Dr. Jan Fischer


Institut für Theoretische Physik
Universität Regensburg
D-93040 Regensburg
office:PHY 4.1.29

email:view address

tel: +49 941 943 2025
fax:+49 941 943 4382

Short Biography

I obtained my Diploma in Physics from the University of Freiburg i.Br. (Germany) in 2006.
My diploma thesis is entitled "Projection-operator methods for the analysis of spin-bath dynamics" and was supervised by Heinz-Peter 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 post-doctoral 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
      » Spin-orbit and hyperfine interactions
      » Spin and charge transport in two-dimensional electron and hole systems

Analytical methods for solving spin problems
      » Markovian and non-Markovian master equations
      » Semiclassical and diagrammatic descriptions of mesoscopic systems


Show all abstracts.

1.  Hybridization and spin decoherence in heavy-hole 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 III-V semiconductor quantum dot and interacting with a narrowed nuclear-spin bath. We show that band hybridization leads to an exponential decay of hole-spin superpositions due to hyperfine-mediated nuclear pair flips, and that the accordant single-hole-spin 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 hyperfine-mediated nuclear-pair-flip processes so strongly that hole-spin quantum dots may be operated beyond the ‘ultimate limitation’ set by the hyperfine interaction which is present in other spin-qubit candidate systems.

2.  Free-induction 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 free-induction 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 free-induction-decay envelope, which is present even for a purely isotropic hyperfine interaction without spin echoes and for a single nuclear species. We give sub-leading corrections to the decoherence rate, and show that, in general, the decoherence rate has a non-monotonic dependence on electron Zeeman splitting, leading to a pronounced maximum. These results illustrate the limitations of methods that make use of leading-order effective Hamiltonians and re-exponentiation of short-time expansions for a strongly-interacting system with non-Markovian (history-dependent) dynamics.

3.  Hyperfine interaction and electron-spin 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 nuclear-spin interactions of a single electron confined to a carbon nanotube or graphene quantum dot. While the conduction-band states in graphene are p-type, the accordant states in a carbon nanotube are sp-hybridized 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 mu-eV, and that the associated electron-spin decoherence time can be expected to be several tens of microseconds or longer, depending on the abundance of spin-carrying 13C nuclei. Furthermore, we find that the hyperfine-induced 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 quasi-two-dimensional quantum dots, as well as electron-spin 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 quasi-two-dimensional III-V semiconductor quantum dot and the resulting dephasing of heavy-hole 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 Ising-like, resulting in unique and interesting decoherence properties, which might provide a crucial advantage to using dot-confined 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 well-defined 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 spin-bath dynamics have been investigated in the non-Markovian limit, giving super-exponential or power-law decay of correlation functions. We calculate the relevant decoherence time T_2 explicitly for free-induction 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 non-Markovian dynamics in spin baths
Jan Fischer and Heinz-Peter Breuer.
Phys. Rev. A 76, 052119 (2007); arXiv:0708.0410.

The dynamics of an open quantum system is usually studied by performing a weak-coupling and weak-correlation expansion in the system-bath interaction. For systems exhibiting strong couplings and highly non-Markovian 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 Nakajima-Zwanzig and the time-convolutionless 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.