Unveiling bosons in an ultra-short single electron pulse.

Quantum dynamics is very sensitive to dimensionality. While two-dimensional electronic systems form Fermi liquids, one-dimensional systems -- Tomonaga-Luttinger liquids -- are described by purely bosonic excitations, even though they are initially made of fermions. With the advent of coherent single electron sources, the quantum dynamics of such a liquid is now accessible at the single electron level. Here, we report on time-of-flight measurements of ultra-short single electron charge pulses injected into a quasi one-dimensional quantum conductor. We change the confinement potential from the one-dimensional Tomonaga-Luttinger liquid limit to the multi-channel Fermi liquid. Our measurements show that the plasmon velocity can be varied over almost an order of magnitude, even in the quantum regime where the pulses carry one or less electrons. These results are in quantitative agreement with a parameter-free theory and demonstrate a powerful new probe for directly investigating real-time dynamics of fractionalisation phenomena in low-dimensional conductors.