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The ω-SQUIPT as a tool to phase-engineer Josephson topological materials


Multi-terminal superconducting Josephson junctions based on the proximity effect offer the opportunity to tailor non-trivial quantum states in nanoscale weak links. These structures can realize exotic topologies in several dimensions1, for example, artificial topological superconductors that are able to support Majorana bound states2,3, and pave the way to emerging quantum technologies4,5,6,7 and future quantum information schemes8. Here we report the realization of a three-terminal Josephson interferometer based on a proximized nanosized weak link. Our tunnelling spectroscopy measurements reveal transitions between gapped (that is, insulating) and gapless (conducting) states that are controlled by the phase configuration of the three superconducting leads connected to the junction. We demonstrate the topological nature of these transitions: a gapless state necessarily occurs between two gapped states of different topological indices, in much the same way that the interface between two insulators of different topologies is necessarily conducting9. The topological numbers that characterize such gapped states are given by superconducting phase windings over the two loops that form the Josephson interferometer. As these gapped states cannot be transformed to one another continuously without passing through a gapless condition, they are topologically protected. The same behaviour is found for all of the points of the weak link, confirming that this topology is a non-local property. Our observation of the gapless state is pivotal for enabling phase engineering of different and more sophisticated artificial topological materials1,4,5,6,7.

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Figure 1: The ω-SQUIPT: a three-terminal double-loop Josephson interferometer based on the proximity effect.
Figure 2: Topological classes of the ω-SQUIPT.
Figure 3: Low-temperature magnetic flux behaviour of the two types of ω-SQUIPTs.
Figure 4: Temperature evolution of the topological transitions.

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The European Research Council under the European Union's Seventh Framework Program (FP7/2007-2013)/ERC Grant agreement No. 615187-COMANCHE and MIUR-FIRB2013 – Project Coca (Grant No. RBFR1379UX) are acknowledged for partial financial support. The work of E.S. is funded by a Marie Curie Individual Fellowship (MSCA-IFEF-ST No. 660532-SuperMag). The work of F.S.B was partially supported by the Spanish Ministerio de Economia y Competitividad under Project No. FIS2014-55987-P.

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S.D. fabricated the samples. E.S. and F.V. performed the measurements. F.V. analysed the data, and carried out the simulations. F.S.B developed the numerical code to calculate the conductance spectra. Y.V.N. developed the theory of the Josephson topological states. F.G. conceived the experiment. All authors discussed the results and their implications equally at all stages, and all the authors wrote the manuscript.

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Correspondence to F. Giazotto.

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The authors declare no competing financial interests.

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Strambini, E., D'Ambrosio, S., Vischi, F. et al. The ω-SQUIPT as a tool to phase-engineer Josephson topological materials. Nature Nanotech 11, 1055–1059 (2016).

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