Radiative cooling of a spin ensemble
Radiative cooling of a spin ensemble
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DOI：
10.1038/s41567-020-0872-2
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发表时间：
2020-04-20
期刊：
影响因子：
19.6
通讯作者：
Bertet, P.
中科院分区：
文献类型：
--
作者：
Albanese, B.;Probst, S.;Bertet, P.
固体中的电子自旋通常通过与主晶格中的声子耦合来弛豫它们的能量。通过在腔中与微波光子的耦合作为另一种弛豫路径，证明了自旋可以被冷却到晶格温度以下，物理系统通过与环境的能量交换达到热平衡，对于固体中的自旋，相关的环境几乎总是它们的主晶格。然而，最近的研究（1）由珀塞尔（2）的观察激发，已经表明，如果自旋-腔耦合足够大（例如对于小模体积腔），进入微波腔的辐射发射如何成为自旋的主要弛豫路径。在这种情况下，腔电磁场取代晶格作为主导环境，通过设计合适的腔场，邀请独立于晶格的自旋温度来控制自旋温度的前景。在这里，我们报告正是这样的控制自旋温度，说明了一种新的和通用的方法，以增加电子自旋极化超过其热平衡值（称为超极化）。通过在不同温度下在电阻负载之间切换腔输入，我们可以控制电子自旋极化，将其冷却到晶格温度以下。我们的演示使用硅中的施主自旋耦合到超导微谐振器，我们观察到自旋极化增加了两倍以上。这种方法提供了一种通过动态核自旋极化增强电子自旋共振或核磁共振信号的一般途径（3，4）。
Electron spins in solid usually relax their energy through the coupling with phonons in the host lattice. By using the coupling to microwave photons in a cavity as an alternative relaxation path, it is demonstrated that spins can be cooled below the lattice temperature.Physical systems reach thermal equilibrium through energy exchange with their environment, and for spins in solids the relevant environment is almost always their host lattice. However, recent studies(1) motivated by observations by Purcell(2) have shown how radiative emission into a microwave cavity can become the dominant relaxation path for spins if the spin-cavity coupling is sufficiently large (such as for small-mode-volume cavities). In this regime, the cavity electromagnetic field overrides the lattice as the dominant environment, inviting the prospect of controlling the spin temperature independently from that of the lattice, by engineering a suitable cavity field. Here, we report on precisely such control over spin temperature, illustrating a novel and universal method to increase the electron spin polarization above its thermal equilibrium value (termed hyperpolarization). By switching the cavity input between resistive loads at different temperatures we can control the electron spin polarization, cooling it below the lattice temperature. Our demonstration uses donor spins in silicon coupled to a superconducting microresonator and we observe more than a twofold increase in spin polarization. This approach provides a general route to signal enhancement in electron spin resonance, or nuclear magnetic resonance through dynamical nuclear spin polarization(3,4).