**Large-Scale Quantum Phenomena, & the Decoherence Enigma (PHAS Colloquium)** - Philip Stamp

We are all accustomed to macroscopic quantum phenomena in superfluids and superconductors- things like superflow, the Meissner and Hess-Fairbank effects, the Josephson effects, etc.- or macroscopic quantum tunneling and coherence in SQUIDs. They have also been discussed in theories of the early universe. But can one envisage large-scale quantum phenomena in any other system - or are these just special cases? This down-to-earth question touches upon deep issues – the nature of quantum mechanics, how quantum superpositions are suppressed during measurements, the transition from quantum to classical physics, etc., If the answer is yes, the way is open to large-scale entanglement, and the practical development of quantum information processing systems (and much else besides!).
I will describe in simple terms the central issue here – the question of what are physical mechanisms of ‘decoherence’, the process in which phase correlations in the dynamics of a quantum system are suppressed by interaction with the environment. We will find that decoherence in the low-energy dynamics of a quantum system is mostly caused by what is called the ‘spin bath’, which describes everything from nuclear spins to ‘defects’ in solids. Some very pretty experiments which I will describe in magnetic and superconducting systems have confirmed this, and so we now know at least in principle how to suppress most decoherence. This also opens the way to exploration of designs for ‘qubit networks’, which can be made from microscopic spins. On the way to this I will also explain a rather elegant and simple description of decoherence in terms of ‘quantum walks’ of a particle on a graph.