Negative currents in Fabry-P\'erot cavities are caused by interfering paths

It has been predicted that the time-dependent current in a Fabry-P\'erot cavity can turn negative even if the applied voltage pulses are always positive. It has also been suggested that the negative currents are related to interfering paths, however, an analytic description of this surprising phenomenon has so far been missing. Here we make use of Floquet scattering theory to demonstrate that the negative currents indeed are caused by the interference of scattering paths through the cavity with different numbers of roundtrips. Our analytic result can be tested in future experiments as it predicts exactly how the effect should be washed out by an increasing temperature. We show that a similar phenomenon is expected for the heat current, which may also turn negative.