DECOHERENCE and DISENTANGLEMENT in MACROSCOPIC SYSTEMS

物理学目前建立在描述小系统的量子力学和描述大质量物体的广义相对论的基础上。每种理论都取得了非凡的成功。到目前为止，还不可能通过实验来对抗它们。近年来，实验将量子力学推向了宏观领域。量子计算机——由固态材料或原子自旋制成的大型“量子位”阵列——涉及原子的宏观数量。量子计算面临的最大问题是“退相干”。量子比特之间微妙的相关性被它们周围的环境破坏，从而破坏了计算的机制。因此，我们需要更好地理解涉及退相干的机制，以及如何抑制它们。退相干机制也与物理学中的基本问题有关。在光力学装置中进行量子叠加的实验很快就能实现，其质量将超过普朗克质量（20微克）。这将测试量子引力的关键特征，并揭示量子力学或广义相对论是否会失效。一些人认为自然界的“内在退相干”机制将会介入；其他人则认为退相干可能导致经典物理学的出现。该项目将发展退相干机制的动力学理论，用于量子计算和量子引力测试。这些机制包括与光子、声子、电子、缺陷、核和顺磁杂质、引力自由度的传统耦合，以及更奇特的机制。“传统”理论将应用于涉及半导体中自旋阵列以及某些稀土和过渡金属系统的量子计算设计。它还将用于研究与引力波探测器相关的传统引力退相干机制。更奇特的机制包括最近的量子引力的“相关世界线”理论，以及半经典的引力理论——这些理论，以及预测的由引力引起的量子力学的崩溃，将在光力学系统上进行测试。这些系统的常规退相干也将被计算，以将其与外来机制分开。最明显的副产品/结果将是(i)抑制量子计算中的退相干的能力——这种结果对实用量子计算机的设计至关重要——以及（ii）量子引力基础实验的定量预测。其他衍生成果将包括更好地理解量子相变（受退相干的强烈影响），这对强相关凝聚态系统的理论非常感兴趣。对加拿大来说，最明显的好处是实用量子计算的发展——这是未来的关键技术，像这样的研究将是必不可少的。

Physics is currently built on Quantum Mechanics, which describes small systems, and General Relativity, which describes massive bodies. Each theory has had extraordinary success. Until now it has been impossible to confront them experimentally.      In recent years experiments have pushed Quantum Mechanics into the macroscopic realm. Quantum computers - large arrays of 'qubits' made from either solid-state materials or atomic spins - involve macroscopic numbers of atoms. The biggest problem confronting quantum computation is 'decoherence', ie., the mechanism whereby the delicate correlations between qubits are destroyed by the environment around them, ruining the computation. Thus we need a better understanding of the mechanisms involved in decoherence, and how to suppress them.      Decoherence mechanisms are also linked to fundamental questions in physics. Experiments involving quantum superpositions in optomechanical devices will soon be possible with masses larger than the Planck mass of 20 micrograms. These will test key features of quantum gravity - and reveal whether either quantum mechanics or General Relativity breaks down. Some argue that an 'intrinsic decoherence' mechanism in Nature will then intervene; others that decoherence may lead to the emergence of classical physics.      This project will develop a theory of the dynamics of decoherence mechanisms, for quantum computation and for tests of quantum gravity. The mechanisms include conventional couplings to photons, phonons, electrons, defects, nuclear and paramagnetic impurities, and gravitational degrees of freedom, as well as more exotic mechanisms. The 'conventional' theory will be applied to quantum computation designs involving arrays of spins in semiconductors as well as in certain rare earth and transition metal systems. It will also be used to look at conventional gravitational decoherence mechanisms, relevant to gravitational wave detectors.       The more exotic mechanisms include the recent 'correlated worldline' theory of quantum gravity, as well as semiclassical theories of gravity - these theories, and the predicted breakdown of quantum mechanics caused by gravity, will be tested on optomechanical systems. Conventional decoherence for these systems will also be calculated, to separate this from the exotic mechanisms.      The most obvious spinoffs/outcomes will be (i) the ability to suppress decoherence in quantum computation - such results are crucial to the design of practical quantum computers - and (ii) quantitative predictions for fundamental experiments in quantum gravity. Other spinoffs will include a better understanding of quantum phase transitions (strongly influenced by decoherence), of great interest for the theory of strongly-correlated condensed matter systems.      The most obvious benefit to Canada is in practical quantum computation development - a key future technology, where research like this will be essential.