Precision spectroscopy of the 2S-4P transition in atomic hydrogen (L. Maisenbacher) / Does gravity have to be quantum? (Dr. A. Tilloy)

Precision spectroscopy of the 2S-4P transition in atomic hydrogen (M.Sc. L. Maisenbacher)

Precision measurements of atomic hydrogen (H) have long been successfully used to extract funda­mental constants and to test bound-state quantum electrodynamics. Both the Rydberg constant R∞ and the proton root mean square charge radius rp are determined to a large degree by H spectroscopy, requiring the measurement of at least two transition frequencies. With the very precisely measured 1S-2S transition fre­quency [1] serving as a corner stone, the current limitation of this extraction is the measurement precision of other H transition frequencies. Moreover, rp extracted from the H spectroscopy world data disagrees by 4 standard deviations with the much more precise value extracted from spectroscopy of muonic hydrogen (µp) [2]. Using a cryogenic beam of H atoms optically excited to the initial 2S state, we measured the 2S-4P transition in H with a relative uncertainty of 4 parts in 1012 [3]. We motivate an asymmetric fit function, which eliminates line shifts from quantum interference of neighboring atomic resonances. Combining our result with the 1S-2S transition frequency yields the values of the Rydberg constant R∞ = 10973731:568076(96)m-1 and rp = 0:8335(95) fm. Our rp value is 3.3 combined standard deviations smaller than the previous H world data, but in good agreement with the µp value.

[1] C. G. Parthey et al., Physical Review Letters 107 (2011) 203001.
[2] A. Antognini et al., Science 339 (2013) 417.
[3] A. Beyer, L. Maisenbacher, A. Matveev et al., Science 358 (2017) 79.

Does gravity have to be quantum? (Dr. Antoine Tilloy)

Theoretical physicists have been restlessly trying to quantize gravity, leveraging more and more elaborate methods in the last 50 years. But is this even needed? Could gravity not just be classical at the fundamental level? Surprisingly, this possibility is still open. It is not forbidden by known experimental results nor by theoretical arguments (although some pseudo "no-go" theorems have historically been proposed). Interestingly, the implications of the quantum or classical nature of gravity are best discussed in the Newtonian limit and there is no need to go to general relativity to understand what is at stake. After critically discussing the historical arguments in favor of the quantization of gravity, I will present simple Newtonian models (with a quantum optics intuition), in which a fundamentally classical gravitational force cohabits with quantum matter. Free of conceptual paradoxes and so far not experimentally falsified, these models make the classical gravity option at least defensible. Fortunately, although separating different approaches to quantum gravity may forever be out of experimental reach (requiring ludicrously large accelerators), discriminating between semiclassical and fully quantum models seems doable in the near future; and only with table top experiments.

*Despite what the title might suggest, this talk will require absolutely no prior familiarity with quantum gravity and rely only on elementary non-relativistic quantum mechanics.