Giant counter-rotating oscillations on the attosecond timescale

We predict an unexplored type of ultrastrong coupling between atoms and intense ultraviolet light that leads to giant population oscillations on the attosecond timescale. These counter-rotating oscillations stem from the interference of energy-conserving and nonconserving photon absorption and emission processes via Rydberg states. They can be of similar amplitude as the elementary femtosecond Rabi oscillations between the two strongly coupled states. The effect, which is beyond the two-level atom, is nonreciprocal: It only affects the excited state, while the ground state is unaffected. We propose that two-photon Rabi oscillations (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>1</a:mn><a:msup><a:mrow><a:mi>s</a:mi></a:mrow><a:mn>2</a:mn></a:msup></a:mrow></a:math>–<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mn>1</b:mn><b:mi>s</b:mi><b:mn>3</b:mn><b:mi>d</b:mi></b:mrow></b:math>) in helium are suitable for the generation of this type of ultrastrong coupling with realistic pulses. We use a combination of Floquet theory and effective Hamiltonian theory to test our predictions against simulations.