We propose an experimental setup to observe coherent elastic neutrino-atom scattering (CE$\nu$AS) using electron antineutrinos from tritium $\beta^-$ decay and a liquid helium target. In this scattering process with the whole atom, which has not been observed so far, the electrons tend to screen the weak charge of the nucleus as seen by the electron antineutrino probe. The interference between the nucleus and the electron cloud produces a sharp dip in the recoil spectrum at atomic recoil energies of about 9 meV, reducing sizably the number of expected events with respect to the coherent elastic neutrino-nucleus scattering case. This technique allows, in principle, to measure the weak mixing angle at energy scales never reached before. In addition, such a low-energy experiment could provide a sensitive test of the neutrino magnetic moment, setting, potentially, a stronger limit than the current ones.