The size of macroscopic superpositions is dictated by quantum mechanics itself

There is nothing in the principles of quantum mechanics that excludes the possibility of preparing macrsocopic objects (such as cats, mugs, or even planets) in a superposition of two, or more, classically distinct states.  And yet we never observe such macroscopic superpositions in our everyday experience.  Several proposals have been put forth that attempt to explain this phenomenon: the most famous of these proposals are modifications to the Schrodinger equation to account for the effects of gravity (the so-called gravitational induced decoherence), and einselection, which deals with the issue by arguing that such macroscopic states are extremely volatile to the effects of environment induced decoherence.

 

In this seminar I will present some preliminary results from ongoing work, in collaboration with Pavel Sekatski and Wolfgang Dür from the University of Innsbruck, that takes a simpler approach to the problem.  Namely, we ask whether the size of macroscopic superpositions can be accounted for by quantum mechanics itself, without invoking extra assumptions on the effects of gravity or environmental decoherence.  One startling result from our approach is the following:  if one wants to prepare Schrödinger’s famous cat, then one would require a reference frame roughly the size of the Earth.  More generally, in order to prepare the GHZ state of N qubits (be it photons or spins) one requires a physical system—acting as a “classical” reference frame relative to which the GHZ state is prepared—whose size is at least quadratic in N.  Finally, I will explore some connections between the resource theories of coherence, reference frames, and themrodynamics, to the problem of quantifying macroscopic superpositions