Boston
A SOLID that can store quantum information by stopping light dead in its
tracks has brought the possibility of a practical quantum computer a step
closer.
Most experiments in quantum computing involve storing information in the
quantum state of an atom. Such states are easily disturbed, leading to a loss of
information. So stabilising quantum particles is a key step on the road to
building a practical quantum computer.
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Last year, two American teams looked set to solve this problem when they
slowed, and eventually froze, a light beam in an atomic gas, thereby storing its
information stably among trillions of atoms
(快猫短视频, 27 January 2001, p 4).
But computer memories made of a gas would be too unwieldy to use.
Now Phil Hemmer of the US Air Force Research Laboratory at Hanscom,
Massachusetts, and his colleagues at MIT, have done the same in a solid. 鈥淭his
is much closer to a device because you can manufacture a crystal,鈥� says Alexey
Turukhin at MIT.
Hemmer and Turukhin used a background lattice of yttrium silicate doped with
ions of the metal praseodymium. Using a laser beam鈥攃alled the coupling
beam鈥攖hey tuned the praseodymium ions to put their electrons into a 鈥渄ark鈥�
state. In this state they were unable to jump to higher energy levels and so
could not absorb light of a particular wavelength. They then sent a second beam
of that wavelength into the solid.
With the energy level jumps blocked, the second beam changes the orientation
of the praseodymium electrons鈥� spin鈥攍osing energy and slowing down in the
process. As the coupling beam is faded out, the secondary beam slows more and
more, until eventually it comes to a complete halt, with its information frozen
in the spin states of the electrons. When the coupling beam is turned back up,
the second light beam springs back to life.
Ron Walsworth, who helped pioneer the approach at the Harvard-Smithsonian
Center for Astrophysics, warns that Hemmer鈥檚 crystal isn鈥檛 quite ready to be
plugged into a quantum computer. 鈥淚t is a solid, but it鈥檚 not your average piece
of gallium arsenide that we already know how to make devices out of,鈥� he says.