Schrödinger's cat has always been an elusive beast, but physicists may have finally trapped one in the laboratory. In the great Austrian physicist's thought experiment, the fate of a cat was linked to the quantum indeterminacy of a radioactive atom—half decayed, half undecayed. The cat, as a result, was both alive and dead. Now physicists at the National Institute of Standards and Technology in Boulder, Colorado, report in this issue ( p. 1131 ) that they have created what one observer calls a “Schrödinger's kitten,” in which they excite an atom into a superposition of quantum states, then ease those two states apart so that the atom appears to be in two distinct physical locations simultaneously.
Schrödinger's cat has always been an elusive beast, but physicists may have finally trapped one in the laboratory. In the great Austrian physicist's thought experiment, the fate of a cat was linked to the quantum indeterminacy of a radioactive atom—half decayed, half undecayed. The cat, as a result, was both alive and dead. Now physicists at the National Institute of Standards and Technology in Boulder, Colorado, report in this issue ( p. 1131 ) that they have created what one observer calls a “Schrödinger's kitten,” in which they excite an atom into a superposition of quantum states, then ease those two states apart so that the atom appears to be in two distinct physical locations simultaneously.
In 1935 Erwin Schrödinger proposed a thought experiment to show the absurd consequences of applying quantum mechanics to everyday objects. Schrödinger imagined placing a cat in a box, along with a vial of deadly poison that is connected to a radioactive atom. If the atom decays, it releases the poison and kills the cat. When the box is closed we do not know if the atom has decayed or not, which means that it can be in a "superposition" of two states (decayed and non-decayed) at the same time. Therefore, protested Schrödinger, the cat can be both dead and alive at the same time – which is clearly absurd. There was obviously, he thought, some boundary between the microscopic world of atoms and quantum mechanics, and the macroscopic world of cats and classical mechanics.
The physicist Erwin Schrödinger described, back in 1935, one aspect of this strange phenomenon. He placed his cat in a sealed box with a piece of radioactive material which might or might not send out a sub-atomic particle. If it did, the Geiger counter was designed to trigger a release of poisonous gas in the box, and thereby kill his cat. The counter was set up to be triggered just enough to give a 50-50 chance of a particle being detected, and therefore the cat an even chance of being poisoned or not. (In those days, this sort of experiment was acceptable, particularly as it was only a thought experiment.) Radioactive behaviour is unpredictable (except in a general, statistical probability kind of way). There is no rule for working out whether or not the Geiger counter will be triggered. If the sub-atomic event does not occur, the cat lives. Otherwise, it dies. Just as a photon (in that famous experiment from ‘real-life’ physics) will quite happily go through both of two parallel slits in a screen – until someone tries to measure or detect it, at which point what the physicists call ‘the collapse of the wave function’ takes place – the particle can both trigger the poison gas and not trigger the poison gas. That is, until someone looks, Schrödinger’s idea is that his cat, for a while, is both alive and dead!
Letters from:
[ Andras Malnasi Csizmadia and Achilles J. Sanchez ][1]
[ Randy Morse ][1]
[ Barbara A. B. Seiders ][1]
[ Paul C. S. Hoaken ][1]
[ Andrew Ahlgren ][1]
When the Schrodinger cat paradox was first proposed in 1935, it was difficult to envision an experimental system in which to model Schrodinger's experiment. But C. Monroe et al. describe such a system in their research article “A ‘Schrodinger cat’ superposition state of an atom” ([24 May, p. 1131][2]). In an accompanying Research News article ([24 May, p. 1101][3]), Gary Taubes writes
> … Erwin Schrodinger described a cat shut up in what he called a “diabolical device”: a closed box also containing a small amount of a radioactive substance. Over the course of an hour, the radioactive substance has a 50-50 chance of decaying. If it does, the decay is detected by a counter, which shatters a flask of deadly acid, killing the cat. If it doesn't, the cat lives.
But for the experiment to be a true paradox, the box must contain only one radioactive atom, as specified by Schrodinger. If there are numerous atoms in the box, it is a statistical certainty that at least one atom (but we cannot know which one) will decay in the course of the experiment, and the unfortunate cat will undoubtedly be killed. If, on the other hand, there is only one or very few atoms in the box, we will not know whether the cat is dead or alive.
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As a naive molecular biologist, I am perplexed by one facet of the problem of Schrodinger's cat as it is usually presented. Why can't the cat be considered an observer and therefore remove the uncertainty about its own life or death status?
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My 12-year-old daughter Phoebe is a student of quantum paradoxes, but she was concerned about an error in the illustration accompanying the Research News article with regard to feline anatomy. Presumably, even Schrodinger's cat would have foreleg “elbows,” not “knees.”
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The title of the Research News article “Schizophrenic atom doubles as Schrodinger's cat—or kitten” perpetuates a misconception about schizophrenia—which is nothing like split personality—and trivializes a potentially disabling mental disorder.
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“Kitten,” as used by Taubes, seems needlessly macroscopic as a metaphor for a single trapped atom. How about “Schrodinger's furball”?
[1]: /lookup/doi/10.1126/science.273.5277.857e
[2]: /lookup/volpage/273/1131
[3]: /lookup/volpage/273/1101
Schrodinger's famous cat has long been misunderstood. According to quantum theory and experiments with entangled systems, an entangled state such as the Schrodinger's cat state is neither a superposition of states of either subsystem nor a superposition of compound states of the composite system, but rather a nonlocal superposition of correlations between pairs of states of the two subsystems. The entangled post-measurement state that results from an ideal measurement is not paradoxical, but is merely a coherent superposition of two statistical correlations at "zero phase angle," i.e. at 100% positive correlation. Thus the state of the radioactive nucleus and Schrodinger's cat is as follows: an undecayed nucleus is 100% positively correlated with an alive cat, and (i.e. superposed with) a decayed nucleus is 100% positively correlated with a dead cat. The superposition consists merely in the fact that both correlations are simultaneously true. Despite many published statements to the contrary, this superposition is not paradoxical. It is in fact what one expects intuitively.
The physicist Erwin Schrödinger described, back in 1935, one aspect of this strange phenomenon. He placed his cat in a sealed box with a piece of radioactive material which might or might not send out a sub-atomic particle. If it did, the Geiger counter was designed to trigger a release of poisonous gas in the box, and thereby kill his cat. The Geiger counter was set up to be triggered just enough to give a 50-50 chance of a particle being detected, and therefore the cat an even chance of being poisoned or not. (In those days, this sort of experiment was acceptable, particularly as it was only a thought experiment.) Radioactive behaviour is unpredictable (except in a general, statistical probability way). There is no way of working out whether or not the Geiger counter will be triggered off. If the sub-atomic event does not occur-the cat lives. Otherwise, it dies. Just as the photon (in Problem 93) will quite happily go through both slits until someone tries to measure or detect it, at which point what the physicists call ‘the collapse of the wave function’ takes place, the particle can both trigger the poison gas and not trigger the poison gas. Until someone looks, Schrödinger’s idea is that his cat, for a while, is both alive and dead!