Quantum Engineering of Strong Field Physics

Strong field physics started in atomic and molecular physics and extended over the past decade to material science. While atoms are isolated systems, in solids many-body effects due to the interaction with the environment have to be considered. As full many-body treatments are extremely challenging, this is usually done by defining an effective dephasing time $T_2$ within the relaxation time approximation. However, the relaxation time approximation causes ''dephasing ionization'', resulting in an unphysical enhancement of ionization by up to orders of magnitude. As such, a more sophisticated model is needed that ideally maintains simplicity and wide applicability of the relaxation time approximation.

In the first part, such a (spin-boson) model will be introduced, based on a coupling between electron and a bosonic harmonic oscillator heat bath. The heat bath can accurately represent phonons, and collective electronic excitations, such as excitons and plasmons. A parameters scan reveals that the unphysical aspects of dephasing ionization disappear; only in exotic materials with strong heat bath coupling novel ionization effects are predicted.

In the second part, we recognize that light can also serve as a heat bath, with the advantage that nature and strength of the coupling can be controlled by pulse energy and photon distribution. Specifically, two-color experiments are discussed with a moderately intense classical laser pulse, and a perturbative quantum field, such as bright squeezed vacuum (BSV), acting as a ''quantum heat bath''. Very moderate BSV fields are found to enhance ionization and HHG by orders of magnitude, thus allowing control of fundamental strong field processes. The lower required intensities remedy a key weakness of strong laser fields; that they distort the very processes they are meant to measure.

In the last part, the quantum properties of harmonic radiation generated by a superposition of classical and perturbative quantum fields are discussed. The central idea is to transfer quantum properties from the perturbative quantum field to the harmonic pulse, thus shifting the generation of quantum pulses from the infrared into the XUV. We show how the entanglement between quantum beam and harmonics can be harnessed to create a variety of non-classical states commonly used in quantum information science, such as high purity single photon states, Schrödinger cat states, and photon added squeezed vacuum states. This opens a path towards engineering the quantum properties of ultrashort high harmonics.