Laser locking to the 199Hg clock transition with 5.4x10(-15)/sqrt(tau) fractional frequency instability

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With Hg atoms confined in an optical lattice trap in the Lamb-Dicke regime, we obtain a spectral line at 265.6 nm in which the full-width at half-maximum is <15Hz. Here we lock an ultrastable laser to this ultranarrow clock transition and achieve a fractional frequency stability of 5.4x10(-15)/sqrt(tau) for tau<=400s. The highly stable laser light used for the atom probing is derived from a 1062.6 nm fiber laser locked to an ultrastable optical cavity that exhibits a mean drift rate of -6.0x10(-17) s(-1) (or -16.9 mHz.s(-1) at 282 THz) over a five month period. A comparison between two such lasers locked to independent optical cavities shows a flicker noise limited fractional frequency instability of 4x10(-16) per cavity.