Optical transport of cold atoms to quantum degeneracy

Abstract

Efficient transport of cold atoms is essential for continuous operation, enabling applications ranging from atomic lasers to continuously operated qubits. However, deep potentials required to overcome vibrations, axial trap nonuniformity and insufficient cooling have limited transport of cold atoms near quantum degeneracy. Here we demonstrate rapid optical transport of cold atoms to Bose-Einstein condensation using a moving optical lattice formed by two Bessel beams. A gas of $3 \times 10^5$ ytterbium atoms at a temperature of $340\,$nK is transported over $34\,$cm in $350\,$ms with efficiency over $60\%$. Furthermore, a degenerate gas of $1 \times 10^5$ atoms with a $40\%$ condensate fraction emerges from the phase synchronization process driven by atomic interactions. This demonstration enables the fast preparation of ultracold atomic beams and large-scale atom arrays for quantum sensing, simulation and computing.

Figures (10)

• Figure 1: Experimental setup. Cold ytterbium atoms are transported from a magneto-optical trap (MOT) chamber to a science chamber over a distance of 34$\,$cm within 350$\,$ms by a moving optical lattice formed by two counter-propagating Bessel beams. The lattice moves at a velocity of $v=\lambda \Delta f/2$ where $\lambda$ is the lattice wavelength and $\Delta f$ is the frequency difference between the two beams. The transport is precisely controlled by tuning the frequency difference. In the final stage of transport, the trap potential is tilted via deceleration and reduced by lowering the power, allowing hotter atoms to spill out and thereby evaporatively cooling the remaining atoms. Time-of-flight (TOF) images are taken after 15$\,$ms and 20$\,$ms of expansion in the MOT after loading and science chambers after phase synchronization, respectively.
• Figure 2: Transport and cooling mechanism. (a,b) The final temperature and transport efficiency for different initial loading temperatures. The overall transport efficiency increases with decreasing initial temperature. The final temperature depends on the final trap depth rather than the initial loading. (c) The round-trip transport efficiency as a function of transport distance for different initial temperatures. The schematic illustrates the mechanisms of atom loss and cooling during the loading phase as well as the effective evaporative cooling in the final stage. (d) The accelerations and final decelerations in units of the gravitational acceleration $g$ are optimized based on the relative atom number after transport. (e) The trap depth and optical lattice depth at different positions in the final cooling stage.
• Figure 3: Emergence of BEC from the phase synchronization. Cold atoms with initially different phases (indicated by different blue colors) are released from the moving lattice to a dipole trap and subsequently synchronized through atomic interactions, resulting in the formation of a Bose-Einstein condensate. Bimodal fits (solid lines) to the momentum distributions at different hold times, with the Gaussian component (dashed lines) representing the thermal atoms, are used to extract the condensate and thermal fractions. The TOF is 20$\,$ms.
• Figure 4: Benchmark of BEC formation(a) The temperature of the thermal cloud decreases and the condensate fraction increases, with increasing hold time. (b) Corresponding condensate fraction and phase-space density (PSD) extracted from fittings are plotted as a function of hold time. (c) The condensate fraction and PSD as a function of the total atom number remaining after the $0.3\,$s hold. Increasing the atom number loaded into the moving lattice leads to a higher final condensate fraction and a larger PSD after synchronization.
• Figure S1: Calibration of the Bessel beam. (a) The measured beam profile of a collimated Gaussian beam with a waist radius of 2.5$\,$mm after transmission through the axicon with an apex angle of $\alpha = 1^{\circ}$ and a refractive index of $n=1.5$. (b) The measured intensity (circles) agrees well with the simulation (solid lines). (c) The beam waist of the Bessel beam is almost constant $\sim50\,\mu$m along with transport.