Majorana-Weyl crossings in topological multiterminal junctions

We analyze the Andreev spectrum in a four-terminal Josephson junction between one-dimensional topological superconductors in class D. We find that a topologically protected crossing in the space of three superconducting phase differences can occur between the two lowest Andreev bound states. This crossing can be detected through the transconductance quantization, in units of $2{e}²/h$, between two voltage-biased terminals. As the crossing occurs away from the Fermi level, our prediction of a quantized response is specific to the nonequilibrium conditions set by the voltage drive. Therefore, it provides an example of a topological signature that cannot be ascribed to a ground-state property. We discuss possible realizations of such junctions with semiconducting crossed nanowires and with quantum-spin Hall insulators.