### Browsing by Author "Li, Jian"

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Item Autler-Townes Effect in a Superconducting Three-Level System(American Physical Society (APS), 2009) Sillanpää, Mika A.; Li, Jian; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Paraoanu, G. S.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWhen a three-level quantum system is irradiated by an intense coupling field resonant with one of the three possible transitions, the absorption peak of an additional probe field involving the remaining level is split. This process is known in quantum optics as the Autler-Townes effect. We observe these phenomena in a superconducting Josephson phase qubit, which can be considered an “artificial atom” with a multilevel quantum structure. The spectroscopy peaks can be explained reasonably well by a simple three-level Hamiltonian model. Simulation of a more complete model (including dissipation, higher levels, and cross coupling) provides excellent agreement with all of the experimental data.Item Decoherence in superconducting quantum circuits(Aalto University, 2012) Li, Jian; Paraoanu, Sorin, Dr.; O.V. Lounasmaa -laboratorio; O.V. Lounasmaa Laboratory; Perustieteiden korkeakoulu; School of Science; Kaivola, Matti, Prof.Superconducting quantum devices have drawn the attention of physicists greatly in recent years, not only because they are one of the most favorable candidates for developing a solid state quantum computer, but also because they can be employed as test systems for understanding quantum mechanics at nearly macro scale. Superconducting quantum devices can be divided into two groups: the superconducting resonators (linear devices), which have equally spaced discrete energy levels, and the artificial atoms (non-linear devices) consisting of Josephson junctions which have unequally spaced discrete energy levels. By combining these two kinds of devices together, a solid state counterpart of optical cavity quantum electrodynamics (QED) known as circuit QED emerges. Experiments of cavity QED and quantum optics can be reproduced in either circuit QED systems or in bare superconducting artificial atoms. As an example, we have observed the Autler-Townes effect in a three-level artificial atom called phase qutrit. More than one decade has passed since the first experimental demonstration of superconducting two-level artificial atom (qubit). However, to build a practical quantum computer with thousands of superconducting qubits there is still a long way to go (we are at the stage of three qubits now). One of the main obstacles preventing us from scaling up a superconducting quantum computer, as well as some other kinds of quantum computers, has been decoherence, which is believed to be dominated by the low frequency noise due to the two-level system (TLS) fluctuators located inside the Josephson junction barrier. We have experimentally simulated the dynamics of a qubit longitudinally coupled to a randomly fluctuating TLS. A phenomenon known as motional averaging has been observed. In this thesis, theoretical models for decoherence in both resonator and artificial atoms have been established and used for explaining phenomena observed in the experiments performed in our laboratory. The phenomenon of decoherence in coupled bipartite systems has been also studied, and besides the already known entanglement sudden death phenomenon, stable entanglement (robust under decoherence) generated from an arbitrary initial state has been found in these systems.Item Decoherence, Autler-Townes effect, and dark states in two-tone driving of a three-level superconducting system(American Physical Society (APS), 2011) Li, Jian; Paraoanu, G. S.; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Sillanpää, Mika A.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe present a detailed theoretical analysis of a multilevel quantum system coupled to two radiation fields and subject to decoherence. We concentrate on an effect known from quantum optics as Autler-Townes splitting, which has been recently demonstrated experimentally [M. A. Sillanpää et al., Phys. Rev. Lett. 103, 193601 (2009)] in a superconducting phase qubit. In the three-level approximation, we derive analytical solutions and describe how they can be used to extract the decoherence rates and to account for the measurement data. Better agreement with the experiment can be obtained by extending this model to five levels. Finally, we investigate the stationary states created