Dr. Jan Fischer
ContactInstitut für Theoretische PhysikUniversität Regensburg D93040 Regensburg Germany

Short Biography
I obtained my Diploma in Physics from the University of Freiburg i.Br. (Germany) in 2006.My diploma thesis is entitled "Projectionoperator methods for the analysis of spinbath dynamics" and was supervised by HeinzPeter Breuer.
From 2006 to 2010, I was a PhD student in the Condensed Matter Theory group at the University of Basel (Switzerland), under the supervision of Daniel Loss.
My PhD thesis is entitled "Spin Decoherence of Electrons and Holes in Semiconductor Quantum Dots".
Since October 2010, I am a postdoctoral researcher in the Complex Quantum Systems group of Klaus Richter at the University of Regensburg (Germany).
Research Interests
Spin interactions and spin dynamics in semiconductor nanostructures» Spin dynamics of electrons and holes in quantum dots
» Spinorbit and hyperfine interactions
» Spin and charge transport in twodimensional electron and hole systems
Analytical methods for solving spin problems
» Markovian and nonMarkovian master equations
» Semiclassical and diagrammatic descriptions of mesoscopic systems
Publications
Show all abstracts.1.  Hybridization and spin decoherence in heavyhole quantum dots 
Jan Fischer and Daniel Loss. Phys. Rev. Lett. 105, 266603 (2010); arXiv:1009.5195.
We theoretically investigate the spin dynamics of a heavy hole confined to an unstrained IIIV
semiconductor quantum dot and interacting with a narrowed nuclearspin bath. We show that band
hybridization leads to an exponential decay of holespin superpositions due to hyperfinemediated nuclear pair flips, and that the accordant singleholespin decoherence time T2 can be tuned over many orders of magnitude by changing external parameters. In particular, we show that, under experimentally accessible conditions, it is possible to suppress hyperfinemediated nuclearpairflip processes so strongly that holespin quantum dots may be operated beyond the â€˜ultimate limitationâ€™ set by the hyperfine interaction which is present in other spinqubit candidate systems.
 
2.  Freeinduction decay and envelope modulations in a narrowed nuclear spin bath 
W. A. Coish, Jan Fischer, and Daniel Loss. Phys. Rev. B 81, 165315 (2010); arXiv:0911.4149.
We evaluate freeinduction decay for the transverse components of a localized
electron spin coupled to a bath of nuclear spins via the Fermi contact
hyperfine interaction. Our perturbative treatment is valid for special
(narrowed) bath initial conditions and when the Zeeman energy of the electron
b exceeds the total hyperfine coupling constant A. Using one unified
and systematic method, we recover previous results reported at short and long
times using different techniques. We find a new and unexpected modulation of
the freeinductiondecay envelope, which is present even for a purely isotropic
hyperfine interaction without spin echoes and for a single nuclear species. We
give subleading corrections to the decoherence rate, and show that, in
general, the decoherence rate has a nonmonotonic dependence on electron Zeeman
splitting, leading to a pronounced maximum. These results illustrate the
limitations of methods that make use of leadingorder effective Hamiltonians
and reexponentiation of shorttime expansions for a stronglyinteracting
system with nonMarkovian (historydependent) dynamics.
 
3.  Hyperfine interaction and electronspin decoherence in graphene and carbon nanotube quantum dots 
Jan Fischer, Bjoern Trauzettel, and Daniel Loss. Phys. Rev. B 80, 155401 (2009); arXiv:0906.2800.
We analytically calculate the nuclearspin interactions of a single electron confined to a carbon nanotube or graphene quantum dot. While the conductionband states in graphene are ptype, the accordant states in a carbon nanotube are sphybridized due to curvature. This leads to an interesting interplay between isotropic and anisotropic hyperfine interactions. By using
only analytical methods, we are able to show how the interaction strength depends on important physical parameters, such as curvature and isotope abundances. We show that for the investigated carbon structures, the 13C hyperfine coupling strength is less than 1 mueV, and that the associated electronspin decoherence time can be expected to be several tens of microseconds or longer, depending on the abundance of spincarrying 13C nuclei. Furthermore, we find that the hyperfineinduced Knight shift is highly anisotropic, both in graphene and in nanotubes of arbitrary chirality.
 
