The famous paradox of Schr=C3=B6dinger's cat starts from principles = of quantum=20 physics and ends with the bizarre conclusion that a cat can be = simultaneously in=20 two physical states =E2=80=93 one in which the cat is alive and the = other in which it is=20 dead. In real life, however, large objects such as cats clearly don't = exist in a=20 superposition of two or more states and this paradox is usually resolved = in=20 terms of quantum decoherence. But now physicists in Canada and = Switzerland argue=20 that even if decoherence could be prevented, the difficulty of making = perfect=20 measurements would stop us from confirming the cat's superposition.
Erwin Schr=C3=B6dinger, one of the fathers of quantum theory, = formulated his=20 paradox in 1935 to highlight the apparent absurdity of the quantum = principle of=20 superposition =E2=80=93 that an unobserved quantum object is = simultaneously in multiple=20 states. He envisaged a black box containing a radioactive nucleus, a = Geiger=20 counter, a vial of poison gas and a cat. The Geiger counter is primed to = release=20 the poison gas, killing the cat, if it detects any radiation from a = nuclear=20 decay. The grisly game is played out according to the rules of quantum = mechanics=20 because nuclear decay is a quantum process.
If the apparatus is left for a period of time and then observed, you = may find=20 either that the nucleus has decayed or that it has not decayed, and = therefore=20 that the poison has or has not been released, and that the cat has or = has not=20 been killed. However, quantum mechanics tells us that, before the = observation=20 has been made, the system is in a superposition of both states =E2=80=93 = the nucleus has=20 both decayed and not decayed, the poison has both been released and not = been=20 released, and the cat is both alive and dead.
Mixing micro and macro
Schr=C3=B6dinger's cat is an example of "micro-macro entanglement", = whereby=20 quantum mechanics allows (in principle) a microscopic object such as an = atomic=20 nucleus and a macroscopic object such as a cat to have a much closer=20 relationship than permitted by classical physics. However, it is clear = to any=20 observer that microscopic objects obey quantum physics, while = macroscopic things=20 obey the classical physics rules that we experience in our everyday = lives. But=20 if the two are entangled it is impossible that each can be governed by = different=20 physical rules.
The most common way to avoid this problem is to appeal to quantum=20 decoherence, whereby multiple interactions between an object and its=20 surroundings destroy the coherence of superposition and entanglement. = The result=20 is that the object appears to obey classical physics, even though it is = actually=20 following the rules of quantum mechanics. It is impossible for a large = system=20 such as a cat to remain completely isolated from its surroundings, and = therefore=20 we do not perceive it as a quantum object.
While not disputing this explanation, Christoph Simon and a colleague = at the=20 University of Calgary, and another at the University of Geneva, have = asked what=20 would happen if decoherence did not affect the cat. In a thought = experiment=20 backed up by computer simulations, the physicists consider pairs of = photons (A=20 and B) generated from the same source with equal and opposite = polarizations,=20 travelling in opposite directions. For each pair, photon A is sent = directly to a=20 detector, but photon B is duplicated many times by an amplifier to make = a=20 macroscopic light beam that stands in for the cat. The polarizations of = the=20 photons in this light beam are then measured.
Two types of amplifier
They consider two different types of amplifier. The first measures = the state=20 of photon B, which has the effect of destroying the entanglement with A, = before=20 producing more photons with whatever polarization it measures photon B = to have.=20 This is rather like the purely classical process of observing the Geiger = counter=20 to see whether it has detected any radiation, and then using the = information to=20 decide whether or not to kill the cat. The second amplifier copies = photon B=20 without measuring its state, thus preserving the entanglement with = A.
The researchers ask how the measured polarizations of the photons in = the=20 light beam will differ depending on which amplifier is used. They find = that, if=20 perfect resolution can be achieved, the results look quite different. = However,=20 with currently available experimental techniques, the differences cannot = be=20 seen. "If you have a big system and you want to see quantum features = like=20 entanglement in it, you have to make sure that your precision is = extremely=20 good," explains Simon. "You have to be able to distinguish a million = photons=20 from a million plus one photons, and there is no current technology that = would=20 allow you to do that."
Quantum-information theorist Renato Renner of ETH Zurich is = impressed: "Even=20 if there was no decoherence, this paper would explain why we do not see = quantum=20 effects and why the world appears classical to us, which is a very = fundamental=20 question of course." But, he cautions, "The paper raises a very = fundamental=20 question and gives us an answer in an interesting special case, but = whether it=20 is general remains to be seen."
The research will be published in Physical Review = Letters.
What is happening while oppening the box?
Edited by Jarek Duda on Nov 23, 2011 5:04 PM. = =09