DECOHERENCE and DISENTANGLEMENT in MACROSCOPIC SYSTEMS

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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. **