  In the presence of an external magnetic field, the surface plasmon polariton
that exists at the metal-dielectric interface is believed to support a
unidirectional frequency range near the surface plasmon frequency, where the
surface plasmon polariton propagates along one but not the opposite direction.
Recent works have pointed to some of the paradoxical consequences of such a
unidirectional range, including in particular the violation of the
time-bandwidth product constraint that should otherwise apply in general in
static systems. Here we show that such a unidirectional frequency range is
nonphysical, using both a general thermodynamic argument, and a detailed
calculation based on a nonlocal hydrodynamic Drude model for the metal
permittivity. Our calculation reveals that the surface plasmon-polariton
remains bidirectional for all frequencies. This work overturns a long-held
belief in nonreciprocal photonics, and highlights the importance of quantum
plasmonic concepts for the understanding of nonreciprocal plasmonic effects.
