A promising alternative to a CPT resonance is a three-photon absorption resonance (an "N-resonance")[1]. As in the case of the CPT resonance, there are two optical fields involved. A weak probe field, E_P, is resonant with the transition between the higher-energy hyperfine level of the ground electronic state |F=2, m_F=0> and an excited state, and optically pumps the atoms into the lower hyperfine level |F=1, m_F=0>. The strong drive field, E_D, is far-detuned from either optical transitions; if the frequency difference between E_P and E_D is equal to the hyperfine frequency, a two-photon resonance is created, driving the atoms coherently from state |F=1, m_F=> to |F=2, m_F=0>, followed by a one-photon absorption from field E_P. Such three-photon nonlinear process creates the narrow resonance in probe transmission, shown in Fig.1. Roughly speaking, the three-photon N-resonance is an all-optical realization of the double-resonance technique, where the direct coupling of two ground hyperfine states by a microwave fields produces a dip in a resonant optical field.

Attractive features of N-resonances for atomic frequency standards include high resonance contrast, leading-order light-shift cancellation, more freedom in the choice of optical transitions, and less sensitivity than coherent population trapping (CPT) resonances to high buffer gas pressures [2,3], which may provide improved short and medium term frequency stability compared to miniature CPT clocks.

References

[1] A. S. Zibrov, C. Y. Ye, Y. V. Rostovtsev, A. B. Matsko, and M. O. Scully, Phys. Rev. A 65, 043817 (2002).
[2] S. Zibrov, I. Novikova, D. F. Phillips, A. V. Taichenachev, V. I. Yudin, R. L. Walsworth, and A. S. Zibrov, Phys. Rev. A 72, 011801(R) (2005).
[3] I. Novikova, D. F. Phillips, A. S. Zibrov, R. L. Walsworth, A. V. Taichenachev, and V. I. Yudin, Opt. Lett. 31, 622 (2006); ibid 2353 (2006).