QUANTUM computers are having an identity crisis鈥攖hey may not be quantum
computers after all. Though a quantum computer was created last year, it appears
the experiment lacked that 鈥渦ncertain something鈥�. By rights the computer
shouldn鈥檛 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 鈥渆ntangled鈥� 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.
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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.
鈥淭he states aren鈥檛 entangled; they鈥檙e incredibly jumbled up,鈥� says Caves.
So why did the chloroform computer work at all? Caves鈥檚 colleague John
Smolin, a physicist at IBM in New York, suspects Chuang鈥檚 chloroform has
simulated a quantum computer, though he doesn鈥檛 know how. Or maybe the
experiment hints there are other ways of doing quantum calculations that we
don鈥檛 yet understand. 鈥淭he hope is that you can do some sort of quantum
computing without entanglement,鈥� says Smolin. 鈥淣obody鈥檚 got a deep understanding
of what that means.鈥�
Chuang is also intrigued. 鈥淨uantum computing is being done from a different
resource that we don鈥檛 understand so far,鈥� he says.