Abstract: The spin-boson model, describing a two-level system coupled to a bath of harmonic oscillators, is a generic model for quantum dissipation, with manifold applications. It has also been studied as a simple example for an impurity quantum phase transition. Here we present a detailed study of a U(1)-symmetric two-bath spin-boson model, where two different components of an SU(2) spin 1/2 are coupled to separate dissipative baths. Non-trivial physics arises from the competition of the two dissipation channels, resulting in a variety of phases and quantum phase transitions. We employ a combination of analytical and numerical techniques to determine the properties of both the stable phases and the quantum critical points. In particular, we find a critical intermediate-coupling phase which is bounded by a continuous quantum phase transition which violates the quantum-to-classical correspondence.

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