A “Schrödinger’s pet” machine


Our machine for turning any input quantum state into a coherent superposition has just appeared on PRL.

The superposition principle is at the heart of quantum mechanics, and the Schrödinger’s cat case is the prototype of the paradoxical effects of extending its predictions to our everyday world. Numerous attempts, to varying degrees of success, have been made to produce such superpositions between a given -possibly macroscopic- quantum state and its most distinct state, an orthogonal state. However, the main ingredient of the story is not the cat inside the box, but rather the quantum superposition of two operations: ‘kill it’ or ‘leave it alive’. Once realized, whatever creature is in the box will be put in a superposition of the ‘dead’ and ‘alive’ orthogonal states.

Here, rather than trying to produce a specific state superposition, as is the standard method of investigating the paradox, we approach it from this general operational viewpoint by proposing and experimentally demonstrating a universal strategy to produce linear superpositions of an arbitrary initial state and its orthogonal. Constituting a sort of Schrödinger’s machine, able to turn a whole zoo of input states into coherent superpositions, our scheme can also produce arbitrary continuous-variable optical qubits, which may prove practical for implementing innovative quantum technologies and measurement tasks.