Testing relativity with optical clocks

State-of-the-art optical atomic clocks reach fractional frequency uncertainties at the 1e-18 level. These accurate quantum sensors make it possible to test fundamental physics with unprecedented precision. I will report on recent tests of relativity with ytterbium ion optical clocks at PTB that led to hundredfold improved spacetime anisotropy limits and twentyfold improved limits for fractional temporal variations of the fine structure constant. Furthermore, I will present experimental results obtained on the Fermi gas Sr optical clock platform at JILA that open up a new path to quantum engineering of spontaneous decay, one of the fundamental decoherence mechanisms affecting optical clock performance.