  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.

Atomic Physics