  We demonstrate a vapor cell atomic clock prototype based on continuous-wave
(CW) interrogation and double-modulation coherent population trapping (DM-CPT)
technique. The DM-CPT technique uses a synchronous modulation of polarization
and relative phase of a bi-chromatic laser beam in order to increase the number
of atoms trapped in a dark state, i.e. a non-absorbing state. The narrow
resonance, observed in transmission of a Cs vapor cell, is used as a narrow
frequency discriminator in an atomic clock. A detailed characterization of the
CPT resonance versus numerous parameters is reported. A short-term frequency
stability of $3.2 \times 10^{-13} \tau^{-1/2}$ up to 100 s averaging time is
measured. These performances are more than one order of magnitude better than
industrial Rb clocks and comparable to those of best laboratory-prototype vapor
cell clocks. The noise budget analysis shows that the short and mid-term
frequency stability is mainly limited by the power fluctuations of the
microwave used to generate the bi-chromatic laser. These preliminary results
demonstrate that the DM-CPT technique is well-suited for the development of a
high-performance atomic clock, with potential compact and robust setup due to
its linear architecture. This clock could find future applications in industry,
telecommunications, instrumentation or global navigation satellite systems.
