Quantum mechanical derivation of radio-frequency-driven coherent beam oscillations in storage rings

The study of the approach to the quantum ground state and the possibility to detect displacements of macroscopic bodies close to the quantum limit represent pressing challenges in modern physics. In the recent experiment of the JEDI Collaboration at the COSY storage ring, the coherent oscillations of a deuteron beam were detected with an amplitude of only one order of magnitude above the limit of the Heisenberg uncertainty principle of about 40 nm for the one-particle betatron motion. On the other hand, the much discussed search for the permanent electric dipole moment of the proton with an ultimate sensitivity of ${10}^{-29}\mathrm{e}\mathrm{cm}$ requires control of the position of the beam center of gravity with an accuracy of $\approx 5\mathrm{pm}$. In this paper, we develop the full quantum mechanical treatment of the coherent beam oscillations with ultrasmall amplitudes. In agreement with the Ehrenfest theorem, we find a continuity of the description of the coherent betatron motion from the large classical amplitudes down to the deep quantum region below the one-particle Heisenberg limit. We argue that quantum mechanics does not preclude control of the beam center with subpicometer accuracy.