in the experiment and show that their structure is close to that of dark states.Item Dynamical Autler-Townes control of a phase qubit(Nature Publishing Group, 2012) Li, Jian; Paraoanu, G. S.; Cicak, Katarina; Altomare, Fabio; Park, Jae I.; Simmonds, Raymond W.; Sillanpää, Mika A.; Hakonen, Pertti J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceRouters, switches, and repeaters are essential components of modern information-processing systems. Similar devices will be needed in future superconducting quantum computers. In this work we investigate experimentally the time evolution of Autler-Townes splitting in a superconducting phase qubit under the application of a control tone resonantly coupled to the second transition. A three-level model that includes independently determined parameters for relaxation and dephasing gives excellent agreement with the experiment. The results demonstrate that the qubit can be used as a ON/OFF switch with 100 ns operating time-scale for the reflection/transmission of photons coming from an applied probe microwave tone. The ON state is realized when the control tone is sufficiently strong to generate an Autler-Townes doublet, suppressing the absorption of the probe tone photons and resulting in a maximum of transmissioItem 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 Multiwavelength Evidence for Quasi-periodic Modulation in the Gamma-Ray Blazar PG 1553+113(2015) Ackermann, M.; Ajello, M.; Albert, A.; Atwood, W.B.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Becerra Gonzalez, J.; Bellazzini, R.; Bissaldi, E.; Blandford, R.D.; Bloom, E.D.; Bonino, R.; Bottacini, E.; Bregeon, J.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G.A.; Cameron, R.A.; Caputo, R.; Caragiulo, M.; Caraveo, P.A.; Cavazzuti, E.; Cecchi, C.; Chekhtman, A.; Chiang, J.; Chiaro, G.; Ciprini, S.; Cohen-Tanugi, J.; Conrad, J.; Cutini, S.; "D'Ammando", F.; de Angelis, A.; de Palma, F.; Desiante, R.; Di Venere, L.; Domi´nguez, A.; Drell, P.S.; Favuzzi, C.; Fegan, S.J.; Ferrara, E.C.; Focke, W.B.; Fuhrmann, L.; Fukazawa, Y.; Fusco, P.; Gargano, F.; Gasparrini, D.; Giglietto, N.; Giommi, P.; Giordano, F.; Giroletti, M.; Godfrey, G.; Green, D.; Grenier, I.A.; Grove, J.E.; Guiriec, S.; Harding, A.K.; Hays, E.; Hewitt, J.W.; Hill, A.B.; Horan, D.; Jogler, T.; Jóhannesson, G.; Johnson, A.S.; Kamae, T.; Kuss, M.; Larsson, S.; Latronico, L.; Li, Jian; Li, Liang; Longo, F.; Loparco, F.; Lott, B.; Lovellette, M.N.; Lubrano, P.; Magill, J.; Maldera, S.; Manfreda, A.; Max-Moerbeck, W.; Mayer, M.; Mazziotta, M.N.; McEnery, J.E.; Michelson, P.F.; Mizuno, T.; Monzani, M.E.; Morselli, A.; Moskalenko, I.V.; Murgia, S.; Nuss, E.; Ohno, M.; Ohsugi, T.; Ojha, R.; Omodei, N.; Orlando, E.; Ormes, J.F.; Paneque, D.; Pearson, T.J.; Perkins, J.S.; Perri, M.; Pesce-Rollins, M.; Petrosian, V.; Piron, F.; Pivato, G.; Porter, T.A.; Rainò, S.; Rando, R.; Razzano, M.; Readhead, A.; Reimer, A.; Reimer, O.; Schulz, A.; Sgrò, C.; Siskind, E.J.; Spada, F.; Spandre, G.; Spinelli, P.; Suson, D.J.; Takahashi, H.; Thayer, J.B.; Thompson, D.J.; Tibaldo, L.; Torres, D.F.; Tosti, G.; Troja, E.; Uchiyama, Y.; Vianello, G.; Wood, K.S.; Wood, M.; Zimmer, S.; Berdyugin, A.; Corbet, R.H.D.; Hovatta, T.; Lindfors, E.; Nilsson, K.; Reinthal, R.; Sillanpää, A.; Stamerra, A.; Takalo, L.O.; Valtonen, M.J.; Metsähovi Radio ObservatoryItem Optomechanical measurement of a millimeter-sized mechanical oscillator approaching the quantum ground state(2017-10-12) Santos, J. T.; Li, Jian; Ilves, J.; Ockeloen-Korppi, C. F.; Sillanpaa, M.; Department of Applied Physics; Quantum Computing and Devices; Quantum Nanomechanics; Aalto University; University of GlasgowCavity optomechanics is a tool to study the interaction between light and micromechanical motion. Here we observe optomechanical physics in a truly macroscopic oscillator close to the quantum ground state. As the mechanical system, we use a mm-sized piezoelectric quartz disk oscillator. Its motion is coupled to a charge qubit which translates the piezo-induced charge into an effective radiation-pressure interaction between the disk and a microwave cavity. We measure the thermal motion of the lowest mechanical shear mode at 7 MHz down to 30 mK, corresponding to roughly 10 2 quanta in a 20 mg oscillator. We estimate that with realistic parameters, it is possible to utilize the back-action cooling by the qubit in order to control macroscopic motion by a single Cooper pair. The work opens up opportunities for macroscopic quantum experiments.