Proposal to test quantum mechanics against macroscopic realism using continuous variable entanglement : a definitive signature of a Schrödinger cat

In the Schrödinger-cat gedanken experiment a cat is in a quantum superposition of two macroscopically distinct states, alive and dead. The paradoxical interpretation of quantum mechanics is that the cat is not in one state or the other, alive or dead, immediately prior to its measurement. Because of this apparent defiance of macroscopic reality, quantum superpositions of states macroscopically distinct have generated much interest. Here we address the issue of proving a contradiction with macroscopic reality objectively, through the testable predictions of quantum mechanics. We consider the premises of macroscopic reality (that the “cat” is either “dead” or “alive”, the measurement indicating which) and macroscopic locality (that simultaneous measurements some distance away cannot induce the macroscopic change, “dead” to “alive” and vice versa, to the “cat”). The predictions of quantum mechanics for certain states, generated using states exhibiting continuous variable entanglement, are shown to be incompatible with the predictions of all theories based on this dual premise. Our proof is along the lines of Bell’s theorem, but where all relevant measurements give macroscopically distinct results. Schrödinger [1] raised the issue of the existence and interpretation of the quantum superposition of two macroscopically distinct states in his famous Schrodinger-cat gedanken experiment. A particle is in a quantum superposition of having escaped the nucleus, or otherwise. The presence of the particle outside the nucleus will trigger a lethal device that will kill a cat located in a box. An observer later looks into the box to determine the state of the cat, whether dead or alive. The application of quantum mechanics, to all stages of the sequence of interactions, would ultimately predict the cat to be in a superposition of a state |1〉, where the cat is dead, and a state | − 1〉, where the cat is alive. It is a basic feature of quantum mechanics that the quantum superposition state (|+〉+ |−〉)/ √ 2 cannot be regarded as classical mixture, where the system is considered to be in state |+〉 with probability 1/2, or in state |−〉 with probability 1/2. Yet to say in this case that the cat cannot be considered to be dead or alive prior to its measurement, here the observer opening the box to view the state of the cat, would seem nonsensical.