Thermometry and cooling of a Bose gas to 0.02 times the condensation temperature
Thermometry and cooling of a Bose gas to 0.02 times the condensation temperature
复制标题
Bose 气体的测温和冷却至冷凝温度的 0.02 倍
DOI：
10.1038/nphys3408
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发表时间：
2015
期刊：
影响因子：
19.6
通讯作者：
D. Stamper
中科院分区：
文献类型：
--
作者：
Ryan Olf;Fang Fang;G. E. Marti;A. MacRae;D. Stamper
尽管量子气体被冷却到非常低的温度，每个粒子的熵仍然比它们应该模拟的凝聚态物质系统的熵大。利用磁振子可以产生低温、低熵的气体。被困量子气体可以被冷却到令人印象深刻的低温1,2，但尚不清楚它们的熵是否低到足以实现d波超导和磁有序等现象3。对于三维晶格和二维晶格中的玻色子，量子磁有序的每个粒子的估计临界熵分别为~ 0.3kB和~ 0.03kB，晶格捕获费米气体的n<s:1>有序值与此相似5。在这里，我们报告了通过成像热化磁振子（原子气体的自旋激发）的动量分布，对简并Rb气体进行可靠的单次温度测量。我们记录的平均温度比玻色-爱因斯坦凝聚温度低50倍，表明平衡状态下每个粒子的熵为~ 0.001kB，比以前在稀释原子气体中的最佳熵低近两个数量级，远低于玻色-哈伯德系统反铁磁有序的临界熵。磁振子可以通过在热化过程中吸收能量和增强蒸发冷却来降低系统的温度，从而允许在深阱中产生低熵气体。
Despite the very low temperatures quantum gases are cooled to, the entropy per particle remains larger than that of the condensed-matter systems they are supposed to emulate. Using magnons one can produce low-temperature, low-entropy gases. Trapped quantum gases can be cooled to impressively low temperatures1,2, but it is unclear whether their entropy is low enough to realize phenomena such as d-wave superconductivity and magnetic ordering3. Estimated critical entropies per particle for quantum magnetic ordering are ∼0.3kB and ∼0.03kB for bosons in three- and two-dimensional lattices, respectively4, with similar values for Néel ordering of lattice-trapped Fermi gases5. Here we report reliable single-shot temperature measurements of a degenerate Rb gas by imaging the momentum distribution of thermalized magnons, which are spin excitations of the atomic gas. We record average temperatures fifty times lower than the Bose–Einstein condensation temperature, indicating an entropy per particle of ∼0.001kB at equilibrium, nearly two orders of magnitude lower than the previous best in a dilute atomic gas2,6 and well below the critical entropy for antiferromagnetic ordering of a Bose–Hubbard system. The magnons can reduce the temperature of the system by absorbing energy during thermalization and by enhancing evaporative cooling, allowing the production of low-entropy gases in deep traps.