The temporal properties of an electron beam are decisive for modern ultrafast electron microscopy and for the quantum optics of the free electron in laser fields. Here, we report a time-domain interferometer that measures and distinguishes the pure and ensemble coherences of a free-electron beam in a transmission electron microscope via symmetry-breaking shifts of photon-order sideband peaks. This result is a free-electron analog to the reconstruction of attosecond busts and photoemission delays in optical attosecond spectroscopy. We find a substantial pure electron coherence that is connected to the thermodynamics of the emitter material and a lower ensemble coherence that is governed by space-charge effects. Pure temporal coherences above 5 fs are measured at $>{10}^9$ electrons per second in a high-brightness beam.

• Received 12 May 2021
• Revised 26 August 2021
• Accepted 16 September 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.165501