Institut Néel, CNRS, associé à l'Université J. Fourier, BP 166, 38042 Grenoble, France.
Philos Trans A Math Phys Eng Sci. 2012 Sep 28;370(1975):4487-516. doi: 10.1098/rsta.2012.0218.
Mesoscopic physics is a sub-discipline of condensed-matter physics that focuses on the properties of solids in a size range intermediate between bulk matter and individual atoms. In particular, it is characteristic of a domain where a certain number of interacting objects can easily be tuned between classical and quantum regimes, thus enabling studies at the border of the two. In magnetism, such a tuning was first realized with large-spin magnetic molecules called single-molecule magnets (SMMs) with archetype Mn(12)-ac. In general, the mesoscopic scale can be relatively large (e.g. micrometre-sized superconducting circuits), but, in magnetism, it is much smaller and can reach the atomic scale with rare earth (RE) ions. In all cases, it is shown how quantum relaxation can drastically reduce classical irreversibility. Taking the example of mesoscopic spin systems, the origin of irreversibility is discussed on the basis of the Landau-Zener model. A classical counterpart of this model is described enabling, in particular, intuitive understanding of most aspects of quantum spin dynamics. The spin dynamics of mesoscopic spin systems (SMM or RE systems) becomes coherent if they are well isolated. The study of the damping of their Rabi oscillations gives access to most relevant decoherence mechanisms by different environmental baths, including the electromagnetic bath of microwave excitation. This type of decoherence, clearly seen with spin systems, is easily recovered in quantum simulations. It is also observed with other types of qubits such as a single spin in a quantum dot or a superconducting loop, despite the presence of other competitive decoherence mechanisms. As in the molecular magnet V(15), the leading decoherence terms of superconducting qubits seem to be associated with a non-Markovian channel in which short-living entanglements with distributions of two-level systems (nuclear spins, impurity spins and/or charges) leading to 1/f noise induce τ(1)-like relaxation of S(z) with dissipation to the bath of two-level systems with which they interact most. Finally, let us mention that these experiments on quantum oscillations are, most of the time, performed in the classical regime of Rabi oscillations, suggesting that decoherence mechanisms might also be treated classically.
介观物理是凝聚态物理的一个分支，专注于固体在大块物质和单个原子之间的尺寸范围内的性质。特别是，它的特点是在一定数量的相互作用物体可以很容易地在经典和量子领域之间进行调谐的区域，从而能够在两者的边界进行研究。在磁性中，这种调谐首先是通过称为单分子磁体（SMM）的大自旋磁性分子来实现的，其原型为 Mn(12)-ac。一般来说，介观尺度可以相对较大（例如微米级超导电路），但在磁性中，它要小得多，并且可以通过稀土（RE）离子达到原子尺度。在所有情况下，都表明量子弛豫如何能够大大降低经典的不可逆性。以介观自旋系统为例，根据 Landau-Zener 模型讨论了不可逆性的起源。该模型的经典对应物被描述，特别是可以直观地理解量子自旋动力学的大多数方面。如果介观自旋系统（SMM 或 RE 系统）被很好地隔离，它们的自旋动力学就会变得相干。它们的 Rabi 振荡的阻尼研究可以通过不同的环境浴，包括微波激励的电磁浴，来获得最相关的退相干机制。这种在自旋系统中很明显的退相干现象，在量子模拟中很容易恢复。即使存在其他竞争的退相干机制，也可以在其他类型的量子比特中观察到，例如量子点中的单个自旋或超导环中的单个自旋。与分子磁体 V(15)一样，超导量子比特的主要退相干项似乎与非马尔可夫通道相关，其中与二能级系统（核自旋、杂质自旋和/或电荷）的短寿命纠缠导致 S(z)的 τ(1)-样弛豫，并与它们相互作用最密切的二能级系统浴发生耗散。最后，让我们提到，这些量子振荡实验大多数是在 Rabi 振荡的经典区域进行的，这表明退相干机制也可以用经典方法来处理。
