A neutral atom frequency reference in the deep UV with 10 −15 range uncertainty We present an assessment of the (6s 2 ) 1 S0 ↔ (6s7s) 3 P0 clock transition frequency in 199 Hg with an uncertainty reduction of nearly three orders of magnitude and demonstrate an atomic quality factor, Q, of ∼1014 . The 199 Hg atoms are confined in a vertical lattice trap with light at the newly determined magic wavelength of 362.5697±0.0011 nm and at a lattice depth of 20 ER. The atoms are loaded from a single stage magneto-optical trap with cooling light at 253.7 nm. The high Q factor is obtained with an 80 ms Rabi pulse at 265.6 nm. The frequency of the clock transition is found to be 1 128 575 290 808 162.0 ± 6.4 (sys.) ± 0.3 (stat.) Hz (fractional uncertainty = 5.7×10 −15 ). Neither an atom number nor second order Zeeman dependence have yet to be detected. Only three laser wavelengths are used for the cooling, lattice trapping, probing and detection.

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High-resolution spectroscopy has been carried out on the 199 Hg 1 S 0 − 3 P 0 spin and dipole forbidden transition, where the atoms are confined in a vertical one-dimensional optical lattice trap using light at the magic wavelength. We describe various characteristics of the resulting line spectra and assess the strength of the Lamb-Dicke confinement. Through a series of absolute frequency measurements of the 199 Hg clock transition with respect to the LNE-SYRTE primary frequency standard, recorded over a 3-month period, we demonstrate a statistical fractional uncertainty of 2.5 × 10 −16 . We include details relating to the generation of ultraviolet light at three wavelengths necessary for the experiment: 253.7 nm for cooling and detection, 265.6 nm for clock transition probing, and 362.570 nm for lattice trapping, along with further aspects related to the magic wavelength evaluation.