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LOW TEMPERATURE PHYSICS RESEARCH

NANO group

R. Danneau, D. Gunnarsson, P. Hakonen, L. Korhonen, L. Lechner, A. Paila, A. Puska, J. Salmilehto, J. Sarkar, M. Sillanpää, M. Tomi, and F. Wu

Visitors: S. Andresen, J.C. Cuevas, L. Foa, Yu. Makhlin, E. Sonin, and M. Wiesner

Collaborators: M. Craciun, J. Hassel, E. Kauppinen, A. Morpurgo, A. Nazibulin, P. Queipo, S. Russo, H. Seppä, E. Thuneberg, T. Tsuneta, and J. Tuorila

The research work of the NANO group is focused on three areas: 1) Mesoscopic quantum amplifiers and qubits, 2) Quantum transport in carbon nanotubes and graphene, and 3) Current fluctuations and fast dynamics in quantum circuits. In all of these categories, our measurements are centered at microwave frequencies, involving reflection measurements for qubits, transmission measurements for AC-conductance, and two channel noise recording for cross correlation studies. After the completion of a new pulse tube based dilution refrigerator, we have now three setups for low-noise microwave studies at 1 - 10 GHz: two down to 25 mK and one for 4.2 K.

During the past years our experimental efforts have become progressively collaborative in character within the domain of the European Union ruled research funding. Two projects are ongoing: one of them, coordinated by P. Hakonen, is an IST-STREP dealing with carbon nanotubes and the second one is an INTAS-project, coordinated by G. Schön, which deals with quantum information. In carbon nanotubes and graphene, we have also started a collaboration with Nokia. Most of our samples, especially the Josephson junction devices, are made in our own in-house semiclean room. As we are not producing any carbon nanotubes ourselves, all our nanotube samples have been obtained on collaborative basis of some sort, either in European projects or within other collaborations. Also, our graphene samples so far have been obtained via collaboration.

In the field of mesoscopic quantum amplifiers and qubits, we have further developed the dispersive charge detection techniques deviced by our group recently for studies of the quantum measurement. We have fabricated novel on-chip ultra-small capacitors which eliminated spurious circuit elements that hampered our earlier studies. This enabled us, for example, to observe genuine sideband cooling and heating in a coupled superconducting qubit + oscillator system for the first time. We also continued our studies of Landau-Zener interference, and investigated the involved non-adiabaticity effects by observing changes in the so-called Stokes phase.

LANDAU-ZENER INTERFEROMETRY IN A COOPER PAIR BOX

D. Gunnarsson, P. Hakonen, Yu. Makhlin, A. Paila, J. Sarkar, and M. Sillanpää

Landau-Zener (LZ) interference is a quantum-mechanical phenomenon in a phase coherent system, taking place at the intersection of two energy levels that repel each other due to a weak interaction. In our measurements of qubit LZ interferometry, we employ the Josephson (quantum) capacitance to determine the state of a Cooper pair box. During the past year, we have paid attention to the Stokes phase that depend on the non-adiabaticity factor of the drives, 2 2/ħ , where 2 is the energy gap and is the speed (energy rate of change) at which the crossing point is traversed.

Annual Report 2007

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