Linking Pump-Probe Experiments and Quantum Simulations: A Linear Response Framework.

Response functions are a fundamental aspect of physics; they represent the link between experimental observations and the underlying quantum many-body state. However, this link is often under-appreciated, as the Lehmann formalism for obtaining response functions in linear response has no direct link to experiment. A previously proposed linear response framework for obtaining response functions on quantum computers restored this link by making the experiment an inextricable part of the quantum simulation. This method is frequency- and momentum-selective, ancilla-free, and encompassing a larger pool of operators that can be directly measured. Another powerful, prospective application of this method entails directly simulating a pump-probe experiment where the system is driven out of equilibrium by an ultrafast laser pulse and probed afterwards to measure its non-equilibrium Green's functions. Using this, and putting the functional derivative approach into action, we investigate the phenomenon of confinement within condensed-matter physics, which involves the emergence of a startling attractive potential between the quasi-particles of a spin-system that alters the dynamics and the spreading of correlations. We study this non-trivial occurrence in long-range Ising models as a compelling example by measuring both their longitudinal and transverse non-equal time correlation functions.