Hugo Ribeiro
ContactDepartment of PhysicsUniversity of Basel Klingelbergstrasse 82 CH4056 Basel, Switzerland

Curriculum vitae
 04/2013present
 01/201203/2013
 01/2012
 05/200801/2012
 09/200704/2008
 03/2007
 20022007
Postdoctoral Fellow, Physics, University of Basel, Switzerland Condensend Matter Theory and Quantum Computing. Prof. Dr. Daniel Loss 

Postdoctoral Fellow, Physics, University of Konstanz, Germany Condensend Matter Theory Group and Quantum Information. Prof. Dr. Guido Burkard 

PhD, Physics, University of Konstanz, Germany  
Graduate Student, Department of Physics, University of Konstanz, Germany Condensend Matter Theory Group and Quantum Information. Prof. Dr. Guido Burkard 

Graduate Student, Department of Physics, RWTH Aachen, Germany Institute of Theoretical Physics C. Prof. Dr. Guido Burkard 

Master of Science, Physics, EPFL, Switzerland Institute of Theoretical Physics. Prof. Dr. Dmitri Ivanov 

Undergraduate Student, Physics, EPFL, Switzerland 
Research interests
 Electron spin qubits in seminconductor quantum dots
 Theory of decoherence
 Nuclear spins in semiconductor quantum dots
 LandauZener(StückelbergMajorana) Interferometry
Publications
Show all abstracts.1.  Nuclear Spin Diffusion Mediated by Heavy Hole Hyperfine NonCollinear Interactions 
Hugo Ribeiro, Franziska Maier, and Daniel Loss. arXiv:1403.0490
We show that the hyperfine mediated dynamics of heavy hole states confined in neutral self assembled quantum dots leads to a nuclear spin diffusion mechanism. It is found that the oftentimes neglected effective heavy hole hyperfine noncollinear interaction is responsible for the low degree of nuclear spin polarization in neutral quantum dots. Moreover, our results demonstrate that after pumping the nuclear spin state is left in a complex mixed state, from which it is not straightforward to deduce the sign of the Isinglike interactions.
 
2.  Quantum limit for nuclear spin polarization in semiconductor quantum dots 
Julia Hildmann, Eleftheria Kavousanaki, Guido Burkard, and Hugo Ribeiro. arXiv:1310.7819
A recent experiment [E. A. Chekhovich et al., Phys. Rev. Lett. 104, 066804 (2010)] has demonstrated that high nuclear spin polarization can be achieved in selfassembled quantum dots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not achieved as expected from a classical rate equation. Here, we theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be achieved due to formation of a nuclear dark state. Moreover, we show that the maximal degree of polarization depends on structural properties of the quantum dot.
 
3.  Interplay of charge and spin coherence in LandauZenerStückelbergMajorana interferometry 
Hugo Ribeiro, J. R. Petta, and Guido Burkard. Phys. Rev. B 87, 235318 (2013).
We study LandauZener dynamics in a double quantum dot filled with two electrons, where the spin states can become correlated with charge states and the level velocity can be tuned in a timedependent fashion. We show that a correct interpretation of experimental data is only possible when finitetime effects are taken into account. In addition, our formalism allows the study of partial adiabatic dynamics in the presence of phononmediated hyperfine relaxation and chargenoiseinduced dephasing. Our findings demonstrate that charge noise severely impacts the visibility of LandauZenerStückelbergMajorana interference fringes. This indicates that charge coherence must be treated on an equal footing with spin coherence.
 
4.  Nuclear spins keep coming back 
Hugo Ribeiro and Guido Burkard. Nature Materials 12(6), 469471 (2013).
Semiconducting quantum dots have been extensively investigated with the idea of using single spins for quantum computing. While access to single electrons and their spins has become routine, the challenges posed by nuclear spins remain ever present.
 
