Can things really be in two places at once?

Can objects be in two places at the same time? Even if we take the best theory of atomic physics for granted – quantum mechanics – the idea is difficult to reconcile with our experiences of everyday life. Is there a reason why macroscopic objects (such as cats, or the moon) cannot also be placed into superpositions: To exist in paradoxical quantum states? A rigorous experimental method to decide the question has been proposed by Leggett and Garg – one which can exclude theories that describe an object as being either here or there (rather than both here and there, which is what quantum theory tells us). In collaboration with the quantum spin dynamics group, we were able to perform such an experiment. It was reported in Nature Communications at the beginning of 2012.

Our chosen quantum system was a Phosphorous nucleus – and in the same way that Schrodinger proposed to place a cat into a superposition of ‘alive’ and ‘dead’, we wanted to place the nucleus in a superposition of ‘spin up’ and ‘spin down’. By using a nearby electron spin to measure the nuclear spin in a non-invasive way, it was possible to show inescapable evidence that the nuclear spin state was indeed in a superposition of both up and down at the same time. This was possible thanks to an extension of Leggett and Garg’s proposal designed to apply to imperfect measurement procedures. Most would consider this system microscopic, and therefore not be terribly surprised by the results – but the experiment will inspire future work probing larger and more complex objects.

This work involved George Knee and Simon Benjamin from QuNaT as well as many of our collaborators.