上海科技大学知识管理系统

The isomer $^{229\text{m}}$Th is the most promising candidate for clocks based on the nuclear transition for its lowest excitation energy of only $8.10\pm0.17$ eV. Various experiments and theories have been focusing on how to trigger the transition between the ground state and isomeric state, among which the electronic bridge (EB) is one of the most efficient ways. We propose in this paper a new electronic bridge mechanism via two-photon excitation based a quantum optics for a two-level nuclear quantum system.  The long-lived $7s_{1/2}$ electronic shell state of $^{229\text{m}}$Th$^{3+}$ with a lifetime of around 0.6s is chosen as the initial state and the atomic shells($7s-10s$) as virtual states could be achieved in a two-photon process. When the virtual states return back to the initial state $7s_{1/2}$ , there would be some chance to trigger the nucleus $^{229}$Th$^{3+}$ to its isomeric state $^{229\text{m}}$Th$^{3+}$ via EB. It is found two lasers at moderate intensity ((10$^{10}$  - 10$^{14}$) W/m$^2$) with the photon energies near the optical range are expected to populate the isomer at a saturated rate around 10$^9/s$ which is much higher than the other mechanisms ever seen. We believe this two-photon EB scheme would provide a hint to realize the nuclear clocks and deserve a check through a series of experiments with ordinary lasers in laboratories.