Ab initio low-energy dynamics of superfluid and solid 4He

We have extracted information about real time dynamics of 4He systems from noisy imaginary-time correlation functions f (τ) computed via quantum Monte Carlo (QMC): production and falsification of model spectral functions s (ω) are obtained via a survival-to-compatibility with f (τ) evolutionary process, based on genetic algorithms. Statistical uncertainty in f (τ) is promoted to be an asset via a sampling of equivalent f (τ) within the noise, which give rise to independent evolutionary processes. In the case of pure superfluid 4He we have recovered from exact QMC simulations sharp quasiparticle excitations with spectral functions displaying also the multiphonon branch. As further applications, we have studied the impuriton branch of one 3He atom in liquid 4He and the vacancy-wave excitations in hcp solid 4He finding an unexpected rotonlike feature.