Laser refrigeration on the nanoscale: From nanocryostats to quantum optomechanics

当被固体材料吸收的入射光随后以更高的能量发射时，发生激光制冷。光子相对于激发波长的这种蓝移发射通过将固体中的振动（声子）的内能转换为经由荧光离开固体的光子而导致内部冷却。这个过程降低了固体的熵和温度。一种材料，掺杂稀土镱离子的氟化钇锂（YLF），已被证明是该工艺的最佳体材料，最近我们首次将这种纳米级材料大幅冷却至130 K。这项原理验证实验发表在《自然光子学》（doi：10.1038/s41566-017-0005-3）上，通过使用光学悬浮将粒子与环境隔离并冷却它来完成。该计划旨在利用我们在两个密切相关的研究领域的发展。第一个项目将开发纳米尺度的激光制冷，目的是利用纳米光子学技术制造微型低温制冷机，以加强冷却，并确定使用这种材料可以达到的最低最低温度。我们的目标是将固体的温度降低到80 K以下，目标是达到10 K。通过使用1-D和3-D光子结构修改发射过程，我们将增强冷却，同时更有效地利用入射光。这种可以纳米制造的设备，使用激光而不直接物理接触，将很快在冷却电子设备，探测器和新的量子技术中找到应用。第二条链将探讨应用这项技术的基础量子力学领域内的悬浮光学力学。在这里，冷却和操纵悬浮纳米粒子的质心运动的能力现在被确立为探索宏观量子力学的一个有前途的工具。这将允许观察非经典运动状态和创造长寿命的宏观量子态，并为引力在量子力学中的作用提供测试基础。然而，虽然我们和其他研究人员已经设法消除了几乎所有的退相干源，但粒子内部温度的控制尚未掌握，但激光制冷提供了很大的希望。因此，在该计划的这一部分中，我们的目标是将悬浮粒子用于NV粒子实验，以增加自旋和运动退相干时间，并使用激光制冷在掺杂有其他稀有离子的固体中产生窄线宽，以探索悬浮纳米晶体的多普勒和边带冷却。虽然坚定地立足于英国，该计划将汇集在英国的实验和理论研究人员与国际合作者谁将有助于使该计划取得成功。这包括具有激光制冷，稀土掺杂材料制造和表征，激光纳米和微米光学制造，量子光学和量子光学力学专业知识的研究人员。

Laser refrigeration occurs when incident light absorbed by a solid material is subsequently emitted at higher energy. This blue-shifted emission of photons with respect to the excitation wavelength leads to internal cooling by converting the internal energy of the vibrations (phonons) in the solid to photons which leave the solid via fluorescence. This process lowers the entropy and temperature of the solid. One material, yttrium lithium fluoride (YLF) doped with rare earth ytterbium ions, has been shown to be the best bulk material for this process, and very recently we demonstrated the first dramatic cooling this nanoscale material to 130 K. This proof-of-principle experiment, published in Nature Photonics (doi:10.1038/s41566-017-0005-3), was accomplished by using optical levitation to both isolate the particle from the environment and to cool it. This programme aims to capitilize on our development in two strongly interlinked strands of research. The first will develop laser refrigeration on the nanoscale, with the aim of fabricating a microscale cryogenic refrigerator using the techniques of nanophotonics to enhance the cooling and to determine the lowest minimum temperature that can be achieved using this material. We aim to lower the temperature of the solid well below 80 K with the aim to reach 10 K. By modifying the emission process using 1-D and 3-D photonic structures, we will enhance cooling while more efficiently utilising the incident light. Such a device, that can be nanofabricated, and uses laser light without direct physical contact, will rapidly find applications in cooling electronics, detectors and new quantum technologies to cryogenic temperatures . A second strand will explore application of this technology to foundational quantum mechanics within the field of levitated optomechanics. Here, the ability to cool and manipulate the centre-of-mass motion of levitated nanoparticles is now established as a promising tool for exploring macroscopic quantum mechanics. This will allow the observation of non-classical states of motion and the creation of long-lived macroscopic quantum states, as well as a providing a testing ground for the role of gravity within quantum mechanics. However, while we and other researchers have managed to remove almost all sources of decoherence, it is the control of particle internal temperature that has yet to be mastered but for which laser refrigeration offers great promise. In this part of the programme we therefore aim to refrigerate levitated particles for use in NV nanocrystal experiments to increase both spin and motional decoherence time, and use laser refrigeration to produce narrow line widths in solids doped with other rare ions to explore both Doppler and sideband cooling of levitated nanocrystals. Although firmly based in the UK, this programme will bring together both experimental and theoretical researchers in UK with international collaborators who will help to make this programme a success. This includes researchers with expertise in laser refrigeration, rare earth doped materials fabrication and characterisation, laser nano and micro optical fabrication, quantum optics and quantum optomechanics.