Ultrastrong time-dependent light-matter interactions are gauge-relative

Understand

Time-dependent light-matter interactions are a widespread means by which to describe controllable experimental operations.

• They can be viewed as an approximation in which a third system - the control system - is treated as external within the Hamiltonian.
• We demonstrate that this results in non-equivalence between gauges.
• We provide a physical example in which each different non-equivalent model coincides with a gauge-invariant description applied in a different experimental situation.