Splitting of a Cooper pair by a pair of Majorana bound states

Johan Nilsson, A. R. Akhmerov, C. W. J. Beenakker

Published 2008-06-30, updated 2008-08-15Version 2

Majorana bound states are spatially localized superpositions of electron and hole excitations in the middle of a superconducting energy gap. A single qubit can be encoded nonlocally in a pair of spatially separated Majorana bound states. Such Majorana qubits are in demand as building blocks of a topological quantum computer, but direct experimental tests of the nonlocality remain elusive. Here we propose a method to probe the nonlocality by means of crossed Andreev reflection, which is the injection of an electron into one bound state followed by the emission of a hole by the other bound state (equivalent to the splitting of a Cooper pair over the two states). We have found that, at sufficiently low excitation energies, this nonlocal scattering process dominates over local Andreev reflection involving a single bound state. As a consequence, the low-temperature and low-frequency fluctuations $\delta I_{i}$ of currents into the two bound states $i=1,2$ are maximally correlated: $\overline{\delta I_{1}\delta I_{2}}=\overline{\delta I_{i}^{2}}$.