Thanks to the breakthrough technologies originating respectively from the world of ultracold quantum gases and that of femtosecond laser frequency combs during the last decades, methods of precision spectroscopy have advanced to the point where atomic/molecular transition frequencies can be determined with an astonishing precision (down to a few parts in 10e-18), to such an extent that detecting the influence of fundamental new physics at the eV energy scale is now within reach. In this frame, more and more challenging experiments are underway, aiming at testing nature symmetries and constants with unprecedented sensitivity. In particular, in the last few years a strong interest has focused on the possibility that what we know as the fundamental physical constants might show variations over cosmological time scales. Such an effect arises quite naturally in modern theories (String-type) attempting either to establish a Grand Unification Theory (GUT) beyond the Standard Model or to reconcile this latter and General Relativity in a Theory Of Everything (TOE). Since variation of dimensional constants cannot be distinguished from that of the units, it makes more sense to consider changes of dimensionless parameters. The prime target is the fine structure constant α, which defines the scale of quantum electrodynamics. The second prominent quantity is represented by the proton-to-electron mass ratio β=mp/me, which characterizes the strength of the strong interaction in terms of the electro-weak one. While the temporal stability of α is conveniently probed through atomic transitions, the β ratio is more accurately addressed with molecular systems.
The main goal of SUPREMO is to constrain, over a-few-year timescale, the fractional temporal variation of the proton-to-electron mass ratio at a level of 10e-15/yr by means of a spectroscopic frequency measurement on a beam of cold stable molecules. This is extracted from a buffer-gas-cooling (BGC) source and then subjected to a high-precision spectroscopic interrogation scheme, like two-photon excitation in the optical domain. The probe source is an ultra-narrow-linewidth infrared laser, phase-locked to a specially-developed optical frequency comb that is ultimately referenced to the Cs primary standard via an optical fiber link.