An efficient method to generate near-ideal hollow beams of different shapes for box potential of quantum gases

TL;DR

The paper addresses the challenge of achieving homogeneous quantum gases by overcoming density inhomogeneity in harmonic traps. It introduces a hybrid optical approach that combines fixed optics for pre-shaping Gaussian light into hollow beams with a digital micromirror device (DMD) for fine-tuning, yielding near-ideal box potentials of various shapes. The authors demonstrate ring- and square-shaped hollows with ultra-steep inner boundaries, achieving power-law exponents up to $\alpha \approx 104$ and residual center light below $2\%$, while boosting the peak-intensity–efficiency product $I_{\rm peak}\eta$ relative to conventional methods. This programmable, efficient hollow-beam design enables nearly uniform two-dimensional quantum gases and versatile geometries for exploring quantum many-body physics, potentially in conjunction with an accordion lattice architecture.

Abstract

Ultracold quantum gases are usually prepared in conservative traps for quantum simulation experiments. The atomic density inhomogeneity, together with the consequent position-dependent energy and time scales of cold atoms in traditional harmonic traps, makes it difficult to manipulate and detect the sample at a better level. These problems are partially solved by optical box traps of blue-detuned hollow beams. However, generating a high-quality hollow beam with high light efficiency for the box trap is challenging. Here, we present a scheme that combines the fixed optics, including axicons and prisms, to pre-shape a Gaussian beam into a hollow beam, with a digital micromirror device (DMD) to improve the quality of the hollow beam further, providing a nearly ideal optical potential of various shapes for preparing highly homogeneous cold atoms. The highest power-law exponent of potential walls can reach a value over 100, and the light efficiency from a Gaussian to a hollow beam is also improved compared to direct optical shaping by a mask or a DMD. Combined with a one-dimensional optical lattice, a nearly ideal two-dimensional uniform quantum gas with different geometrical boundaries can be prepared for exploring quantum many-body physics to an unprecedented level.

Figures (3)

• Figure 1: (a) Schematic diagram of the near-ideal hollow beam generation system. (b) The experimental setup of the ring-shaped hollow beam. (c) The experimental setup of the square-shaped hollow beam.
• Figure 2: The image of the hollow beam with (b,e) and without (a,d) DMD optimization pictured by the Pi camera at the atom position. For both ring-shaped (a,b) and square-shaped (d,e) cases, the DMD perfectly tailors the residual light inside the hollow beam and realizes a clear and sharp inner boundary. (c) The intensity profile of the ring beam (blue line) along the vertical diameter [white dashed line in (b)]. A power-law fit (red line) gives an exponent of $\alpha = 80 \pm 7$. (f) The intensity profile of the square-shaped beam (blue line) along a vertical cut through the origin [white dashed line in (e)]. A power-law fit (red) gives an exponent of $\alpha = 104 \pm 9$.
• Figure 3: The product of peak-intensity and power efficiency $I_{\rm peak} \cdot \eta$ obtained by different methods, as a function of the ratio between the inner and outer radii of the ring-shaped beam. The blue solid line represents the theoretically calculated result for our proposed scheme, the green dashed line denotes the result from the masking method, and the red dotted line is obtained by using DMD only. The result of DMD is lower than that of the masking method by a factor of 2.7, owing to the diffraction efficiency of DMD of 38%. Our experimental result (black dot) agrees well with the theoretical prediction and beats the other two methods by a factor of 7.3 at $r_{\rm in}/r_{\rm out}=0.85$.