Figure 2.

Quantum interference in free‐electron‐light interactions at moderate coupling strengths, resulting in elimination of the zero‐loss peak by destructive quantum interference, and enhancement of satellite peaks by constructive quantum interference. a) An incoming shaped free electron (modeled as a quantum electron wavepacket (QEW)), with input spectrum shown in (b), scatters off a classical light field with a dimensionless coupling strength $\mathcal{G}=0.7$, resulting in an output electron spectrum where the zero‐loss peak completely vanishes. If quantum interference contributions are neglected (unfilled bars with blue outlines in (c)), which occurs, for instance, when there is no fixed phase relation between the input eigenstates, the zero‐loss peak dominates. d) In contrast, for an unshaped incoming QEW (input spectrum shown in (e)), the output QEW spectrum in the presence and absence of quantum interference coincide (as shown in (f)), which implies that quantum interference contributions vanish for unshaped input QEWs. (g,h) compare the output QEW spectrum in the presence and absence of quantum interference, respectively, as a function of $\mathcal{G}$. The contribution of quantum interference is already substantial even for weaker interactions of about $\mathcal{G}\approx 0.1$. As shown in (h), the complete suppression of gain/loss peaks away from the zero‐loss peak can be achieved for coupling strengths $\mathcal{G}\gtrsim 1$.