It's very strange, but it's not quantum

 作者:翟蠼     |      日期:2019-03-08 05:12:00
By Charles Seife QUANTUM computers are having an identity crisis—they may not be quantum computers after all. Though a quantum computer was created last year, it appears the experiment lacked that “uncertain something”. By rights the computer shouldn’t have worked at all. The probabilistic laws of quantum mechanics allow a quantum computer to do things that would be impossible for a normal computer. For instance, last April, Isaac Chuang of IBM in San Jose, California, and Neil Gershenfeld of the Massachusetts Institute of Technology created a quantum computer that solved a problem by asking just one question rather than many (This Week, 18 April 1998, p 10). However, in a forthcoming issue of Physical Review Letters, Carlton Caves of the University of New Mexico and his colleagues say they are unsure why quantum computation worked. Gershenfeld and Chuang used magnetic fields to manipulate atoms in liquid chloroform. But the problem, says Caves, is that the chloroform atoms were not in “entangled” states. Such atoms share a common fate. Manipulate one atom in an entangled pair, and you automatically affect the other. Entanglement is at the very centre of the algorithms of a quantum computer. However, because the chloroform was at room temperature, the atoms could not have been entangled as Gershenfeld and Chuang believed. The thermal motion of the atoms would have mixed up their quantum states and ruined any entanglement. “The states aren’t entangled; they’re incredibly jumbled up,” says Caves. So why did the chloroform computer work at all? Caves’s colleague John Smolin, a physicist at IBM in New York, suspects Chuang’s chloroform has simulated a quantum computer, though he doesn’t know how. Or maybe the experiment hints there are other ways of doing quantum calculations that we don’t yet understand. “The hope is that you can do some sort of quantum computing without entanglement,” says Smolin. “Nobody’s got a deep understanding of what that means.” Chuang is also intrigued. “Quantum computing is being done from a different resource that we don’t understand so far,