Fluctuations arising from Heisenberg's uncertainty principle enable quantum systems to exhibit phase transitions even at zero temperature. For example, a superfluid-to-insulator transition has been observed for weakly interacting bosonic atomic gases. Here the authors report a novel type of quantum phase transition in a strongly interacting, one-dimensional quantum gas of bosonic caesium atoms. The results open up the experimental study of ultracold gases in a new regime.

New methods are emerging that aim to image chemical reactions as they occur using X-ray diffraction, electron diffraction or laser-induced recollision, but spectral selection cannot be used to monitor the reacting molecules for these methods. These authors show that this apparent limitation offers opportunities for recollision-based high-harmonic spectroscopy, in which unexcited molecules can act as local oscillators against which structural and electronic dynamics is observed on an attosecond timescale.