Daniel Becker
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Research Interests
 Realtime Nonequilibrium Transport through Interacting, Magnetic Quantum Dot Systems
 Micromechanical Devices Interacting with Superconducting Qubits
 Interactions in OneDimensional Bose Gases
Curriculum Vitae
 physics studies:
 Universität Kaiserslautern (2001‒2002)
 Universität Hamburg (2002‒2006)
 Diplomarbeit under supervision of Prof. Dr. D. Pfannkuche in the condensed matter theorie group of the Universität Hamburg (2006)
 Doctorate student under supervision of Prof. Dr. D. Pfannkuche (2006‒2011)
 Doctorate degree in physics (2011)
 Postdoctoral researcher in the group of Prof. Dr. Christoph Bruder at the Universität Basel (since 2011)
Publications
Show all abstracts.1.  Iterative path integral summation for nonequilibrium quantum transport 
Stephan Weiss, Roland Hützen, Daniel Becker, Jens Eckel, Reinhold Egger, and Michael Thorwart. arXiv:1304.6919
We have developed a numerically exact approach to compute realtime path integral expressions for quantum transport problems out of equilibrium. The scheme is based on a deterministic iterative summation of the path integral (ISPI) for the generating function of nonequilibrium observables of interest, e.g., the charge current or dynamical quantities of the central part. Selfenergies due to the leads, being nonlocal in time, are fully taken into account within a finite memory time, thereby including nonMarkovian effects. Numerical results are extrapolated first to vanishing (Trotter) time discretization and, second, to infinite memory time...
 
2.  Dynamic Generation of Topologically Protected SelfCorrecting Quantum Memory 
Daniel Becker, Tetsufumi Tanamoto, Adrian Hutter, Fabio L. Pedrocchi, and Daniel Loss. Phys. Rev. A 87, 042340 (2013)
We propose a scheme to dynamically realize a quantum memory based on the toric code. The code is generated from qubit systems with typical twobody interactions (Ising, XY, Heisenberg) using periodic, NMRlike, pulse sequences. It allows one to encode the logical qubits without measurements and to protect them dynamically against the time evolution of the physical qubits. A weakly coupled cavity mode mediates a longrange attractive interaction between the stabilizer operators of the toric code, thereby suppressing the creation of thermal anyons. This significantly increases the lifetime of the memory compared to the code with noninteracting stabilizers. We investigate how the fidelity, with which the toric code is realized, depends on the period length T of the pulse sequence and the magnitude of possible pulse errors. We derive an optimal period T_opt that maximizes the fidelity.
 
3.  Strategy for implementing stabilizerbased codes on solidstate qubits 
Tetsufumi Tanamoto, Vladimir M. Stojanović, Christoph Bruder, and Daniel Becker. Phys. Rev. A 87, 052305 (2013)
We present a method for implementing stabilizerbased codes with
encoding schemes of the operator quantum error correction paradigm,
e.g., the "standard" fivequbit and CSS codes, on solidstate qubits
with Ising or XYtype interactions. Using pulse
sequences, we show how to dynamically generate the effective dynamics of the
stabilizer Hamiltonian, the sum of an appropriate set of stabilizer
operators for a given code. Within this approach, the encoded states
(ground states of the stabilizer Hamiltonian) can be prepared without
measurements and preserved against both the time evolution governed by
the original qubit Hamiltonian, and errors caused
by local sources.
 
4.  Preserving universal resources for oneway quantum computing 
Tetsufumi Tanamoto, Daniel Becker, Vladimir M. Stojanović, and Christoph Bruder. Phys. Rev. A 86, 032327 (2012)
The common spin Hamiltonians such as the Ising, XY, or Heisenberg model do
not have ground states that are the graph states needed in measurementbased
quantum computation. Various highlyentangled manybody states have been
suggested as a universal resource for this type of computation, however, it is
not easy to preserve these states in solidstate systems due to their short
coherence times. Here we propose a scheme for generating a Hamiltonian that has
a cluster state as ground state. Our approach employs a series of pulse
sequences inspired by established NMR techniques and holds promise for
applications in many areas of quantum information processing.
 
5.  Nonequilibrium quantum dynamics of the magnetic Anderson model 
Daniel Becker, Stephan Weiss, Michael Thorwart, and Daniela Pfannkuche. New J. Phys. 14, 073049 (2012)
We study the nonequilibrium dynamics of a spinful singleorbital quantum dot
with an incorporated quantum mechanical spin1/2 magnetic impurity. Due to the
spin degeneracy, double occupancy is allowed, and Coulomb interaction together
with the exchange coupling of the magnetic impurity influence the dynamics. By
extending the iterative summation of realtime path integrals (ISPI) to this
coupled system, we monitor the timedependent nonequilibrium current and the
impurity spin polarization to determine features of the timedependent
nonequilibrium dynamics. We particulary focus on the deep quantum regime,
where all time and energy scales are of the same order of magnitude and no
small parameter is available. We observe a significant influence of the
nonequilibrium decay of the impurity spin polarization both in the presence
and in the absence of Coulomb interaction. The exponential relaxation is faster
for larger bias voltages, electronimpurity interactions and temperatures. We
show that the exact relaxation rate deviates from the corresponding
perturbative result. In addition, we study in detail the impurity's back action
on the charge current and find a reduction of the stationary current for
increasing coupling to the impurity. Moreover, our approach allows us to
systematically distinguish meanfield Coulomb and impurity effects from the
influence of quantum fluctuations and flipflop scattering, respectively. In
fact, we find a local maximum of the current for a finite Coulomb interaction
due to the presence of the impurity.
 
