Quantum Computing - Kwantumcomputer
Quantum computer solves problem,
|Photo by L. Brian Stauffer
Kwiat, right, a John Bardeen Professor of Electrical
and Computer Enginering and Physics, and graduate
student Onur Hosten have found an exotic way
of determining an answer to an algorithm – without
ever running the algorithm.
Ill. — By combining quantum computation
and quantum interrogation, scientists at the University
of Illinois at Urbana-Champaign have found an exotic
way of determining an answer to an algorithm – without
ever running the algorithm.
Using an optical-based quantum computer, a research
team led by physicist Paul Kwiat has presented the
first demonstration of “counterfactual computation,” inferring
information about an answer, even though the computer did not run. The researchers
report their work in the Feb. 23 issue of the journal Nature.
Quantum computers have the potential for solving certain types of problems much
faster than classical computers. Speed and efficiency are gained because quantum
bits can be placed in superpositions of one and zero, as opposed to classical
bits, which are either one or zero. Moreover, the logic behind the coherent nature
of quantum information processing often deviates from intuitive reasoning, leading
to some surprising effects.
“It seems absolutely bizarre that counterfactual computation – using information
that is counter to what must have actually happened – could find an answer without
running the entire quantum computer,” said Kwiat, a John Bardeen Professor of Electrical
and Computer Engineering and Physics at
Illinois. ”But the nature of quantum interrogation makes this amazing feat possible.”
Sometimes called interaction-free measurement, quantum interrogation is a technique
that makes use of wave-particle duality (in this case, of photons) to search
a region of space without actually entering that region of space.
Utilizing two coupled optical interferometers, nested within a third, Kwiat's
team succeeded in counterfactually searching a four-element database using Grover's
quantum search algorithm.
“By placing our photon in a quantum superposition of running and not running
the search algorithm, we obtained information about the answer even when the
photon did not run the search algorithm,” said graduate student Onur Hosten,
lead author of the Nature paper. “We also showed theoretically how to obtain
the answer without ever running the algorithm, by using a ‘chained Zeno' effect.”
Through clever use of beam splitters and both constructive and destructive interference,
the researchers can put each photon in a superposition of taking two paths. Although
a photon can occupy multiple places simultaneously, it can only make an actual
appearance at one location. Its presence defines its path, and that can, in a
very strange way, negate the need for the search algorithm to run.
“In a sense, it is the possibility that the algorithm could run which prevents
the algorithm from running,” Kwiat said. “That is at the heart of quantum interrogation
schemes, and to my mind, quantum mechanics doesn't get any more mysterious than
While the researchers' optical quantum computer cannot be scaled up, using
these kinds of interrogation techniques may make it possible to reduce errors
in quantum computing, Kwiat said. “Anything you can do to reduce the errors
will make it more likely that eventually you'll get a large-scale quantum computer.”
In addition to Kwiat and Hosten, co-authors of the Nature paper are graduate
students Julio Barreiro, Nicholas Peters and Matthew Rakher (now at the University
of California at Santa Barbara). The work was funded by the Disruptive Technologies
Office and the National Science Foundation.
To reach Paul Kwiat, call 217-333-9116; e-mail: firstname.lastname@example.org .
James E. Kloeppel, Physical Sciences Editor
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