Coherent Schrödinger’s cat still confounds

Diagram showing the experiment proposed by Christoph Simon and colleagues. Photon A moves from the source to the left where its polarization can be measured. Photon B moves to the right where it is amplified into a macroscopic beam.

Schrödinger’s cat is an example of “micro-macro entanglement”, whereby quantum mechanics allows (in principle) a microscopic object such as an atomic nucleus and a macroscopic object such as a cat to have a much closer relationship than permitted by classical physics. However, it is clear to any observer that microscopic objects obey quantum physics, while macroscopic things obey the classical physics rules that we experience in our everyday lives. But if the two are entangled it is impossible that each can be governed by different physical rules.

The most common way to avoid this problem is to appeal to quantum decoherence, whereby multiple interactions between an object and its surroundings destroy the coherence of superposition and entanglement. The result is that the object appears to obey classical physics, even though it is actually following the rules of quantum mechanics. It is impossible for a large system such as a cat to remain completely isolated from its surroundings, and therefore we do not perceive it as a quantum object.

While not disputing this explanation, Christoph Simon and a colleague at the University of Calgary, and another at the University of Geneva, have asked what would happen if decoherence did not affect the cat. In a thought experiment backed up by computer simulations, the physicists consider pairs of photons (A and B) generated from the same source with equal and opposite polarizations, travelling in opposite directions. For each pair, photon A is sent directly to a detector, but photon B is duplicated many times by an amplifier to make a macroscopic light beam that stands in for the cat. The polarizations of the photons in this light beam are then measured.

They consider two different types of amplifier. The first measures the state of photon B, which has the effect of destroying the entanglement with A, before producing more photons with whatever polarization it measures photon B to have. This is rather like the purely classical process of observing the Geiger counter to see whether it has detected any radiation, and then using the information to decide whether or not to kill the cat. The second amplifier copies photon B without measuring its state, thus preserving the entanglement with A.

The researchers ask how the measured polarizations of the photons in the light beam will differ depending on which amplifier is used. They find that, if perfect resolution can be achieved, the results look quite different. However, with currently available experimental techniques, the differences cannot be seen. “If you have a big system and you want to see quantum features like entanglement in it, you have to make sure that your precision is extremely good,” explains Simon. “You have to be able to distinguish a million photons from a million plus one photons, and there is no current technology that would allow you to do that.”

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