Thermometry via Light Shifts in Optical Lattices

For atoms or molecules in optical lattices, conventional thermometry methods are often unsuitable due to low particle numbers or a lack of cycling transitions. However, a differential spectroscopic light shift can map temperature onto the line shape with a low sensitivity to trap anharmonicity. We study narrow molecular transitions to demonstrate precise frequency-based lattice thermometry, as well as carrier cooling. This approach should be applicable down to nanokelvin temperatures. We also discuss how the thermal light shift can affect the accuracy of optical lattice clocks.

Figure 1

(a) ${}^{88}\mathrm{Sr}$ atoms are trapped and cooled in a 1D optical lattice, and subsequently photoassociated on the narrow 689 nm intercombination line to create ultracold diatomic molecules. The molecules are then probed along the lattice axis in the LD and RSB regimes. (b) The carrier ($C$) and blue and red sideband (SB) transitions between long-lived electronic states in an approximately harmonic trap are indicated.

Figure 2

(a) An optical spectrum of ${\mathrm{Sr}}_2$ molecules in a state-insensitive lattice. The central carrier transition and the first-order red and blue SBs are visible (the SB signals are enhanced via longer probing times). The axial trap frequency $f_x\sim 80\mathrm{kHz}$ is found from the SB spacing [17], while $f_r\sim 0.6\mathrm{kHz}$. (b) The carrier line shape in a state-sensitive lattice, including light-induced shift and broadening. The average light shift $W/h$ and the temperature-independent contribution to the light shift $W_0/h$ are indicated. (The natural logarithm of the data was taken prior to fitting, to account for linear probe absorption.) Zero detuning of the probe laser on the horizontal axes in (a) and (b) corresponds to zero lattice light shift. (c) The dependence of $W/h$, $W_0/h$, $f_x$, and ${\mathrm{\Gamma }}_{\mathit{C}}$ on the lattice light power.

Figure 3

Carrier thermometry of ultracold molecules (stars) is compared with an alternative technique that uses SB areas (circles), for (a) $v=-1$ and (b) $v=-2$ molecules. The notation ($v,J$) specifies the vibrational level and total angular momentum of the molecule. The molecular potentials $X$ and $1_g$ [7] are separated by an optical frequency.

Figure 4

(a) Molecule temperatures at various lattice light powers via carrier thermometry, along with the initial atom temperatures via TOF expansion imaging. (b) Molecule temperature versus the PA pulse duration. (c) Molecule temperature versus the lattice hold time. (d) Carrier cooling of molecules in a weakly state-sensitive lattice.