Schrödinger's cat, the enduring icon of
quantum mechanics, has been defied. By making constant but weak
measurements of a quantum system, physicists have managed to probe a
delicate quantum state without destroying it – the equivalent of taking a
peek at Schrodinger's metaphorical cat without killing it. The result
should make it easier to handle systems such as quantum computers that exploit the exotic properties of the quantum world.
Quantum objects have the bizarre but
useful property of being able to exist in multiple states at once, a
phenomenon called superposition. Physicist Erwin Schrödinger illustrated
the strange implications of superposition by imagining a cat in a box
whose fate depends on a radioactive atom.
Because the atom's decay is governed by quantum mechanics – and so only
takes a definite value when it is measured – the cat is, somehow, both
dead and alive until the box is opened.
Superposition could, in theory, let
quantum computers run calculations in parallel by holding information in
quantum bits. Unlike ordinary bits, these qubits don't take a value of 1
or 0, but instead exist as a mixture of the two, only settling on a
definite value of 1 or 0 when measured.
But this ability to destroy
superpositions simply by peeking at them makes systems that depend on
this property fragile. That has been a stumbling block for wouldbe
quantum computer scientists, who need quantum states to keep it together
long enough to do calculations.
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