Abstract: Interaction with environment causes decoherence of qubits, which is usually an unwanted process. However, instead of worrying about this we can use it to gain valuable information about the dynamics of the environment. The procedure is especially simple when the environment can be assumed to be a source of classical Gaussian noise. When a qubit is driven by a sequence of N dynamical decoupling pulses, the measurement of its coherence at different times and different values of N can be used to reconstruct the spectral density of environmental noise that is affecting the qubit. In the case of linear coupling to Gaussian noise the relation between the coherence and the noise spectrum is very simple, and it has been successfully used to perform noise spectroscopy in many recent experiments. On the other hand, the case of quadratic coupling to Gaussian noise, which is relevant for a qubit at an optimal working point, is much more nontrivial theoretically. A solution allowing for reconstruction of noise spectrum in this case will be given.

About the Speaker: Lukasz Cywinski got his masters degree at the Faculty of Physics of Warsaw University in Poland. He did his graduate thesis under the supervision of professor Lu Jeu Sham, and obtained the PhD title in 2007, after which he worked as a postdoctoral researcher for Professor Sankar Das Sarma in the Condensed Matter Theory Center, University of Maryland at College Park. Since 2009 he is an assistant professor in the group of Professor Tomasz Dietl at the Institute of Physics, Polish Academy of Sciences. He has worked on spin qubit decoherence, spintronics, ultrafast magnetization dynamics, and topological insulators based on narrow-gap semiconductors.