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Department of Physics > LS Grifoni > AG Grifoni

Welcome to the homepage of the Quantum Transport and Dissipation group!

We are a theoretical group interested in the dynamical properties (transient and stationary) of open quantum systems out-of-equilibrium. These include on the one hand quantum transport properties of hybrid nanojunctions, molecular systems in STM configuration or complex quantum dots. On the other hand, the dissipative dynamics of (effective) quantum particles interacting with a surroundings. In recent years we have developed a many-body theory of quantum transport based on the reduced density matrix approach, which we have applied to investigate transport in carbon nanotube based nanojunctions and molecules. Also, we have used non-perturbative field theory approaches to investigate strongly correlated systems in the Kondo regime and interacting one-dimensional systems. Path integral approaches have recently been used to investigate decoherence and relaxation properties of superconducting qubits interacting with an electromagnetic environment.

Latest News


GRK Seminar

International GRK Workshop

19.09. - 21.09.2018
Thon-Dittmer-Palais, Regensburg, Germany

Probing the strongly driven spin-boson model in a superconducting quantum circuit

L. Magazzu, P. Forn-Díaz, R. Belyansky, J.-L. Orgiazzi, M.A. Yurtalan, M.R. Otto, A. Lupascu, C.M. Wilson and M. Grifoni
Nature Communications 9, 1403 (2018)

Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Such dissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the “ spin ” , interacting with an environment formed by harmonic oscilla- tors. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a super- conducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to- incoherent transition can be achieved by tuning the drive amplitude. An out-of-equilibrium detailed balance relation is demonstrated. These results advance fundamental understanding of open quantum systems and bear potential for the design of entangled light-matter states.

Dark states in a carbon nanotube quantum dot

A. Donarini, M. Niklas, M. Schafberger, N. Paradiso, Ch. Strunk und M. Grifoni
arXiv:1804.02234 (2018)

Illumination of atoms by resonant lasers can pump electrons into a coherent superposition of hyperfine levels which can no longer absorb the light. Such superposition is known as dark state, because fluorescent light emission is then suppressed. Here we report an all-electric analogue of this destructive interference effect in a carbon nanotube quantum dot. The dark states are a coherent superposition of valley (angular momentum) states which are decoupled from either the drain or the source leads. Their emergence is visible in asymmetric current-voltage characteristics, with missing current steps and current suppression which depend on the polarity of the applied source-drain bias. Our results demonstrate for the first time coherent-population trapping by all-electric means in an artificial atom.

Majorana quasiparticles in semiconducting carbon nanotubes

M. Marganska, L. Milz, W. Izumida, Ch. Strunk und M. Grifoni
Phys. Rev. B 97, 075141 (2018)

Engineering effective p-wave superconductors hosting Majorana quasiparticles (MQPs) is nowadays of particular interest, also in view of the possible utilization of MQPs in fault-tolerant topological quantum computation. In quasi-one-dimensional systems, the parameter space for topological superconductivity is significantly reduced by the coupling between transverse modes. Together with the requirement of achieving the topological phase under experimentally feasible conditions, this strongly restricts in practice the choice of systems which can host MQPs. Here, we demonstrate that semiconducting carbon nanotubes (CNTs) in proximity with ultrathin s-wave superconductors, e.g., exfoliated NbSe2, satisfy these needs. By precise numerical tight-binding calculations in the real space, we show the emergence of localized zero-energy states at the CNT ends above a critical value of the applied magnetic field, of which we show the spatial evolution. Knowing the microscopic wave functions, we unequivocally demonstrate the Majorana nature of the localized states. An effective four-band model in the k-space, with parameters determined from the numerical spectrum, is used to calculate the topological phase diagram and its phase boundaries in analytic form. Finally, the impact of symmetry breaking contributions, like disorder and an axial component of the magnetic field, is investigated.

Topology and zero energy edge states in carbon nanotubes with superconducting pairing

W. Izumida, L. Milz, M. Marganska, and M. Grifoni
Phys. Rev. B 96, 125414 – (2017)

We investigate the spectrum of finite-length carbon nanotubes in the presence of onsite and nearest-neighbor superconducting pairing terms. A one-dimensional ladder-type lattice model is developed to explore the low-energy spectrum and the nature of the electronic states. We find that zero energy edge states can emerge in zigzag class carbon nanotubes as a combined effect of curvature-induced Dirac point shift and strong superconducting coupling between nearest-neighbor sites. The chiral symmetry of the system is exploited to define a winding number topological invariant. The associated topological phase diagram shows regions with nontrivial winding number in the plane of chemical potential and superconducting nearest-neighbor pair potential (relative to the onsite pair potential). A one-dimensional continuum model reveals the topological origin of the zero energy edge states: a bulk-edge correspondence is proven, which shows that the condition for nontrivial winding number and that for the emergence of edge states are identical. For armchair class nanotubes, the presence of edge states in the superconducting gap depends on the nanotube's boundary shape. For the minimal boundary condition, the emergence of the subgap states can also be deduced from the winding number.

Apparent Reversal of Molecular Orbitals Reveals Entanglement

Ping Yu, Nemanja Kocic, , Benjamin Siegert, Jascha Repp and Andrea Donarini

The frontier orbital sequence of individual dicyanovinyl-substituted oligothiophene molecules is studied by means of scanning tunneling microscopy. On NaCl/ Cu(111) the molecules are neutral and the two lowest unoccupied molecular states are observed in the expected order of increasing energy. On NaCl/Cu(311), where the molecules are negatively charged, the sequence of two observed molecular orbitals is reversed, such that the one with one more nodal plane appears lower in energy. This experimental results, in open contradiction with a single-particle interpretation, are explained by a many-body theory predicting a strongly entangled doubly charged ground state.

Boundary effects and correlations in one-dimensional systems

June 1-2, 2017

Aim of the workshop is to bring together and stimulate in-depth discussions with leading experimentalists and theoreticians working on emerging phenomena in finite one-dimensional correlated conductors. Focus is on the interplay between topological properties and electron correlations in nanowires, carbon nanotubes, as well as on the edges of two-dimensional topological insulators (TI) and the surfaces states of three-dimensional TI constrictions. Despite the big interest in these systems, the investigation of the role of electron correlations in combination with topological properties is still in its infancy.

22 February 2018