Probing the strongly driven spinboson model in a
superconducting quantum circuit
L. Magazzu, P. FornDíaz, 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 twolevel 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 spinboson model, describing a
twostate 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 spinboson model. We show that the
drive reinforces environmental suppression of quantum coherence, and that a coherentto
incoherent transition can be achieved by tuning the drive amplitude. An outofequilibrium
detailed balance relation is demonstrated. These results advance fundamental understanding
of open quantum systems and bear potential for the design of entangled lightmatter states.
https://www.nature.com/articles/s4146701803626w
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 allelectric 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 currentvoltage characteristics, with missing current steps and current suppression which depend on the polarity of the applied sourcedrain bias. Our results demonstrate for the first time coherentpopulation trapping by allelectric means in an artificial atom.
https://arxiv.org/abs/1804.02234
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 pwave superconductors hosting Majorana quasiparticles (MQPs) is nowadays of particular interest, also in view of the possible utilization of MQPs in faulttolerant topological quantum computation. In quasionedimensional 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 swave superconductors, e.g., exfoliated NbSe2, satisfy these needs. By precise numerical tightbinding calculations in the real space, we show the emergence of localized zeroenergy 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 fourband model in the kspace, 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.
https://epub.uniregensburg.de/37127/1/PhysRevB.97.075141.pdf
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 finitelength carbon nanotubes in the presence of onsite and nearestneighbor superconducting pairing terms. A onedimensional laddertype lattice model is developed to explore the lowenergy 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 curvatureinduced Dirac point shift and strong superconducting coupling between nearestneighbor 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 nearestneighbor pair potential (relative to the onsite pair potential). A onedimensional continuum model reveals the topological origin of the zero energy edge states: a bulkedge 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.
https://journals.aps.org/prb/pdf/10.1103/PhysRevB.96.125414
Apparent Reversal of Molecular Orbitals Reveals Entanglement
Ping Yu, Nemanja Kocic, , Benjamin Siegert, Jascha Repp and Andrea Donarini
The frontier orbital sequence of individual dicyanovinylsubstituted 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 singleparticle interpretation, are explained
by a manybody theory predicting a strongly entangled doubly charged ground state.
https://arxiv.org/abs/1704.02282
Boundary effects and correlations
in onedimensional systems
June 12, 2017
Aim of the workshop is to bring together and stimulate indepth discussions with leading
experimentalists and theoreticians working on emerging phenomena in finite
onedimensional correlated conductors. Focus is on the interplay between topological
properties and electron correlations in nanowires, carbon nanotubes, as well as on the edges
of twodimensional topological insulators (TI) and the surfaces states of threedimensional 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.
