Source: National Natural Science Foundation of China 

Grant No:91536104

Project: Nondestructive measurement of the transition probability in an ytterbium optical clock

Chief Specialist: Prof.Baolong Lv

Abstract: Ytterbium lattice clock is currently one of optical clocks with the highest precision. In contrast with an optical ion clock, optical lattice clocks based on  neutral atoms can interrogate many atoms simultaneously, and thus have much lower quantum projection noises. However, the long-term frequency instability of a lattice clock has so far been limited by the Dick effect, which is still well above the limit set by the quantum projection noise. The Dick effect can be significantly suppressed by reducing the time spent on the preparation of the cold atoms, and hence increasing the duty cycle which is defined by ratio of the interrogation time to the cycle time Tc. Usually, the transition probability of the clock transition is detected by the resonant fluorescence method which also leads to the total loss of the detected atoms due to the heating effects. Enough time must be spent to repeat the cooling process in order to trap new atoms for  the subsequent cycle.This project is aimed to realize the so-called nondestructive measurement of the transition probability for an ytterbium clock. The basic strategy is the measurement of the phase shift of the probe light induced by the atoms in the ground state, by employing a scheme of Mach-Zehnder interferometer. The novelty of our design lies in the weak inter-combination transition which is chosen for the phase detection. Compared with a strong transition, the detection based on this weak transition has not only larger phase shift signals, but also lower photon  scattering rates, and hence negligible atomic losses. Nearly all atoms can thus remain in the optical lattice. Our nondestructive measurement technique, once applied to a Yb clock in the future, will significantly reduce the preparation time of the atomic sample, enable a larger duty cycle (possibly higher than 0.8), and eventually make the averaging time an order of magnitude shorter to reach a stability of the order of E-18.