Cooper Pair Mediated Coherence Between Two Normal Metals

Two electrons bound in a singlet state have long provided a conceptual and pedagogical framework for understanding the nonlocal nature of entangled quantum objects. As bound singlet electrons separated by a coherence length of up to several hundred nanometers occur naturally in conventional BCS superconductors in the form of Cooper pairs, recent theoretical investigations 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 nonlocal processes of elastic cotunneling (EC) and crossed Andreev reflection (CAR). In CAR, the constituent electrons of a Cooper pair are sent into different normal probes while retaining their mutual coherence. In EC, a sub-gap electron approaching the superconductor from one normal probe undergoes coherent, long-range tunneling to the second probe that is mediated by the Cooper pairs in the condensate. We present here experimental evidence for coherent, nonlocal coupling between electrons in two normal metals linked by a superconductor. The coupling is observed in nonlocal resistance oscillations that are periodic in an externally applied magnetic flux, as well as in cross-correlation noise experiments.

In collaboration with Paul Cadden-Zimansky and Jian Wei.