6.  Timedependent transport through a correlated quantum dot with magnetic impurity 
Daniel Becker, Stephan Weiss, Jens Eckel, Michael Thorwart, and Daniela Pfannkuche. J. Phys.: Conf. Ser. 245, 012021 (2010)
We investigate electronic and spin transport through a single level quantum dot with magnetic impurity in a symmetric forward bias setup. On the quantum dot, electrons either interact with each other due to Coulomb interaction or with the spin 1/2 magnetic impurity. For certain configurations, the tunnel coupling to the leads induces an exponential relaxation of the impurity spin, which has been prepared in a polarized state initially. Furthermore, we study the influence of the nonequilibrium transport current on the relaxation dynamics. We obtain the respective numerical result by means of the iterative summation of path integral (ISPI) scheme. Within this approach, observables of interest are calculated from a functional derivative with respect to appropriate source terms in the Keldysh partition function. The realtime path integral extends over all possible paths (i) of the impurity spin and (ii) of the Ising like fluctuating spin fields we have to introduce in order to decouple the quartic interaction term of the Anderson model. The ISPI scheme allows us to sum up all paths including the time nonlocal self energies of the leads.
 
7.  The Different Faces of Coulomb Interaction in Transport Through Quantum Dot Systems 
Benjamin Baxevanis, Daniel Becker, Johann Gutjahr, Peter Moraczewski, and Daniela Pfannkuche. Quantum Materials, Lateral Semiconductor Nanostructures, Hybrid Systems and Nanocrystals, NanoScience and Technology 2010, 79101 (2010)
Transport through quantum dot systems covers a broad range of phenomena ranging from Coulomb blockade oscillations to the Kondo effect. The role of Coulomb interaction in transport processes has many facets. It influences the electronic structure of quantum dot systems, it introduces a strong dependence on the number of charge carriers in the confined system, and, last but not least, it enhances the appearance of spin effects. In this chapter, we review the different faces of Coulomb interaction on the electronic structure of fewparticle quantum dot systems emphasizing the mutual interplay between quantum confinement, dimensionality, and charge interaction.
 
8.  Exact Solution of Strongly Interacting QuasiOneDimensional Spinor Bose Gases 
Frank Deuretzbacher, Klaus Fredenhagen, Daniel Becker, Kai Bongs, Klaus Sengstock, and Daniela Pfannkuche. Phys. Rev. Lett. 100, 160405 (2008)
We present an exact analytical solution of the fundamental system of
quasionedimensional spin1 bosons with infinite deltarepulsion. The
eigenfunctions are constructed from the wave functions of noninteracting
spinless fermions, based on Girardeau's FermiBose mapping, and from the wave
functions of distinguishable spins. We show that the spinor bosons behave like
a compound of noninteracting spinless fermions and noninteracting
distinguishable spins. This duality is especially reflected in the spin
densities and the energy spectrum. We find that the momentum distribution of
the eigenstates depends on the symmetry of the spin function. Furthermore, we
discuss the splitting of the ground state multiplet in the regime of large but
finite repulsion.
 
9.  CoulombBlocked Transport Through a Quantum Dot with SpinSplit Level: Increase of Differential Conductance Peaks by Spin Relaxation 
Daniel Becker and Daniela Pfannkuche. Phys. Rev. B 77, 205307 (2008)
Nonequilibrium transport through a quantum dot with one spinsplit
singleparticle level is studied in the cotunneling regime at low temperatures.
The Coulomb diamond can be subdivided into parts differing in at least one of
two respects: what kind of tunneling processes (i) determine the
singleparticle occupations and (ii) mainly contribute to the current. No
finite systematic perturbation expansion of the occupations and the current can
be found that is valid within the entire Coulomb diamond. We therefore
construct a nonsystematic solution, which is physically correct and
perturbative in the whole cotunneling regime, while smoothly crossingover
between the different regions. With this solution the impact of an intrinsic
spinflip relaxation on the transport is investigated. We focus on peaks in the
differential conductance that mark the onset of cotunnelingmediated sequential
transport. It is shown that these peaks are maximally pronounced at a
relaxation roughly as fast as sequential tunneling. The approach as well as the
presented results can be generalized to quantum dots with few levels.
