We report a correlation between periodic patterns of cold-atom density and field polarization that gives new insight into a nearly 30 year old question about the role of optical phases in laser cooling. The laser field of three intersecting pairs of counter-propagating ${\sigma ^ +} - {\sigma ^ -}$ beams is widely used for cooling of atoms in magneto-optical traps and optical molasses. This six-beam optical lattice differs from four-beam lattices in having two “excess” phase constants of the optical standing waves. These relative time phases are independent degrees of freedom that affect key lattice properties, such as the polarization of the potential wells and the sub-Doppler cooling process. In this work, we create one-dimensional and two-dimensional spatial gradients of the optical phases in a six-beam lattice by slightly tilting the retro-reflected beams. This tilt imparts a long-range periodicity ($\sim {\rm mm}$) to the lattice polarization, creating a superlattice in which atoms are seen to diffuse into a periodic density distribution over several milliseconds. We find that high density is correlated with low field ellipticity; atoms move preferentially into regions where the field is linearly polarized. Such a non-uniform density may be important in cold collision experiments and may have applications in the preparation of atoms for atomic clocks and quantum computation.

中文翻译：

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六光束光学晶格中来自偏振干扰的远距离密度图案

我们报告了冷原子密度的周期性模式与场极化之间的相关性，这为有关光学相在激光冷却中作用的近30年的问题提供了新的见解。三对相交的反向传播$ {\ sigma ^ +}-{\ sigma ^-} $的激光场束被广泛用于冷却磁光阱和光学糖蜜中的原子。该六光束光学晶格与四光束晶格的不同之处在于，它们具有两个“过量”的光学驻波相位常数。这些相对时间阶段是独立的自由度，会影响关键晶格属性，例如势阱的极化和亚多普勒冷却过程。在这项工作中，我们通过略微倾斜后向反射光束来创建六光束晶格中光学相的一维和二维空间梯度。这种倾斜会产生长期的周期性（$ \ sim {\ rm mm} $）形成晶格极化，从而形成一个超晶格，在该超晶格中，原子可在几毫秒内扩散为周期性密度分布。我们发现高密度与低场椭圆率相关。原子优先移动到电场发生线性极化的区域。这种不均匀的密度在冷碰撞实验中可能很重要，并且可能在原子钟和量子计算的原子制备中具有应用。