Publications

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.

Identifying a bath-induced Bose liquid in interacting spin-boson models
Cai, Zi, Schollwöck, Ulrich and Pollet, Lode
Phys. Rev. Lett. , Volume 113,, page: 260403
2014

Abstract: We study the ground state phase diagram of a one-dimensional hard-core bosonic model with nearest-neighbor interactions (XXZ model) where every site is coupled Ohmically to an independent but identical reservoir, hereby generalizing spin-boson models to interacting spin-boson systems. We show that a bath-induced Bose metal phase can occur in the ground state phase diagram away from half filling. This phase is compressible, gapless, and conducting but not superfluid. At haf-filling, only a Luttinger liquid and a charge density wave are found. The phase transition between them is of Kosterlitz-Thouless type where the Luttinger parameter takes a non-universal value.The applied quantum Monte Carlo method can be used for all open bosonic and unfrustrated spin systems, regardless of their dimension, filling factor and spectrum of the dissipation as long as the quantum system couples to the bath via the density operators.

Highly coherent electron beam from a laser-triggered tungsten needle tip
Ehberger, Dominik, Hammer, Jakob, Eisele, Max, Kr, Michael, Noe, Jonathan, H, Alexander and Hommelhoff, Peter
2014

Abstract: We report on a quantitative measurement of the spatial coherence of electrons emitted from a sharp metal needle tip. We investigate the coherence in photoemission using near-ultraviolet laser triggering with a photon energy of 3.1 eV and compare it to DC-field emission. A carbon-nanotube is brought in close proximity to the emitter tip to act as an electrostatic biprism. From the resulting electron matter wave interference fringes we deduce an upper limit of the effective source radius both in laser-triggered and DC-field emission mode, which quantifies the spatial coherence of the emitted electron beam. We obtain (0.80\pm 0.05)\,nm in laser-triggered and (0.55\pm 0.02)\,nm in DC-field emission mode, revealing that the outstanding coherence properties of electron beams from needle tip field emitters are largely maintained in laser-induced emission. In addition, the relative coherence width of 0.36 of the photoemitted electron beam is the largest observed so far. The preservation of electronic coherence during emission as well as ramifications for time-resolved electron imaging techniques are discussed.

Frustrated Quantum Magnetism with Laser-Dressed Rydberg Atoms
W. Glaetzle, Alexander, Dalmonte, Marcello, Nath, Rejish, Gross, Christian, Bloch, Immanuel and Zoller, Peter
Phys. Rev. Lett. , Volume 114(173002)
2014

Abstract: We show how a broad class of lattice spin-1/2 models with angular- and distance-dependent couplings can be realized with cold alkali atoms stored in optical or magnetic trap arrays. The effective spin-1/2 is represented by a pair of atomic ground states, and spin-spin interactions are obtained by admixing van der Waals interactions between fine-structure split Rydberg states with laser light. The strengths of the diagonal spin interactions as well as the "flip-flop", and "flip-flip" and "flop-flop" interactions can be tuned by exploiting quantum interference, thus realizing different spin symmetries. The resulting energy scales of interactions compare well with typical temperatures and decoherence time-scales, making the exploration of exotic forms of quantum magnetism, including emergent gauge theories and compass models, accessible within state-of-the-art experiments.

Rotational state detection of electrically trapped polyatomic molecules
Gl, Rosa, Prehn, Alexander, Rempe, Gerhard and Zeppenfeld, Martin
2014

Abstract: Detecting the internal state of polar molecules is a substantial challenge when standard techniques such as resonance-enhanced multi photon ionization (REMPI) or laser-induced fluorescense (LIF) do not work. As this is the case for most polyatomic molecule species, we here investigate an alternative based on state selective removal of molecules from an electrically trapped ensemble. Specifically, we deplete molecules by driving rotational and/or vibrational transitions to untrapped states. Fully resolving the rotational state with this method can be a considerable challenge as the frequency differences between various transitions is easily substantially less than the Stark broadening in an electric trap. However, making use of a unique trap design providing homogeneous fields in a large fraction of the trap volume, we successfully discriminate all rotational quantum numbers, including the rotational M-substate.

The Ground State Energy of Heavy Atoms: the Leading Correction
Handrek, Michael and Siedentop, Heinz
2014

Abstract: For heavy atoms (large atomic number Z) described by no-pair operators in the Furry picture we find the ground state's leading energy correction. We compare the result with (semi-)empirical values and Schwinger's prediction showing more than qualitative agreement.

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