computer runs quantum algorithm
Technology Research News
Although the incentive to build quantum
computers is strong -- they are potentially many orders of magnitude faster
than today's classical computers -- no one is sure yet that they can be
A team of researchers from the University of Rochester has set its sights
a little lower with a prototype computer that has some elements in common
computing, but operates using interfering lightwaves rather than quantum
While the theory of quantum computers has been proven, there is still
a large and potentially lengthy challenge in figuring out how to build
them. Wave-based computers, on the other hand, could be built right away.
The challenge is in proving that there's any reason to build them.
Although the lightwave computers use some of the same types of algorithms
as quantum computers, they can be built today using technology found in
networks. "The nice thing about optical interference is the technology
is very mature, and it's a lot easier to manipulate light than it is to
manipulate individual atoms," said Ian A. Walmsley, a professor of optics
at the University of Rochester.
The new type of computer would be as fast as quantum computers because
its light beams could be used to examine all at once every possible answer
to a given problem. The question is whether they would be efficient enough
to be practical, meaning they wouldn't require huge amounts of hardware
Although they would not ever reach the efficiency of quantum computers
and so could not be scaled up to handle the largest problems, they might
be more efficient than classical computers, making them useful for some
problems that are beyond the reach of classical computers.
Lightweight interference computing could be used for some quantum computing
applications like search engines that could answer queries to large databases
almost instantaneously, according to Walmsley.
The researchers tested their prototype with a version of a quantum search
algorithm that uses different colors in a beam of light to query every
item in a 50-item database
at the same time. They split the beam in two and sent one of the beams
through a modulator, which shifted the phase, or position of the wave
crest of one of the colors corresponding to the target of the query. The
researchers then recombined the two beams and detected which color experienced
interference due to the phases being out of alignment.
The algorithm did not search any more efficiently than is possible on
a classical computer, but the experiment showed that computations performed
using interference between the quantum mechanical wave states of particles
can also be done using classical lightwave interference, Walmsley said.
The next step for the researchers is finding an algorithm that would make
classical lightwave computing more efficient than classical computing,
Most quantum algorithms that are more efficient than classical algorithms
require the most counterintuitive aspect of quantum mechanics -- entanglement.
In these cases, the quantum computer performs a series of operations on
a set of entangled particles. The key to entanglement's usefulness in
computing is that groups of particles have an exponential number of possible
states and so a relatively small number of them can represent very large
Because lightwaves do not have this property, they cannot use these algorithms.
But there are others that do translate to classical physics. "Some of
the quantum algorithms do not rely on entanglement, at least for their
speed of operation," said Paul Kwiat, a physics professor at the University
of Illinois. "You can certainly do things like the search algorithms just
using interference, and as soon as you can do it just using interference
then quantum or classical, it doesn't make any difference."
However, the efficiency of most quantum algorithms comes from entanglement.
"If you want to have exponential savings in resources so that you don't
need to have a gazillion beam splitters on your table or you don't need
to have all these different distinguishable frequencies, then entanglement
becomes important," he said.
But, not every application needs the power of a full-blown quantum computer.
"A dictionary... is a reasonable kind of database that people search that
has a pretty finite number of elements in it," Kwiat said. "You might
imagine somehow eventually extending these [wave computing] techniques
-- it's not exactly clear how -- but somehow encoding your dictionary
in some sort of hardware, some sort of fixed system which you could then
search very, very quickly," he said.
Researchers are working on quantum algorithms that do not require entanglement
but are more efficient than classical algorithms. The University of Rochester
researchers are planning to adapt these algorithms to their wave-based
computing scheme, said Walmsley.
The challenge for wave-based computing is filling in the conceptual blanks,
he said. "Can we actually implement these efficient algorithms using interference?
How far can we push into the quantum regime? It may be that we can only
get a factor of two or three in efficiency, in which case it's hardly
worth investing a lot of money into this stuff. But maybe we can go further,"
Once the appropriate algorithms are written, it should take a year or
less to implement them using optical technology, he said.
Walmsley's research colleagues were Christophe Dorrer, Matt Anderson,
Pablo Londero, Sascha Wallentowitz and Konrad Banaszek of the University
of Rochester. They presented the research at the Lasers and Electro-Optics/Quantum
Electronics and Laser Science conference held in Baltimore the week of
May 7, 2001. The research was funded by the Department of Defense (DoD).
Timeline: > 1 year
TRN Categories: Quantum Computing; Optical Computing, Optoelectronics
Story Type: News
Related Elements: Technical paper, "Computing with Waves:
All-Optical Single-Query 50-Element Database Search," Lasers and Electro-Optics/Quantum
Electronics and Laser Science conference, Baltimore, May 7-11, 2001
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