text: Our method of Ramsey laser is suitable for any atoms with 
  metastable energy level, as an example, we choose the tran- 
  sition from the metastable state 4*s*4*p*3*P*1 to the ground state 
  4*s*2 1*S*0 of 40Ca to check the striking feature of this laser: sub- 
  natural linewidth. As mentioned in [29], the corresponding 
  natural linewidth of the metastable state 4*s*4*p*3*P*1 is 320Hz. 
  As in the recently proposed active optical clock with atomic 
  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 1 µs. 
  If 
  a bad cavity with κ is on the order of 107Hz, the relation 
  1 is satisﬁed. Then when*g*is on the order of the 
  κ/2 
  magnitude of kHz, which can be easily achieved for current 
  technique [30], from the linewidth expression of Eq.(16) the 
  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 diﬃculty 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 applicable 
  to some special transitions [17]. 
  
  τ− 
  ≫ 