Session X1: Quantum and Classical Phenomena in Josephson Junction Arrays

Quantum Coherence of the Fluxonium Superconducting Artificial Atom

Artificial atoms built from superconducting tunnel junctions illustrate the problem of engineering a controllable electrodynamic quantum system, starting from basic elements. Can circuit architecture mitigate or even eliminate coherence limitations due to defects in the basic electrical constituents? This central question will be discussed from the perspective of recent experimental results of our group, obtained on the fluxonium[1], a novel superconducting quantum circuit. It consists of a Cooper-pair box junction which is shunted by a long array of larger junctions. Immunity to offset charge noise and only a weak sensitivity to flux noise is observed for the qubit transition. The combination of the very large inductance of the array, which has negligible parasitic resistance, and large phase fluctuations of the small junction, distinguishes fundamentally the fluxonium from the flux qubit. Significant improvement of the relaxation time has been obtained, when one compares with qubits of the same family. Finally, fluxonium displays the type of 3-level-atom physics which should prove useful for continuous, high-fidelity monitoring of a state. Work supported by the IARPA, ARO and NSF. \\[4pt] [1] V.E. Manucharyan, Jens Koch, L.I. Glazman and M.H. Devoret, Science 326, 113-116 (2009).   

Measurement of Quantum Phase-Slips in Josephson Junction Chains

Quantum phase-slip dynamics in Josephson junction chains could provide the basis for the realization of a new type of topologically protected qubit [1] or for the implementation of a new current standard [2]. I will present measurements of the effect of quantum phase-slips on the ground state of a Josephson junction chain. We can tune in situ the strength of the phase-slips. These phase-slips are the result of fluctuations induced by the finite charging energy of each junction in the chain. Our measurements demonstrate that a Josephson junction chain under phase bias constraint behaves in a \textit{collective }way [3]. I will also show evidence of coherent phase-slip interference, the so called Aharonov-Casher effect. This phenomenon is the dual of the well known Aharonov-Bohm interference.\\[4pt] In collaboration with I.M. Pop, Institut Neel, C.N.R.S. and Universite Joseph Fourier, BP 166, 38042 Grenoble, France; I. Protopopov, L. D. Landau Institute for Theoretical Physics, Kosygin str. 2, Moscow 119334, Russia and Institut fuer Nanotechnologie, Karlsruher Institut fuer Technologie, 76021 Karlsruhe, Germany; and F. Lecocq, Z. Peng, B. Pannetier, O. Buisson, Institut Neel, C.N.R.S. and Universite Joseph Fourier. \\[4pt] [1] I. M. Pop, O. Buisson, K. Hasselbach, I. Protopopov, W. Guichard and B. Pannetier, Phys. Rev. B, 78, 104504(2008) \\[0pt] [2] W. Guichard and F. Hekking, Phys. Rev. B 81, 064508 (2010) \\[0pt] [3] I. M. Pop, I. Protopopov, F. Lecocq, Z. Peng, B. Pannetier, O. Buisson and W. Guichard, Nature Physics, 6, 589 (2010).   

Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime

Having small sized active and tunable devices operating at frequencies up to the Terahertz (THz) range is one of the goals of modern electronics. However, there is still a lack of good active or passive devices, often referred to as the ``Terahertz gap.'' Intrinsic Josephson junctions formed by the layered crystal structure of high temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ have the potential to operate in this regime. While for a long time the research on THz generation with this type of junctions was carried out with perhaps only modest success, recently synchronous emission, with an estimated output power in the $\mu $W range, of stacks consisting of several hundred intrinsic Josephson junctions was achieved [1]. We report on the investigation of THz electromagnetic wave generation in intrinsic junction stacks (mesas) of different geometries, using a combination of transport measurement, direct electromagnetic wave detection and Low Temperature Scanning Laser Microscopy [2,3]. At high enough input power a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent THz emission. We discuss possible scenarios of the hot spot/wave interaction and its relation to the generation of coherent THz radiation. \\[4pt] [1] L. Ozyuzer, et al., Science \textbf{318}, 1291 (2007). \\[0pt] [2] H.~B. Wang, et al., Phys. Rev. Lett. \textbf{102}, 017006 (2009). \\[0pt] [3] H. B. Wang, et al., Phys. Rev. Lett. \textbf{105}, 057002 (2010).   

Vortex pinning in ferromagnet-superconductor bilayer with tunable domain patterns

Ferromagnet superconductor hybrids provide a fascinating example of systems in which there is a rich interplay between two seemingly incompatible collective phenomena. Particularly interesting is the impact of the ferromagnet on the dynamics of vortices in the superconductor. The magnetic domains control the location of the vortices. Exquisite control of the dynamics can be achieved by careful tuning of the geometry of the magnetic domains. In this talk I will present the results of recent experiments on superconductor(S)-ferromagnet(F) bilayers with a focus on understanding the hitherto unexplained seemingly unpredictable dependence of the critical current density on the parameters of the experiment. In our experiments the S layer is made of niobium, the F layer is a Co/Pt multilayer with perpendicular magnetic anisotropy, and a thin insulating layer in-between eliminates proximity effect. We use various demagnetization procedures to define different domain patterns in the F layer. We show that some domain patterns produce highly inhomogeneous flux penetration and strong vortex confinement at the sample edge, while for others there is remerkable enhancement of the critical current density in excess of 15. This is the highest value reported to date. We have measured, for the first time in a single tunable structure, the dependence of the activation energy for vortex pinning on the domain width, temperature, and magnetic field. In collaboration with L.Y. Zhu, X. M. Cheng and C. L. Chien (Johns Hopkins), Z. Adamus (Polish Acad. Sci.) and M. Konczykowski (Ecole Polytechnique).   

Tuning superconductivity by carrier injection

All high-T$_c$ cuprates are stacking sequences of CuO$_2$ layers and charge reservoir layers consisting of metal oxides. Upon doping the CuO$_2$ layers, antiferromagnetic order is destroyed and metallic conductivity is established. Usually doping is achieved by a non-stoichiometric composition of the charge reservoir layer. However, we already have shown that we can change the carrier concentration of Bi$_2$Sr$_2$CaCu$_2$O$_ {8+\delta}$ single crystals by current injection along the c- axis [1]. Critical temperature, c-axis resistivity and critical current of intrinsic Josephson junctions can be tuned in a large range from underdoping to extreme overdoping. This effect is persistent up to annealing temperatures of approximately 270 K. Using current injection at higher bias, we were able to reduce the carrier concentration again. We investigated in detail the superconducting properties by performing macroscopic quantum tunneling experiments of intrinsic Josephson junctions. The experiments have been carried out repeatedly on samples, whose properties were changed only by current injection. An exponential increase of the critical current density with hole concentration was observed. At the same time, the capacitance of intrinsic Josephson junctions increased significantly. Finally, only by current injection, we were able to convert into the superconducting state a nonsuperconducting, oxygen depleted sample. This work was done in collaboration with Y. Koval, X.Y. Jin, S. Probst, Y. Simsek, C. Steiner (Universit\"at Erlangen), H. B. Wang (NIMS, Tsukuba), and G. Behr, B. B\"uchner (IFW Dresden). \\[4pt] [1] Y. Koval, X.Y. Jin, C. Bergmann, Y. Simsek, L. \"Ozy\"uzer, P. M\"uller, H. B. Wang, G. Behr, B. B\"uchner, Appl. Phys. Lett. \textbf{96}, 082507 (2010).