Narrowline cooling of dysprosium atoms in an optical tweezer array

TL;DR

This study demonstrates narrow-line cooling of single dysprosium atoms in a non-magic optical tweezer array by chirping a narrow 741 nm transition to address red sidebands, achieving substantial ground-state cooling across 75 traps. The approach is validated by release-and-recapture and Raman thermometry measurements, yielding a radial ground-state occupation around 76% and a low temperature spread across the array thanks to polarization-gradient mitigation of light shifts. The work extends narrow-line cooling techniques from alkaline-earth-like species to lanthanides, highlighting the role of tensor polarizability and non-magic trapping dynamics. This establishes motional control in lanthanide tweezer platforms, paving the way for scalable quantum information processing and quantum simulation with Dy and related species.

Abstract

We perform narrowline cooling of single dysprosium atoms trapped in a 1D optical tweezers array, employing the narrow single-photon transition at 741 nm. At the trapping wavelength of 532 nm, the excited state is less trapped than the ground state. To obtain efficient cooling performances, we chirp the frequency of the cooling beam to subsequently address the red sidebands of different motional states. We demonstrate the effectiveness of the cooling protocol through Raman thermometry, which we characterize for our experimental conditions. We obtain an array of 75 atoms close to the motional ground state in the radial direction of the tweezers. Our results demonstrate the possibility to manipulate the motional degree of freedom of dysprosium in optical tweezers arrays, a key ingredient to exploit the potential of lanthanide-based tweezers platforms for quantum science.

Figures (6)

• Figure 1: Chirp cooling and time of flight measurements. a) Sketch of the cooling protocol. We load a 1D array of 75 traps at 532 nm (green) propagating along $y$, with the main polarization axis $\epsilon$ along $x$. The cooling beam (741 nm) propagates along $x$ and the magnetic field is along $y$. We chirp the laser detuning from $\delta_i$ to $\delta_f$ to cool down the hottest atoms first and the coldest atoms last (inset). b) Release and recapture measurements. Survived fraction versus time of flight $t_{\text{tof}}$ without cooling (red), with a fixed-frequency cooling (black) and with chirp cooling (blue). Solid lines are classical simulations to extract the temperature, see text. The dashed line is a numerical simulation of the ground state release and recapture with the Schrödinger equation, rescaled to 97.5 % to take into account the imaging losses Bl23.
• Figure 2: Simulations of the chirp cooling. a) Mean occupation number as a function of cooling time during a frequency ramp from $\delta_i/\omega = -6$ to $\delta_f/\omega = -0.9$ (upper horizontal axis) in $t_{\text{ramp}} =$ 30 ms, for $\alpha_e/\alpha_g = 0.5$. The initial state is thermal with $\langle n \rangle=2.5$. The vertical lines indicate motional transitions corresponding to faster-decreasing steps in $\langle n \rangle$. Thick, dotted and dashed lines indicate $n \rightarrow n-3$, $n \rightarrow n-2$, and $n \rightarrow n-1$ transitions, respectively. Despite being reduced when the Lamb-Dicke parameter $\eta$ is small, transitions with $\Delta n>1$ are still significant when $\langle n\rangle$ is large Yu18Wu23Ho23. (b) Minimum occupation number reached for different ratios $\alpha_e/\alpha_g$. The initial and final detunings of the ramp are the same as in a), and $t_{\text{ramp}}$ is either 30 ms or 50 ms to let the system reach the minimum occupation number. The blue region indicates the uncertainty interval for the polarizability ratio in our experiment. The red region indicates the Raman thermometry measurement averaged over the whole array.
• Figure 3: Raman thermometry. a) Sketch of the Raman transition and energy levels involved, see text. Energies are not in scale for clarity (in the experiment, $\Delta \gg \delta_R$), and the Zeeman splitting is not depicted to highlight the light shift energy $\Delta E_{g_1g_2}$. b) Raman sideband resolved spectra without cooling (red) and after the chirp cooling (blue), together with gaussian fits (solid lines) to extract the sideband populations. c) Light shift of the carrier transition as a function of the magnetic field direction in the $yz$ (green circles) and $xz$ plane (yellow squares), with $P_T = 1.8$ mW. The yellow dashed line is a $\cos^2{\theta}$ fit, while the gray dotted line indicates the magic angle $\theta_m$.
• Figure 4: Homogeneity of the cooling across the array. (a) Light shift of the $\ket{g}\rightarrow \ket{e}$ as a function of the trap position, before (green) and after (blue) the homogenization algorithm, for $P_T = 2.2$ mW. The inset shows a zoom for the homogenized array. (b) Temperature $T$ from a release and recapture experiment as a function of the trap position. Red points are without cooling and blue points are after chirp cooling performed with $P_T = 0.7$ mW. The horizontal dashed line is the energy associated with the ground state, $T_{gs} = \hbar\omega_g/2k_B$, with $\omega_g = 2\pi\times$ 45 kHz the radial trap frequency in the release and recapture sequence.
• Figure 5: Measurement of the excited state polarizability. a) Spectroscopy of the 741 nm transition in different 532 nm trap depths, corresponding to different power per tweezer $P_T$. Thick lines are gaussian fit to the data to extract the corresponding light shifts $\Delta E_{ge}$. b) Light shift $\Delta E_{ge}$ fitted from the spectra in a) versus tweezer power $P_T$. The black line is a linear fit.