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.