text: Though, the natural linewidth of quantum transition was 
  regarded as the ultimate limit to high-resolution laser spec- 
  troscopy [4], several methods of subnatural linewidth spec- 
  troscopy have been proposed to gain subnatural linewidth [3– 
  10]. However, in all these eﬀorts, including optical Ramsey 
  spectroscopy, subnatural line is realized at the expense of a 
  quick reduction in signal-to-noise (SNR) ratio due to the ex- 
  ponential decaying of signal, thus all these schemes can only 
  get the linewidth several times narrower than the atomic nat- 
  ural linewidth. In the past three decades, this situation does 
  not change in the ﬁeld of the precision laser spectroscopy. 
  On the other hand, the thermal noise of the cavity mirrors is 
  the main obstacle for further linewidth reduction of a laser 
  [11, 12], and it is a challenge to substantially reduce this noise 
  further[13]. Recently, a new scheme, called active optical 
  clock [14–18], was proposed to substantially reduce the laser 
  linewidth. With lattice trapped atoms, it is possible to reach 
  mHz linewidth laser based on the mechanism of active optical 
  clock [14, 15, 19]. The principal mechanism of active optical 
  clock is to directly extract light emitted from the ultranarrow 
  atomic transition with a cavity mode linewidth much wider 
  than that of lasing. This bad cavity ensures that any frequency 
  shift due to cavity noise reduces to cavity-pulling eﬀect [15– 
  17], then the thermal noise is not the major obstacle again for 
  reducing the linewidth. This means the bad cavity can play an 
  indispensable role in new subnatural linewidth spectroscopy. 
  In this Letter, we propose a new scheme called Ramsey 
  laser with bad cavity. Distinct from any previous applications 
  of conventional Ramsey separated oscillating ﬁelds method 
  [1], which focuses on the absorption spectrum, we here fo- 