Beat Röthlisberger
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

CV
2004  Bachelor in Physics from the University of Basel 
2006  Master in Physics from the University of Basel 
2006  present  PhD student 
Research interests
 Entanglement production and quantification in multipartite systems
 Optimization on nonEuclidian manifolds
 Topological quantum computing, quantum memories
Publications
Show all abstracts.1.  Incoherent dynamics in the toric code subject to disorder 
Beat Röthlisberger, James R. Wootton, Robert M. Heath, Jiannis K. Pachos, and Daniel Loss. Phys. Rev. A 85, 022313 (2012); arXiv:1112.1613.
We numerically study the effects of two forms of quenched disorder on the
anyons of the toric code. Firstly, a new class of codes based on random
lattices of stabilizer operators is presented, and shown to be superior to the
standard square lattice toric code for certain forms of biased noise. It is
further argued that these codes are close to optimal, in that they tightly
reach the upper bound of error thresholds beyond which no correctable CSS codes
can exist. Additionally, we study the classical motion of anyons in toric codes
with randomly distributed onsite potentials. In the presence of repulsive
longrange interaction between the anyons, a surprising increase with disorder
strength of the lifetime of encoded states is reported and explained by an
entirely incoherent mechanism. Finally, the coherent transport of the anyons in
the presence of both forms of disorder is investigated, and a significant
suppression of the anyon motion is found.
 
2.  libCreme: An optimization library for evaluating convexroof entanglement measures 
Beat Röthlisberger, Jörg Lehmann, and Daniel Loss. Comput. Phys. Comm. 183, 155 (2012); arXiv:1107.4497.
We present the software library libCreme which we have previously used to
successfully calculate convexroof entanglement measures of mixed quantum
states appearing in realistic physical systems. Evaluating the amount of
entanglement in such states is in general a nontrivial task requiring to solve
a highly nonlinear complex optimization problem. The algorithms provided here
are able to achieve to do this for a large and important class of entanglement
measures. The library is mostly written in the Matlab programming language, but
is fully compatible to the free and opensource Octave platform. Some
inefficient subroutines are written in C/C++ for better performance. This
manuscript discusses the most important theoretical concepts and workings of
the algorithms, focussing on the actual implementation and usage within the
library. Detailed examples in the end should make it easy for the user to apply
libCreme to specific problems.
 
3.  A SelfCorrecting Quantum Memory in a Thermal Environment 
Stefano Chesi, Beat Röthlisberger, and Daniel Loss. Phys. Rev. A 82, 022305 (2010); arXiv:0908.4264.
The ability to store information is of fundamental importance to any
computer, be it classical or quantum. Identifying systems for quantum memories
which rely, analogously to classical memories, on passive error protection
('selfcorrection') is of greatest interest in quantum information science.
While systems with topological ground states have been considered to be
promising candidates, a large class of them was recently proven unstable
against thermal fluctuations. Here, we propose new twodimensional (2D) spin
models unaffected by this result. Specifically, we introduce repulsive
longrange interactions in the toric code and establish a memory lifetime
polynomially increasing with the system size. This remarkable stability is
shown to originate directly from the repulsive longrange nature of the
interactions. We study the time dynamics of the quantum memory in terms of
diffusing anyons and support all our analytical results with extensive
numerical simulations. Our findings demonstrate that selfcorrecting quantum
memories can exist in 2D at finite temperatures.
 
4.  Quantum Computing with Electron Spins in Quantum Dots 
Robert Andrzej Żak, Beat Röthlisberger, Stefano Chesi, and Daniel Loss. Lecture notes for Course CLXXI "Quantum Coherence in Solid State Systems" Int. School of Physics "Enrico Fermi", Varenna, July 2008. arXiv:0906.4045
Several topics on the implementation of spin qubits in quantum dots are
reviewed. We first provide an introduction to the standard model of quantum
computing and the basic criteria for its realization. Other alternative
formulations such as measurementbased and adiabatic quantum computing are
briefly discussed. We then focus on spin qubits in single and double GaAs
electron quantum dots and review recent experimental achievements with respect
to initialization, coherent manipulation and readout of the spin states. We
extensively discuss the problem of decoherence in this system, with particular
emphasis on its theoretical treatment and possible ways to overcome it.
 
5.  Numerical evaluation of convexroof entanglement measures with applications to spin rings 
Beat Röthlisberger, Jörg Lehmann, and Daniel Loss. Phys. Rev. A 80, 042301 (2009); arXiv:0905.3106.
We present two readytouse numerical algorithms to evaluate convexroof
extensions of arbitrary purestate entanglement monotones. Their implementation
leaves the user merely with the task of calculating derivatives of the
respective purestate measure. We provide numerical tests of the algorithms and
demonstrate their good convergence properties. We further employ them in order
to investigate the entanglement in particular fewspins systems at finite
temperature. Namely, we consider ferromagnetic Heisenberg exchangecoupled
spin1/2 rings subject to an inhomogeneous inplane field geometry obeying full
rotational symmetry around the axis perpendicular to the ring through its
center. We demonstrate that highly entangled states can be obtained in these
systems at sufficiently low temperatures and by tuning the strength of a
magnetic field configuration to an optimal value which is identified
numerically.
 
6.  Highly Entangled Ground States in Tripartite Qubit Systems 
Beat Röthlisberger, Jörg Lehmann, D. S. Saraga, Philipp Traber, and Daniel Loss. Phys. Rev. Lett. 100, 100502 (2008); arXiv:0705.1710.
We investigate the creation of highly entangled ground states in a system of
three exchangecoupled qubits arranged in a ring geometry. Suitable magnetic
field configurations yielding approximate GHZ and exact W ground states are
identified. The entanglement in the system is studied at finite temperature in
terms of the mixedstate tangle tau. By adapting a steepestdescent
optimization algorithm we demonstrate that tau can be evaluated efficiently and
with high precision. We identify the parameter regime for which the equilibrium
entanglement of the tripartite system reaches its maximum.
