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Quantum Cat Tricks

By Jeffrey Winters
Oct 1, 1996 5:00 AMNov 12, 2019 6:41 AM


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Erwin Schrödinger, the brilliant Austrian physicist who was among the founders of quantum mechanics, once dreamed up a paradoxical thought experiment to highlight one of the stranger aspects of quantum theory. Put a cat in a box, he proposed, along with a vial of poison and a lump of some radioactive element. After a certain period of time, depending on the element used, there’s a fifty-fifty chance that an atom will decay and emit a particle, triggering a device that smashes the vial, releases the poison, and kills the cat. There is, of course, an equal chance that the atom will not decay, thus sparing the cat. But during the entire time, according to quantum mechanics, the atom is simultaneously in the decayed and undecayed states. Not until someone makes a measurement of the atom is it forced into one mode or the other. And the cat? Schrödinger said that one would have to express this situation by having the living and the dead cat mixed, or smeared out (pardon the expression) into equal parts, living and dead.

No one has ever carried out that experiment in all its feline- unfriendly detail. But two physicists at the National Institute of Standards and Technology in Boulder, Colorado, recently did something almost as strange. They managed to coax a single atom to exist in two places at once.

David Wineland and Chris Monroe pulled off this feat using lasers and a magnet to manipulate a beryllium atom inside a vacuum chamber. They first confined the atom inside an electromagnetic field and, with lasers, bounced photons off it until it rested essentially motionless. Using another laser burst, they pumped just enough energy into the atom so that it had an equal chance of assuming either of two quantum states known as spin-up and spin-down, which describe the orientation of the magnetic field of the atom’s electrons. Just as with the hapless cat, the atom, until it is actually measured, exists simultaneously in both states.

Physicists have been creating such odd, mixed states within atoms for years. But what Wineland and Monroe did next was unprecendented. They calculated that a light pulse with a wavelength of exactly 313 billionths of a meter, and of a precise polarization (which describes the direction in which a light wave vibrates), could move the atom in its spin-up state without affecting the spin-down version of the atom: atoms in different quantum states absorb only very specific wavelengths and polarizations of light. The right light, in other words, enabled Wineland and Monroe to tease apart the superimposed versions of the atom. With a laser, they pushed the spin-up version of the atom about 80 billionths of a meter away from its spin-down self--a distance some ten times larger than the original beryllium atom.

Wineland and Monroe’s research, as esoteric as it seems, may one day find a practical application. The two physicists are interested--as are many researchers--in the feasibility of something called a quantum computer. Atoms in such a computer would replace transistors and other electronic components, greatly shrinking the size and increasing the power of computers. In a quantum computer, one atom could simultaneously represent a zero and a one in the binary language of computers. In conventional computers, each number of binary code must be stored separately.

To build a quantum computer requires precise control of just the sort of strange quantum effects Wineland and Monroe are now studying.

But quantum states are fragile--the slightest disturbance destroys them. In Wineland and Monroe’s relatively simple experiment, for example, the separated spin-up and spin-down states collapse back into a single atom if the lasers are not tuned just right, or if some stray radiation trickles in. This fragility may make building a quantum computer- -containing thousands of atoms--an engineering nightmare. Still, the researchers are optimistic. Fundamentally it’s not a problem, says Wineland. We can go a long way from here.

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