beam [15], the velocity of the atoms in thermal atomic beam is
about 500m/s, and the length of the interaction region is about
1mm, then the time for the atom to traverse each coherent-
interaction region is on the order of magnitude of 1µs. If
a bad cavity with κ is on the order of 10 7 Hz, the relation
κ/2 ≫ τ−1 is satisﬁed. Then when g is on the order of the
magnitude of kHz, which can be easily achieved for current
technique [30], from the linewidth expression of Eq.(16) th e
order of magnitude of linewidth is below 1 Hz. This means
the linewidth of a Ramsey laser can be more than two or-
ders of magnitude narrower than the atomic natural linewidth,
therefore our Ramsey method provides a new subnatural spec-
troscopy technique. And since it is stimulated-emission spec-
trum, it overcomes the di ﬃculty in other subnatural linewidth
spectroscopy schemes where the quick reduction of signal to
noise ratio is a formidable limit. We should point out that
this Ramsey laser does not escape the limitation of all active
optical clock: in order to pump atoms to the excited state ef-
fectively and to be stimulated emit photon during the lifetime
of a metastable state, this new method will only be applicabl e
to some special transitions [17].