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Abstract
The separation between two electrons bound in a Cooper pair in a conventional superconductor can extend up to several hundred nanometres. A new study shows that these long-range interactions can reach beyond the confines of a superconductor itself to coherently couple electrons in two normal metals either side of the superconductor. Two electrons bound in a singlet state have long provided a conceptual and pedagogical framework for understanding the non-local nature of entangled quantum objects. As bound singlet electrons separated by a coherence length of up to several hundred nanometres occur naturally in conventional Bardeen–Cooper–Schrieffer superconductors in the form of Cooper pairs, recent theoretical investigations1,2,3,4,5,6,7,8,9 have focused on whether electrons in spatially separated normal-metal probes placed within a coherence length of each other on a superconductor can be quantum mechanically coupled by the singlet pairs. This coupling is predicted to occur through the non-local processes of elastic cotunnelling and crossed Andreev reflection. In crossed Andreev reflection, the constituent electrons of a Cooper pair are sent into different normal probes while retaining their mutual coherence. In elastic cotunnelling, a sub-gap electron approaching the superconductor from one normal probe undergoes coherent, long-range tunnelling to the second probe that is mediated by the Cooper pairs in the condensate. Here, we present experimental evidence for coherent, non-local coupling between electrons in two normal metals linked by a superconductor. The coupling is observed in non-local resistance oscillations that are periodic in an externally applied magnetic flux.Cite this article
Cadden-Zimansky, P., Wei, J. & Chandrasekhar, V. Cooper-pair-mediated coherence between two normal metals. Nature Phys 5, 393–397 (2009). https://doi.org/10.1038/nphys1252 