Neutral Atom Frequency Reference in the Deep Ultraviolet with $\mathrm{\text{Fractional Uncertainty}}=5.7\times {10}^{-15}$

We present an assessment of the ($6s^2$) ${}^{1}S_0\leftrightarrow (6s6p){}^{3}P_0$ clock transition frequency in ${}^{199}\mathrm{Hg}$ with an uncertainty reduction of nearly 3 orders of magnitude and demonstrate an atomic quality factor $Q$ of $\sim {10}^{14}$. The ${}^{199}\mathrm{Hg}$ atoms are confined in a vertical lattice trap with light at the newly determined magic wavelength of $362.5697\pm 0.0011\mathrm{nm}$ and at a lattice depth of $20E_R$. 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. We find the frequency of the clock transition to be $1128575290808162.0\pm 6.4(\mathrm{syst})\pm 0.3(\mathrm{stat})\mathrm{Hz}$ (i.e., with $\mathrm{\text{fractional uncertainty}}=5.7\times {10}^{-15}$). Neither an atom number nor second order Zeeman dependence has yet been detected. Only three laser wavelengths are used for the cooling, lattice trapping, probing, and detection.

Figure 1

a) Drawing of the vacuum system showing the 3D MOT (left side) and 2D MOT chambers. Also visible is the upper mirror of the optical lattice cavity above the 3D MOT chamber (note that fl. refers to fluorescence). (b) Timing sequence for the cooling light, probe light, and 3D MOT magnetic field gradient. The lattice light remains on continuously throughout the cycle. $\Delta t_l=80\mathrm{ms}$ and $T_p=50\mathrm{ms}$ for most measurements here; $\Delta t_d=9\mathrm{ms}$.

Figure 2

(a) Partial level scheme for ${}^{199}\mathrm{Hg}$ with hyperfine splitting of the ground and excited states. The 265.6 nm radiation is used to probe the ${}^{1}S_0\leftrightarrow {}^{3}P_0$ clock transition ($\Gamma /2\pi \approx 100\mathrm{mHz}$), while 253.7 nm radiation is used for cooling and detection. (b) A spectrum of the ${}^{199}\mathrm{Hg}$ clock $\sigma$ transition with $m_F=\pm 1/2\to m_F=\pm 1/2$ Zeeman components overlapped. ${\lambda }_L=362.573\mathrm{nm}$ and $\mathrm{FWHM}=140\mathrm{Hz}$. ${\nu }_C$ is defined in the text. (c) Line profile of the ${}^{199}\mathrm{Hg}$ $\sigma$ transition showing the separated Zeeman components with $\mathrm{FWHM}=120\mathrm{Hz}$. (d) Frequency separation of the Zeeman components for the ultranarrow $\pi$ and broadened $\sigma$ transitions versus bias $B$ field. The line gradients are $3.1\pm 0.2\mathrm{Hz}\mu {\mathrm{T}}^{-1}$ and $11.1\pm 1.7\mathrm{Hz}\mu {\mathrm{T}}^{-1}$, respectively.

Figure 3

Ground state fraction versus probe laser detuning frequency for (a) 50 and (b) 80 ms Rabi pulses. $\Omega T_p=1.45\pi$ and $1.36\pi \mathrm{rad}$ for the curve fits (solid lines), respectively. $B=0$ and ${\lambda }_L\approx {\lambda }_m$ for (a) and (b).

Figure 4

(a) Differential light shift versus lattice frequency for the ${}^{199}\mathrm{Hg}$ clock transition. The inset shows measurements made over a larger frequency range. The Stark-shift free frequency (wavelength) is $826.8546\pm 0.0024\mathrm{THz}$ ($362.5697\pm 0.0011\mathrm{nm}$) and the slope is $-57\mathrm{mHz}{E}_R^{-1}{\mathrm{GHz}}^{-1}$. (b) Clock transition frequency versus lattice wavelength with changes of lattice power. (1W implies $3.5E_R$ for the trap depth.)

Figure 5

Line-center frequency versus (a) relative atom number, where $N_0=2.5\times {10}^3$ and (b) MOT cooling light intensity, $s_0$. (c) Frequency shift at $N_0$ versus measurement day. (d) Line-center frequency versus probe power.

Figure 6

Clock transition frequency measured over a three month period with respect to the SYRTE primary frequency standard for ${\lambda }_m=362.5697\mathrm{nm}$. The weighted mean is ${\nu }_{\mathrm{c}}-238.2\pm 0.3\mathrm{Hz}$. ${\nu }_{\mathrm{c}}=1128575290808400\mathrm{Hz}$.