Measuring single qubits: "This is where Korotkov and his peers come in. Their system would possibly be a more practical way of distinguishing between a system that exhibits classical behaviors and quantum behaviors. The test proposed by Korotkov and his colleagues is one that makes use of quantum back action, a concept that many in quantum mechanics choose to ignore. “Such an experiment would prove that you can’t explain this principle in terms of classical physics.” Plus, using such a test would detect a system that “is required to evolve in the direction of the result, giving you information gradually.” And a system that does that, a system affected by observation, is an indicator of a quantum system. "