Condensed-Matter Physics & Materials Science Seminar

In the talk I will present and discuss our recent experiments that demonstrate quantum tunneling and energy level quantization of Josephson vortices in long Josephson nano-junctions. A single vortex behaves as a particle with a spatial extent of several micrometers which tunnels through a potential barrier created in a long Josephson junction. In order to obtain low enough effective mass of the vortex, long junctions have to be made of sub-micron width. However, for such narrow junctions the local theory based on the sine-Gordon equation breaks down as the fields outside the junction contribute significantly to the vortex energy. We investigated the classical dynamics on vortices and electromagnetic cavity modes in few hundred micron long and some hundred nanometer wide Josephson junctions. We have fabricated and measured long Al-AlOx-Al and Nb-AlOx-Nb Josephson junctions of width ranging from 4000 nm to 100 nm. Experiments clearly indicate the effects of stray electromagnetic fields outside the junction. We find that the characteristic vortex velocity increases as the junction width decreases. The nonlocal dispersion of Josephson plasma oscillations leads to a radiation of Cherenkov waves by fast vortices, in agreement with our analysis and simulations. At mK temperatures, we probe the quantum properties of individual vortices by investigating the statistics of individual tunneling events. Measurements of vortex escape from a potential well in the presence of microwave radiation indicate discrete energy levels of the vortex. Both the tunneling rate and the energy level separation can be tuned in experiment by varying the applied magnetic field. The experiments on annular long junction in the absence of a single trapped vortex demonstrate quantum dissociation of a vortex "molecule", consisting of a bound state of vortex and antivortex. Engineering an energy profile for a vortex in a long Josephson junction opens an opportunity of designing vortex qubits.