A laser hits an atom, and tears out an electron. The brighter the light, the stronger the kick, and the faster the electron flies away. That's how the familiar picture works, where light is a smooth wave. But what if the light isn't smooth but 'squeezed,' with quantum fluctuations, like the trembling of a string? In a new study, physicists show that then the electron reacts quite differently.

Quantum fluctuations are random deviations of the field from its average. In squeezed light, they are asymmetric: the field more often deviates in one direction or the other. The authors found that when such light ionizes an atom, it seems to pick two fields of equal strength but opposite sign. The electron feels their combined magnitude, not the average brightness. Therefore, its longitudinal momentum grows not linearly with intensity but as $I^{2/3}$—more slowly.

If you catch each photon individually, the picture changes. The two paths, with positive and negative fields, interfere, as in the double-slit experiment, but the slits are separated in time by half a wave period. Then the electron momentum shows characteristic oscillations that depend on the parity of the photon number. This means that from the electron's motion, you can infer the quantum statistics of the light.

Thus, the longitudinal momentum becomes a new tool in quantum optics of strong fields. It encodes both the photon statistics and the coherence of intense quantum light.