Four-second optical coherence between different atomic species, and the search for new physics with atomic clocks*

The extreme precision and accuracy of today’s optical atomic clocks can be used to look for very small deviations from the predictions of the Standard Model, offering a tool to search for beyond Standard Model (BSM) physics complementary to particle accelerators.  These searches are based on measuring the frequency ratio of two atomic transitions that depend differently on interactions with BSM particles or fields.  In this talk, I will present frequency ratio measurements between atomic clocks based on Al⁺, Hg⁺, Sr, and Yb atoms, and the use of these measurements to constrain the coupling of ultralight scalar dark matter candidates to the Standard Model.  The precision of traditional, incoherent frequency ratio measurements and resulting constraints on BSM physics are limited by the coherence time of the lasers used to probe the atomic transitions.  We have recently demonstrated a new, coherent frequency ratio measurement technique that removes this limitation and achieved a record for the precision of frequency ratio measurements between different atomic species.