# Quantum Computation and Information Seminar

## 30/10/2015, 14:00 — 15:00 — Room P9, Mathematics Building Hendrik Bluhm, Aachen University

### Spin qubits in gate defined GaAs quantum dots – from dephasing due to nuclear spins to scalable control

GaAs based quantum dots have played a pioneering role in the experimental development of electron spin qubits. However, one challenge is decoherence due to the unavoidable interaction of the electrons with nuclear spins.

I will discuss experiments that elucidate the role of quadrupolar coupling and g-factor anisotropy for this decoherence channel and thus complete our understanding of the intricate dynamics arising from the coupling to nuclear spins, known as the central spin problem. Contrary to what one might expect when considering spin diffusion, we find that quadrupolar effect degrade coherence via decorrelating transverse nuclear field that are coupled to the electron in second order.

In a second part, I will discuss how careful tuning of gate operations can lead to high gate fidelities ($\gt 98\%$ experimentally, $\gt 99.8 \%$ predicted based on best measured coherence time) in the face of real life experimental constraints.  Our feedback-based tuning method is of particular relevance for encoded spin qubits that employ several spins to encode a single qubit in order to simplify control or decoherence suppression as these are often not well suited for standard Rabi control.

I will conclude with an outlook of how a scalable quantum computer based on semiconductor spin qubits might look like and what challenges have to be overcome to reach that long term goal.

Supported by: Phys-Info (IT), SQIG (IT), CeFEMA and CAMGSD, with funding from FCT, FEDER and EU FP7, specifically through the Doctoral Programme in the Physics and Mathematics of Information (DP-PMI), FCT strategic projects PEst-OE/EEI/LA0008/2013 and UID/EEA/50008/2013, IT project QuSim, project CRUP-CPU CQVibes, the FP7 Coordination Action QUTE-EUROPE (600788), and the FP7 projects Landauer (GA 318287) and PAPETS (323901).