Lisa Grossman in New Scientist:
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 would-be quantum computer scientists, who need quantum states to keep it together long enough to do calculations.