Probing Cooper pair momentum by quasiparticle steering with planar Josephson junctions

The Cooper pair momentum in a superconductor is associated with a phase gradient of the superconducting order parameter. In general, this momentum is small compared to the Fermi momentum, which makes it challenging to measure. Josephson junctions, however, enable the creation of large phase gradients and transfer of the Cooper pair momentum to quasiparticles via Andreev reflection. In this work we demonstrate that Andreev bound states propagating along ballistic planar Josephson junctions eject into an adjacent normal region at a phase-controlled angle that scales as $\Theta \sim \sqrt{\Delta/ \mu}$, where $\Delta$ is the superconducting gap and $\mu$ is the chemical potential. This angle parametrically exceeds the conventional Cooper pair momentum scale $\Delta/ \mu$, and thus this phenomenon is sizeable even within the Andreev approximation regime $\Delta / \mu \ll 1$. Our results establish phase-controlled quasiparticle ejection as a kinematic probe of condensate momentum transfer: unlike existing probes that detect the Doppler energy shift, the signal appears as a momentum-space deflection of emitted quasiparticles.