physics: The ultimate no-brainer
PHILIP BALL

John Cage's composition 4' 33" is a piano piece that is performed by not performing. The pianist goes through all the preliminary motions, but sits in silence for 4 minutes and 33 seconds - after which, one assumes, the audience applauds. Two scientists are now aiming to produce a computer that works in the same spirit - to give the answer without ever being switched on1.

Like Cage's piece, this feat is not about doing nothing; it is about doing nothing in the time normally allotted for doing something. "Due time must be allowed for the machine not to run," say Graeme Mitchison of the University of Cambridge and Richard Jozsa of the University of Bristol, UK.

The researchers insist that this is not some surrealist form of computer science, but rather that it should truly be possible to determine the outcome of a computation while the machine stays off. The computer they have in mind is no ordinary desktop model but a quantum computer.

In theory, a quantum computer would be much faster than existing computers. It would exploit the principles of quantum mechanics to achieve massively parallel processing, performing many logic operations at the same time.

So far, demonstrations of quantum computing have been limited to the most rudimentary of calculations, involving only two or three bits of information. Extending this to the thousands or millions of bits needed for the computer to be practically useful is a huge challenge. Although there have been several suggestions for how a quantum computer might be built, no one is even close to doing it.

But imagine, say Mitchison and Josza, that such a computer exists. Quite apart from streamlining information technology, they point out, this hypothetical machine would highlight, "in a particularly poignant way," the counter-intuitive nature of quantum physics.

Quantum laws allow for the bizarre phenomenon of 'counterfactuality': one can glean information about a quantum event that did not actually take place.

Quantum systems can exist in two incompatible states at once, a condition known as 'superposition'. The most famous example is Schrödinger's cat, which can be both alive and dead if its fate is determined by a quantum superposition of two possible outcomes. A quantum computer uses such superpositions to enlarge its computational power. A superposition generally collapses into one state or the other if measured - we can never actually 'see' a superposition.

Michison and Jozsa describe a scheme for probing all the possible states of a quantum computer, including that in which all the 'switches' are 'off' - that is, in which the computer is not turned on.

In other words, although the computer's quantum circuits existed in a superposition of states while it was performing a computation, it is possible to collapse this into an identical state to that in which it never ran at all - and, in so doing, to obtain the answer to the computation.

One interpretation of quantum superpositions is that they represent alternative worlds. When the collapse occurs, we end up in one of these worlds and the other becomes irrelevant to our own reality. But that parallel world nevertheless has, in some sense, a real existence. In this picture, a multiplicity of worlds is continually being generated by quantum events.

Mitchison and Jozsa's 'counterfactual computation' essentially taps into worlds in which the computer did run in order to extract the result into a world in which it didn't.


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Mitchison, G. & Jozsa, R. Counterfactual computation. Proceedings of the Royal Society London A 457, 1175–1193 (2001).

Team makes first quantum computer chip

Monday, 7 May 2001 14:52 (ET)


Team makes first quantum computer chip


NEW SOUTH WALES, Australia, May 7 (UPI) -- A team of scientists from the
United States and Australia has accomplished a remarkable feat -- controlled
placement of single phosphorous atoms on a silicon surface to fabricate the
world's first quantum computer chips.

The ten-member team of physicists and materials scientists from the
University of New South Wales, Los Alamos National Laboratory, and
University of Maryland first coated an ultra-clean, ultra-pure silicon
surface with a thin layer of hydrogen. Using the sub-nanometer-size tip of a
scanning tunneling microscope, the team carefully removed a precisely-spaced
array of individual hydrogen atoms, forming small holes in the surface of
the hydrogen coating.

They next sent a stream of phosphine gas over the silicon. Individual
phosphorous molecules from the gas settled into the hydrogen holes, bonding
with the exposed silicon. Finally, the team drove off the hydrogen layer by
heating it or bombarding the hydrogen atoms with free electrons, leaving a
planned array of phosphorous atoms dotting the silicon surface.

"The nuclear or electron spins of the single phosphorus atoms form
qubits," the basic building blocks of a solid-state quantum computer,
explained Jeremy O'Brien, one of the team's researchers from the Center for
Quantum Computer Technology at the University of New South Wales. "We have
shown it is possible to fabricate an atomically precise linear array of
phosphorus atoms on a silicon surface for the fabrication of a
silicon-based, solid state quantum computer."

Other approaches to "qubit" construction include ion trap and nuclear
magnetic resonance systems, O'Brien explained. Neither method, however,
offers the scalability or ease of mass-production inherent in the
phosphorous-silicon method.

Team researcher Marilyn Hawley told United Press International the spin
interactions between the phosphorous atoms communicate information directly
to the silicon, and phosphorous is a normal "dopant," or commonly-used
silicon additive. Nonetheless, "nobody else has tried this approach before,"
Hawley explained from her office at the Los Alamos National Laboratory in
Los Alamos, New Mexico. Researchers at the University of Illinois have used
a scanning tunneling microscope to move molecules before, however.

"We charge the tip, which has atomic-scale resolution, to induce
vibrations on the hydrogen atoms that break their bonds with the silicon,"
Hawley explained. The technique creates the equivalent of minute molecular
tweezers that literally "pluck" the individual hydrogen atoms from the
silicon surface, creating tiny holes for the deposition of phosphorous.

Arthur Rheingold, director of the University of Delaware's x-ray
crystallographic facility and an inorganic chemist who focuses on
solid-state materials, told UPI the use of this novel technique to marry
phosphorous and silicon is "more than sound. From a solid-state materials
point of view, the successful end result of this work does not surprise me,"
Rheingold said from Newark.

The technique's inventor, University of Maryland physicist Bruce Kane,
says one last challenge remains.

"We have to figure out a way to actually encapsulate the phosphorous
qubits within the silicon's crystalline lattice, without moving the
phosphorous atoms from their carefully constructed array" Kane told UPI from
College Park. "Right now they are just sitting on the surface."

The team's results are due to appear in the journal Physical Review B by
early summer.