Keldysh number

The Keldysh number is a parameter used in physics to describe how an electron interacts with a strong electric field within a material. It compares the energy scale of tunneling (quantum leap) processes to the energy of oscillations caused by the field. A low Keldysh number indicates dominant tunneling behavior (semi-classical regime), whereas a high value suggests multiphoton absorption is more relevant (quantum regime). Essentially, it helps scientists understand whether electrons escape a material more like they are tunneling through barriers or absorbing multiple photons, guiding the analysis of light-matter interactions in strong-field physics.