A Quantum Paradox

The idea that quantum mechanics applies to everything in the universe, even to us humans, can lead to some strange conclusions. Consider this variant of the iconic Schrödinger cat thought experiment that Nobel laureate Eugene P. Wigner came up with in 1961 and David Deutsch of the University of Oxford elaborated on in 1986.

Suppose that a very able experimental physicist, Alice, puts her friend Bob inside a room with a cat, a radioactive atom and cat poison that gets released if the atom decays. The point of having a human there is that we can communicate with him. (Getting answers from cats is not that easy.) As far as Alice is concerned, the atom enters into a state of being both decayed and not decayed, so that the cat is both dead and alive. Bob, however, can directly observe the cat and sees it as one or the other. Alice slips a piece of paper under the door asking Bob whether the cat is in a definite state. He answers, “yes.”

Note that Alice does not ask whether the cat is dead or alive because for her that would force the outcome or, as physicists say, “collapse” the state. She is content observing that her friend sees the cat either alive or dead and does not ask which it is.

Because Alice avoided collapsing the state, quantum theory holds that slipping the paper under the door was a reversible act. She can undo all the steps she took. If the cat was dead, it would now be alive, the poison would be in the bottle, the particle would not have decayed and Bob would have no memory of ever seeing a dead cat.

And yet one trace remains: the piece of paper. Alice can undo the observation in a way that does not also undo the writing on the paper. The paper remains as proof that Bob had observed the cat as definitely alive or dead.

That leads to a startling conclusion. Alice was able to reverse the observation
because, as far as she was concerned, she avoided collapsing the state; to her, Bob was in just as indeterminate a state as the cat. But the friend inside the room thought the state did collapse. That person did see a definite outcome; the paper is proof of it. In this way, the experiment demonstrates two seemingly contradictory principles. Alice thinks that quantum mechanics applies to macroscopic objects: not just cats but also Bobs can be in quantum limbo. Bob thinks that cats are only either dead or alive.

Doing such an experiment with an entire human being would be daunting, but physicists can accomplish much the same with simpler systems. Anton Zeilinger and his colleagues at the University of Vienna take a photon and bounce it off a large mirror. If the photon is reflected, the mirror recoils, but if the photon is transmitted, the mirror stays still. The photon plays the role of the decaying atom; it can exist simultaneously in more than one state. The mirror, made up of billions of atoms, acts as the cat and as Bob. Whether it recoils or not is analogous to whether the cat lives or dies and is seen to live or die by Bob. The process can be reversed by reflecting the photon back at the mirror. On smaller scales, teams led by Rainer Blatt of the University of Innsbruck and by David J. Wineland of the National Institute of Standards and Technology in Boulder, Colo., have reversed the measurement of vibrating ions in an ion trap.

In developing this devious thought experiment, Wigner and Deutsch followed
in the footsteps of Erwin Schrödinger, Albert Einstein and other theorists who
argued that physicists have yet to grasp quantum mechanics in any deep way. For decades most physicists scarcely cared because the foundational issues had no effect on practical applications of the theory. But now that we can perform these experiments for real, the task of understanding quantum mechanics has become all the more urgent. —V.V.

(Source: physics.utoronto.ca)