Quantum interference of Cooper pair split quasiparticles

In a normal metal (N) placed in contact with a superconductor (S), electrons in the normal metal with energy E less than the gap Δ of the superconductor cannot be transmitted into the superconductor. Instead, the electron pairs with another electron with energy -E in a process called Andreev reflection. The two electrons participating in Andreev reflection need not come from the same normal metal, but can come from two different normal metals, so long as the distance between the two normal metals in the superconductor is less than the superconducting coherence length ξs, in a process called crossed Andreev reflection (CAR). The reverse process, where a Cooper pair splits into two electrons, each propagating in a different normal metal, is sometimes called Cooper-pair splitting. CAR has been demonstrated in nonlocal measurements on NSN devices [1]. In a normal metal ring, we show that the Cooper pair split quasiparticles remain phase coherent over micron size length scales by measuring quantum h/2e oscillations of the nonlocal voltage as a function of an applied magnetic field. Surprisingly, the phase of the nonlocal oscillations is reversed with respect to local measurements on the same loop.

[1] P. Cadden-Zimansky, PRL 97, 237003 (2006).