I recently gave a seminar for WG2 on our recent preprint “Quantum signatures of proper time in optical ion clocks,” written in collaboration with Gabriel Sorci, Dietrich Leibfried, Christian Sanner, and Igor Pikovski . Dennis suggested I start a discussion on this work; feel free to contribute in the replies, I will be glad to discuss further. 

In our paper, we show that optical ion clocks can be used to probe quantum signatures of proper time that cannot be explained by a semiclassical description (e.g. one in which the clock’s evolution occurs with respect to a fixed external parameter). We derive observable signatures in optical ion clocks that require a “quantisation” of proper time through dependence on the momentum operator, tau = tau(p-hat^2). We show that by preparing initial squeezed motional states of the clock, the interferometric visibility of the internal states (i.e. “the clock”) observably reduces, an effect that results from entanglement between the internal and external degrees of freedom of the clock. We also show that this quantum treatment of proper time leads to anomalous excitation of the clock’s motion when initially prepared in its ground state, leading to a new frequency shift dubbed “quantum second-order Doppler shift”.

Albert raised some good questions following the seminar, and the discussion can be viewed on the recording of the seminar on the RQI-Cost YouTube page. One question was whether state-engineering the ground and excited states of the clock to follow the same trajectory can “mitigate” the visibility loss effect. 

https://arxiv.org/abs/2509.09573