Evidence for a Bose–Einstein condensate in liquid 4He from quantum evaporation
Evidence for a Bose–Einstein condensate in liquid 4He from quantum evaporation
复制标题
来自量子蒸发的液体 4He 中玻色-爱因斯坦凝聚态的证据
作者：
A. Wyatt
玻色-爱因斯坦凝聚（BEC）是一种纯量子现象，其中宏观数量的相同原子占据相同的单粒子态。在碱金属原子的低密度气体中直接证明了BEC之后，对这一现象的兴趣大大增加。因此，值得重新考虑液态4He的情况，它被普遍接受有这样的冷凝物，但缺乏类似的直接证据。尽管如此，依赖于凝聚体存在的理论模型已经被证明成功地解释了这个系统的许多性质，BEC被认为是在液体4He中观察到的超流性和量子化涡旋的惊人现象的基础。因此，目前的问题不是这个系统中是否存在凝聚态，而是如何以清晰而简单的方式证明它的存在。在这里，我认为，较早的测量蒸发从液体4He所造成的准直光束的声子提供了这样一个演示。如果假设原子最初平行于液体表面的动量为零，则蒸发原子的角分布的计算值与测量值吻合得很好-如果原子来自冷凝物，则这是可以预期的。这种量子蒸发过程也为产生短波长的相位相干原子束提供了可能性。
Bose–Einstein condensation (BEC) is a purely quantum phenomenon whereby a macroscopic number of identical atoms occupy the same single-particle state. Interest in this phenomenon has grown considerably following the direct demonstration of BEC in low-density gases of alkali metal atoms. It is therefore worth reconsidering the case of liquid 4He, which is generally accepted to have such a condensate, but for which similarly direct evidence is lacking. Nevertheless, theoretical models that depend on the existence of a condensate have proved successful at explaining many of the properties of this system, and BEC is considered to underlie the striking phenomena of superfluidity and quantized vorticity observed in liquid 4He. So the current issue is not whether there is a condensate in this system, but how to demonstrate its existence in a clear and simple way. Here I argue that an earlier measurement of evaporation from liquid 4He caused by a collimated beam of phonons provides such a demonstration. The calculated angular distribution of evaporated atoms agrees well with that measured if it is assumed that the atoms initially had zero momentum parallel to the surface of the liquid—this is to be expected if the atoms originate from a condensate. This process of quantum evaporation also opens the possibility for creating beams of phase-coherent atoms of short wavelength.