Ultralow long-term drift of cryogenic optical reference resonators

Summary form only given. Laser frequency stabilization is central to high resolution spectroscopy, to the development of optical frequency standards, and for fundamental tests based on clock-clock comparisons. Usually, glass ceramic reference cavities operated near room temperature are used. These exhibit a drift. We have recently demonstrated that cryogenic sapphire reference cavities can be used to stabilize lasers. Here, we report direct proof of an ultralow long-term drift of this system by comparison of a cryogenic cavity with a completely independent molecular reference. The molecular reference is the hyperfine transition of molecular iodine (I/sub 2/).