Séminaire de groupe
Quantum phase fluctuations in Josephson junction systems 
Gianluca Rastelli 
Fachbereich Physik, Universitaet Konstanz (Allemagne) 
jeudi 31 janvier 2013 , 14h00 
Salle de séminaire du groupe de Physique Statistique 
Coherent Quantum PhaseSlip (CQPS) processes in superconducting systems are an active research topic. They have been studied theoretically [16] and experimentally, in 1D Josephson junction chains [7,8] and in superconducting nanowires [9,10]. In 1D Josephson junction networks biased with externally applied magnetic fluxes, CQPS provides a coupling between macroscopic states with different circulating supercurrent. In such systems, CQPSs play a fundamental role for testing quantum mechanics in macroscopic separables systems [11] or for the realization of a nontrivial quantum collective order [12] which may be exploited to realize qubits topologically protected against decoherence [13]. CQPSs are also of main interest for the realization of a fundamental current standard in quantum metrology [1]. In the first part of the talk, I shall discuss CQPSs in Josephson junction rings biased with a magnetic flux and with an arbitrary (long) range for the electrostatic interaction between the N superconducting islands. By analyzing the maximal supercurrent sustained by the ring, I shall show that finitesize systems have signatures of the superconducting/insulator transition eventually occurring in the thermodynamic limit [14]. I shall also present a comparison between the theory and the experimental observations reported by the group of Grenoble. I will discuss the experimental evidences for CQPSs in homogeneous Josephson chains [15] as well as the formation of a coherent chargestate induced by CQPSs in chains containing one weak Josephson element [16]. The last observation opens the route towards the achievement of the current Shapiro steps. As the CQPS is the dual process of the charge (Cooper pairs) tunneling across an insulating barrier, the circuits containing a single CQPS element are predicted to exhibit current plateau (dual Shapiro steps) when they are irradiated by microwaves. But such sharp features in the IV characteristic have eluded a direct and clear observation up to now. One possible explaination may be the effect of the quantum fluctuations in the dynamics of the coherent chargestate. Such fluctuations are expected to smear the steps in the IV curves, thereby complicating their experimental detection. <br> In the second part, I shall present a quantum approch to study the dynamics of the coherent chargestate under microwave irradiation and in presence of dissipation [17]. In the last part, I shall present a systematic comparison between Josephson junction chains and super conducting nanowires having, respectively, a weak Josephson element or nanoconstriction where QPSs are pinned [18]. Using this analogy, I shall discuss the crossover between coherent vs incoherent QPSs. <br> [1] J. E. Mooij and C. J. P. M. Harmans, New. J. Phys 7, 219 (2005). <br> [2] J. E. Mooij and Y. V. Nazarov, Nat. Physics 2, 169 (2006). <br> [3] A. M. Hriscu and Yu. V. Nazarov, Phys. Rev. Lett 106, 077004 (2011); Phys. Rev. B 83, 174511 (2011). <br> [4] M. Vanevic and Yu. V. Nazarov, Phys. Rev. Lett 108, 198002 (2012). <br> [5] K. A. Matveev, A. I. Larkiv, and L. I. Glazman Phys. Rev. Lett 89, 096802 (2002). <br> [6] G. Rastelli, I. M. Pop, W. Guichard, and F. W. J. Hekking, arXiv:1201.0539. <br> [7] I. M. Pop et al. Nat. Phys. Lett. 6 589 (2010); I. M. Pop et al., Phys. Rev. B 85, 094503 (2012). <br> [8] V. E. Manucharyan, N. A. Masluk, et al. Phys. Rev. B 85, 025521 (2012). <br> [9] O. V. Astafiev, L. B. Ioffe et al. Nat. Letter 484, 355 (2012). <br> [10] J. S. Lehtinen, K. Zakharov, and K. Yu. Arutyunov, Phys. Rev. Lett. 109, 187001 (2012).<br> [11] V. E. Manucharyan, J. Koch et al., eprint arXiv:0910.3039. <br> [12] L. B. Ioffe, M. V. Feigel’man, Phys. Rev. B (2002); B. Dou¸cot, J. Vidal, Phys. Rev. Lett. 88, 227005 (2002). <br> [13] S. Gladchenko, D. Olaya et al., Nat. Physics 5, 48 (2009). <br> [14] R. Fazio, H. van der Zant, Phys. Rep. 355, 235 (2001). <br> [15] T. Weissl, G. Rastelli et al. (to be submitted) <br> [16] G. Rastelli, T. Weissl et al. (to be submitted) <br> [17] G. Rastelli, F. W. J. Hekking (to be submitted) <br> [18] G. Rastelli, M. Vaneni´c et al. (in preparation) <br> 
