The Kondo effect in ultracold atoms

杂质模型的简单性，加上杂质对系统涌现特性的重要作用，使它们成为物理学前沿开放性问题研究的一个富有成效的起点。我们建议用超冷原子混合物来实现Kondo杂质模型作为开放性问题的实验基准，如高度相关系统的动力学性质以及非费米液体的出现。“单通道”Kondo模型描述了局部杂质与自旋为1/2的费米子海洋的反铁磁耦合。在高温下杂质可以被扰动处理，但在低温下杂质将费米子束缚成单重态。因此，在基态中，杂质被近藤屏蔽云完全从其他费米子中屏蔽掉。近藤温度区域的转变不能用微扰方法来描述，是非微扰方法的标准基准问题。在“多通道”Kondo模型中，情况发生了巨大变化，杂质与自旋为1/2的费米子的多个通道耦合。在这种情况下，基态是自旋单重态的叠加，每个单重态由杂质和来自其中一个通道的费米子形成。这种简并导致通道之间的强量子相关性，与杂质竞争单线态形成。这些量子关联甚至可以在非费米液态的形成中宏观地表现出来。激发态在这种状态下失去了它们的费米子性质。这些新的准粒子的出现及其性质的预测仍然是基础物理学的一个悬而未决的问题。本提案的目标是建立原子混合物作为研究近藤模型的新平台。这些非常干净和可调的系统的实验参数具有很高的通用性，它们现在被建立为各种模型系统的基准测试平台。尽管有这种吸引力，但迄今为止还没有关于近藤模型的实验结果。多通道费米海将由大量具有大自旋的费米子云来实现。它们将被困在一个弱的光学陷阱中，这样自旋就可以自由翻转。自旋杂质将由几个玻色子原子来实现，这些玻色子原子定位在深光阱的基态，这样它的运动就被冻结了。交换相互作用将由于玻色子和费米子之间的自旋改变碰撞而产生。我们期望相互作用强度足够高，以获得与费米温度相同的近藤温度，这是实验上可以达到的。近藤效应将通过玻色子和费米子之间的局部自旋关联来观察。我们将通过猝灭后相关的非费米子传播来检测非费米液体的行为。

The simplicity of impurity models, combined with the often fundamental importance of the impurity on the emergent properties of the system, makes them a fruitful starting point for studies of open questions at the forefront of physics. We propose to implement the Kondo impurity model with ultracold atomic mixtures as experimental benchmark for open questions such as dynamical properties of highly correlated systems as well as the emergence of non-Fermi liquids. The 'single-channel' Kondo model describes the antiferromagnetic coupling of a localized impurity to a sea of spin-1/2 Fermions. The impurity can be treated perturbatively at high temperatures, but for low temperatures the impurity binds a Fermion into a singlet. In the ground state, the impurity is therefore entirely screened from the other Fermions by the Kondo screening cloud. The transition in the region of the Kondo temperature cannot be described by perturbative approaches and is a standard benchmark problem for non-perturbative methods. The situation changes dramatically in the 'multi-channel' Kondo model, where the impurity is coupled to more than one channel of spin-1/2 Fermions. In this case, the ground state is the superposition of spin singlets, each formed by the impurity and a Fermion from one of the channels. This degeneracy leads to strong quantum correlations between the channels, competing for the singlet formation with the impurity. These quantum correlations can even manifest themselves macroscopically in the formation of a non-Fermi liquid state. The excitations are losing their fermionic character in such a state. The emergence of these new quasi-particles and the prediction of their properties remains an open problem of fundamental physics. The goal of this proposal is to establish atomic mixtures as a new platform for investigating the Kondo model. The experimental parameters of these extremely clean and tunable systems enjoy a high versatility and they are now established as a benchmarking platform for a variety of model systems. Despite this attractiveness, no experimental results have been reported so far on the Kondo model. The multi-channel Fermi sea will be realized by a large cloud of Fermions with large spin. They will be trapped in a weak optical trap such that the spin is free to flip. The spin impurity will be realized by a few bosonic atoms that are localized in the ground states of a deep optical trap, such that its motion is frozen out. The exchange interaction will arise due to spin-changing collisions between the Bosons and the Fermions. We expect the interaction strength to be sufficiently high to obtain a Kondo temperature in the order of the Fermi temperature, which is experimentally accessible. The Kondo effect will be observed through local spin correlations between the Bosons and Fermions. We will detect non-Fermi liquid behavior through the non-fermionic propagation of correlations after a quench.