Principle and progress of the ion optical frequency standards

With respect to human history, time keeping is probably up to thousands of years; with the development of science and technology, the accuracy of time keeping becomes more and more accurate. At present, the world's most accurate timing keeping device is the optical frequency standard, which can neither gain nor lose one second in about ten billion years. In this paper, we will review the development and current research progress of the ion optical frequency standards, after which we will briefly introduce the principle and the progress of the calcium ion optical frequency standard and the quantum logic aluminum ion optical frequency standard.