New strategies for the mutual synchronization of very large Josephson junction networks

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The aim of the project is to investigate new strategies for the mutual synchronization of very large Josephson junction networks with the objective to significantly advance sub-mm wavelength radiation sources based on phase-synchronized Josephson junctions (JJs). The sources will consist of large 1D or 2D JJ arrays and, in terms of output power, shall exceed the present state-of-the-art values of several 10 µW by at least an order of magnitude. The sources shall be tunable over a wide range of emission frequencies. We will study arrays based on mainly three different types of JJs. The first type are SNS Josephson junctions (with "S" = superconductor, "N" = normal metal) using NbSi as the barrier layer. With these arrays we will identify proper geometric configurations and synchronization schemes and develop suitable high-frequency designs, e.g. based on travelling wave antennas. We intend to learn how to design Josephson systems with a simple set of oscillation modes that effectively interact with the JJs, as well as suppress parasitic modes by increasing their losses due to the use of strip electrodynamic elements. The second type of JJs are externally shunted SIS ("I" = Insulator) tunnel junctions, which use AlOx or AlN as barrier layers. Arrays based on these JJs additionally allow integration of elements such as harmonic mixers and on-chip detection schemes. The third type of arrays uses YBa2Cu3O7 junctions created by focused helium ion beam patterning. For this line of research both fabrication and the mechanisms of synchronization are less established than for the other ones but the possible gain of using such arrays is that much higher emission frequencies, exceeding 1 THz, are feasible at least in principle. For all arrays we will successively improve and optimize the figure of merits (output power, maximum frequency, linewidth of radiation. Target frequencies will be in the range between 0.2 and 1 THz, maximum output powers in the range of several 100 µW and linewidths in the range of a few MHz or less. The proposed project brings together one German and three Russian groups who all have an international reputation in their field and who have experience in international collaborations. Each group has a recognized track record in a field relevant for project goals, and prior collaborations between the participating groups have been highly successful.