### Browsing by Author "Kumar, K. S."

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Item Motional averaging in a superconducting qubit(2013) Li, Jian; Silveri, M. P.; Kumar, K. S.; Pirkkalainen, J.-M.; Vepsäläinen, A.; Chien, W.C.; Tuorila, J.; Sillanpää, M.A.; Hakonen, P.J.; Thuneberg, E.V.; Paraoanu, G.S.; Department of Applied Physics; Quantum Nanomechanics; University of OuluSuperconducting circuits with Josephson junctions are promising candidates for developing future quantum technologies. Of particular interest is to use these circuits to study effects that typically occur in complex condensed-matter systems. Here we employ a superconducting quantum bit—a transmon—to perform an analogue simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance spectroscopy. By modulating the flux bias of a transmon with controllable pseudo-random telegraph noise we create a stochastic jump of its energy level separation between two discrete values. When the jumping is faster than a dynamical threshold set by the frequency displacement of the levels, the initially separate spectral lines merge into a single, narrow, motional-averaged line. With sinusoidal modulation a complex pattern of additional sidebands is observed. We show that the modulated system remains quantum coherent, with modified transition frequencies, Rabi couplings, and dephasing rates. These results represent the first steps towards more advanced quantum simulations using artificial atoms.Item Stimulated Raman adiabatic passage in a three-level superconducting circuit(2016-02-23) Kumar, K. S.; Vepsäläinen, Antti; Danilin, S.; Paraoanu, G. S.; Department of Applied Physics; Superconducting Qubits and Circuit QEDThe adiabatic manipulation of quantum states is a powerful technique that opened up new directions in quantum engineering-enabling tests of fundamental concepts such as geometrical phases and topological transitions, and holding the promise of alternative models of quantum computation. Here we benchmark the stimulated Raman adiabatic passage for circuit quantum electrodynamics by employing the first three levels of a transmon qubit. In this ladder configuration, we demonstrate a population transfer efficiency >80% between the ground state and the second excited state using two adiabatic Gaussian-shaped control microwave pulses. By doing quantum tomography at successive moments during the Raman pulses, we investigate the transfer of the population in time domain. Furthermore, we show that this protocol can be reversed by applying a third adiabatic pulse, we study a hybrid nondiabatic-adiabatic sequence, and we present experimental results for a quasi-degenerate intermediate level.Item Stückelberg interference in a superconducting qubit under periodic latching modulation(2015) Silveri, Matti; Kumar, K. S.; Tuorila, J.; Li, J.; Vepsäläinen, A.; Thuneberg, E.V.; Paraoanu, G. S.; Department of Applied Physics; O.V.Lounasmaa-laboratorio; Superconducting Qubits and Circuit QED; Multiscale Statistical and Quantum Physics