If you put a cat inside an opaque box and make his life dependent on a random event, when does the cat die? When the random event occurs, or when you open the box?
Though common sense suggests the former, quantum mechanics — or at least the most common "Copenhagen" interpretation enunciated by Danish physicist Neils Bohr in the 1920s — says it's the latter. Someone has to observe the result before it becomes final. Until then, paradoxically, the cat is both dead and alive at the same time.
UC Berkeley physicists have for the first time showed that, in fact, it’s possible to follow the metaphorical cat through the whole process, whether he lives or dies in the end.
"Gently recording the cat's paw prints both makes it die, or come to life, as the case may be, and allows us to reconstruct its life history," said Irfan Siddiqi, UC Berkeley associate professor of physics, who is senior author of a cover article describing the result in the July 31 issue of the journal Nature.
The Schrödinger’s cat paradox is a critical issue in quantum computers, where the input is an entanglement of states — like the cat's entangled life and death — yet the answer to whether the animal is dead or alive has to be definite.
"To Bohr and others, the process was instantaneous — when you opened the box, the entangled system collapsed into a definite, classical state. This postulate stirred debate in quantum mechanics," Siddiqi said. "But real-time tracking of a quantum system shows that it's a continuous process, and that we can constantly extract information from the system as it goes from quantum to classical. This level of detail was never considered accessible by the original founders of quantum theory."
For quantum computers, this would allow continuous error correction. The real world, everything from light and heat to vibration, can knock a quantum system out of its quantum state into a real-world, so-called classical state, like opening the box to look at the cat and forcing it to be either dead or alive. A big question regarding quantum computers, Siddiqi said, is whether you can extract information without destroying the quantum system entirely.
"This gets around that fundamental problem in a very natural way," he said. "We can continuously probe a system very gently to get a little bit of information and continuously correct it, nudging it back into line, toward the ultimate goal."
*Editor's Note, from Erwin Schrödinger's 1935 essay:
"A cat is penned up in a steel chamber, along with the following diabolical device (which must be secured against direct interference by the cat): in a Geiger counter there is a tiny bit of radioactive substance, so small that perhaps in the course of one hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer which shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The Psi function for the entire system would express this by having in it the living and the dead cat (pardon the expression) mixed or smeared out in equal parts."