5.  Coherent Adiabatic Spin Control in the Presence of Charge Noise Using Tailored Pulses 
Hugo Ribeiro, Guido Burkard, J. R. Petta, H. Lu, and A. C. Gossard. Phys. Rev. Lett. 110, 086804 (2013).
We study finitetime LandauZener transitions at a singlettriplet level crossing in a GaAs double quantum dot, both experimentally and theoretically. Sweeps across the anticrossing in the high driving speed limit result in oscillations with a small visibility. Here we demonstrate how to increase the oscillation visibility while keeping sweep times shorter than T2* using a tailored pulse with a detuning dependent level velocity. Our results show an improvement of a factor of ∼2.9 for the oscillation visibility. In particular, we were able to obtain a visibility of ∼0.5 for Stückelberg oscillations, which demonstrates the creation of an equally weighted superposition of the qubit states.
 
6.  Theory of electron and nuclear spins in IIIV semiconductor and carbonbased dots 
Hugo Ribeiro and Guido Burkard. Quantum Dots: Optics, Electron Transport and Future Applications, edited by A. Tartakovskii (Cambridge Univ. Press) , 277 (2012).  
7.  Harnessing the GaAs quantum dot nuclear spin bath for quantum control 
Hugo Ribeiro, J. R. Petta, and Guido Burkard. Phys. Rev. B 82, 115445 (2010).
We theoretically demonstrate that nuclear spins can be harnessed to coherently control twoelectron spin states in a double quantum dot. Hyperfine interactions lead to an avoided crossing between the spin singlet state and the ms=+1 triplet state, T+. We show that a coherent superposition of singlet and triplet states can be achieved using finitetime LandauZenerStückelberg interferometry. In this system the coherent rotation rate is set by the Zeeman energy, resulting in ∼1 ns single spin rotations. We analyze the coherence of this spin qubit by considering the coupling to the nuclear spin bath and show that T2∗∼16 ns, in good agreement with experimental data. Our analysis further demonstrates that efficient single qubit and twoqubit control can be achieved using LandauZenerStückelberg interferometry.
 
8.  Nuclear State Preparation via LandauZenerStueckelberg transitions in Double Quantum Dots 
Hugo Ribeiro and Guido Burkard. Phys. Rev. Lett. 102, 216802 (2009).
We theoretically model a nuclearstate preparation scheme that increases the coherence time of a twospin qubit in a double quantum dot. The twoelectron system is tuned repeatedly across a singlettriplet levelanticrossing with alternating slow and rapid sweeps of an external bias voltage. Using a LandauZenerStueckelberg model, we find that in addition to a small nuclear polarization that weakly affects the electron spin coherence, the slow sweeps are only partially adiabatic and lead to a weak nuclear spin measurement and a nuclearstate narrowing which prolongs the electron spin coherence. This resolves some open problems brought up by a recent experiment [D. J. Reilly et al., Science 321, 817 (2008).]. Based on our description of the weak measurement, we simulate a system with up to n=200 nuclear spins per dot. Scaling in n indicates a stronger effect for larger n.
 
9.  Single hole and vortex excitations in the doped RokhsarKivelson quantum dimer model on the triangular lattice 
Hugo Ribeiro, Samuel Bieri, and Dmitri Ivanov. Phys. Rev. B 76, 172301 (2007).
We consider the doped RokhsarKivelson quantum dimer model on the triangular lattice with one mobile hole (monomer) at the RokhsarKivelson point. The motion of the hole is described by two branches of excitations: the hole may either move with or without a trapped Z2 vortex (vison). We perform a study of the hole dispersion in the limit where the hole hopping amplitude is much smaller than the interdimer interaction. In this limit, the hole without vison moves freely and has a tightbinding spectrum. On the other hand, the hole with a trapped vison is strongly constrained due to interference effects and can only move via higherorder virtual processes.

Invited Talks
 Interplay of Charge and Spin Coherence in LandauZener Interferometry
APS March Meeting, Baltimore, USA, March, 2013.  Interplay of Charge and Spin Coherence in LandauZener Interferometry
LMU Munich, Germany, October 22, 2012.  Interplay of Charge and Spin Coherence in LandauZener Interferometry
Okinawa Institute of Science and Technology, Okinawa, Japan, September 18, 2012.  Interplay of Charge and Spin Coherence in LandauZener Interferometry
ICPS 2012, Zurich, July 31, 2012.  Nuclear State Preparation and Electron Spin Manipulation via LandauZenerStückelberg Interferometry
University of Princeton, Princeton, USA, March 29, 2011.