4.  Dealing with Decoherence 
Jan Fischer and Daniel Loss. Science 324, 1277 (2009)
The dream of building computers that work according to the rules of quantum mechanics has strongly driven research over the past decade in many fields of basic and applied sciences, including physics, chemistry, and computer science. About 10 years ago, it was shown mathematically that the direct use of quantum phenomena such as interference and entanglement could crucially speed up data searching and prime factorization for encryption. To turn quantum computers into reality, however, many issues in engineering and in basic physics need to be addressed.
 
5.  Spin interactions, relaxation and decoherence in quantum dots 
Jan Fischer, Mircea Trif, W. A. Coish, and Daniel Loss. Solid State Communications 149, 1443 (2009); arXiv:0903.0527.
We review recent studies on spin decoherence of electrons and holes in quasitwodimensional quantum dots, as well as electronspin relaxation in nanowire quantum dots. The spins of confined electrons and holes are considered major candidates for the realization of quantum information storage and processing devices, provided that sufficently long coherence and relaxation
times can be achieved. The results presented here indicate that this prerequisite might be realized in both electron and hole quantum dots, taking one large step towards quantum computation with spin qubits.
 
6.  Spin decoherence of a heavy hole coupled to nuclear spins in a quantum dot 
Jan Fischer, W. A. Coish, D. V. Bulaev, and Daniel Loss. Phys. Rev. B 78, 155329 (2008); arXiv:0807.0386.
We theoretically study the interaction of a heavy hole with nuclear spins in a quasitwodimensional IIIV semiconductor quantum dot and the resulting dephasing of heavyhole spin states. It has frequently been stated in the literature that heavy holes have a negligible interaction with nuclear spins. We show that this is not the case. In contrast, the interaction can be rather
strong and will be the dominant source of decoherence in some cases. We also show that for unstrained quantum dots the form of the interaction is Isinglike, resulting in unique and interesting decoherence properties, which might provide a crucial advantage to using dotconfined hole spins for quantum information processing, as compared to electron spins.
 
7.  Exponential decay in a spin bath 
W. A. Coish, Jan Fischer, and Daniel Loss. Phys. Rev. B 77, 125329 (2008); arXiv:0710.3762.
We show that the coherence of an electron spin interacting with a bath of nuclear spins can exhibit a welldefined purely exponential decay for special (`narrowed') bath initial conditions in the presence of a strong applied magnetic field. This is in contrast to the typical case, where spinbath dynamics have been investigated in the nonMarkovian limit, giving superexponential or powerlaw decay of correlation functions. We calculate the
relevant decoherence time T_2 explicitly for freeinduction decay and find a simple expression with dependence on bath polarization, magnetic field, the shape of the electron wave function, dimensionality, total nuclear spin I, and isotopic concentration for experimentally relevant heteronuclear spin systems.
 
8.  Correlated projection operator approach to nonMarkovian dynamics in spin baths 
Jan Fischer and HeinzPeter Breuer. Phys. Rev. A 76, 052119 (2007); arXiv:0708.0410.
The dynamics of an open quantum system is usually studied by performing a weakcoupling and weakcorrelation expansion in the systembath interaction. For systems exhibiting strong couplings and highly nonMarkovian behavior this approach is not justified. We apply a recently proposed correlated projection superoperator technique to the model of a central spin coupled to a spin bath
via full Heisenberg interaction. Analytical solutions to both the
NakajimaZwanzig and the timeconvolutionless master equation are determined and compared with the results of the exact solution. The correlated projection operator technique significantly improves the standard methods and can be applied to many physical problems such as the hyperfine interaction in a quantum dot.
