Quantum Computational Matter


Speaker: Associate Professor Stephen Bartlett (University of Sydney)
Date: 23rd September 2011

Low-temperature phases of strongly-interacting quantum many-body systems can exhibit a range of exotic quantum phenomena, from superconductivity to fractionalized particles.  One exciting prospect is that the ground or low-temperature thermal state of an engineered quantum system can function as a quantum computer.  The output of the computation can be viewed as a response, or ‘susceptibility’, to an applied input (say in the form of a magnetic field).  For this idea to be sensible, the usefulness of a ground or low-temperature thermal state for quantum computation cannot be critically dependent on the details of the system’s Hamiltonian; if so, engineering such systems would be difficult or even impossible.  A much more powerful result would be the existence of a robust ordered phase which is characterised by its ability to perform quantum computation.
I’ll discuss some recent results on the existence of such a quantum computational phase of matter.  I’ll outline some positive results on a phase of a toy model that allows for quantum computation, including a recent result that provides sufficient conditions for fault-tolerance.  I’ll also introduce a more realistic model of antiferromagnetic spins, and demonstrate the existence of a quantum computational phase in a two-dimensional system.  Together, these results reveal that the characterisation of quantum computational matter has a rich and complex structure, with connections to renormalisation and recently-proposed concepts of ‘symmetry-protected topological order’.