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Quantum copies do new tricks

March 23, 2012

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One of the strange features of quantum information is that, unlike
almost every other type of information, it cannot be perfectly copied. For
example, it is impossible to take a single photon and make a number of
photons that are in the exact same quantum state. This may seem minor, but
it’s not. If perfect copying was possible, it would, among other things,
be possible to send signals faster than the speed of light. This is
forbidden by Einstein’s theory of relativity.

For years, scientists have been experimenting with the idea of
approximate quantum copying. A recent paper published in Physical Review
Letters (PRL), by Sadegh Raeisi, Wolfgang Tittel and Christoph Simon of
the Institute for Quantum Information Science takes another step in that
research.

They showed that it is possible to perfectly recover the original from
the imperfect quantum copies. They also proposed a way that his could be
done in practice.

“Copying classical information is very important in our daily lives,”
says paper co-author Simon. “Think of the prevalence of photocopiers,
faxes, scanners. It was quite surprising for physicists when they realized
that the same thing is not possible for quantum systems, at least not
perfectly. It is then important to study what exactly is possible and what
isn't.”

The research can be used in a variety of ways. First, it shows clearly
that quantum information is preserved when copied. Even though the copies
may be imperfect, the original quantum state can be recovered. In
practical terms, it might lead to a precision measurement technique based
on quantum physics for samples that have very low contrast, such as living
cells.

The Institute for Quantum Information Science is a multidisciplinary
group of researchers from the areas of computer science, mathematics and
physics.

“At the fundamental level our world is governed not by classical
physics, but by quantum physics,” says Simon. “We are trying to understand
the consequences of that for fundamental concepts such as information and
trying to use this understanding to develop new kinds of quantum
technology.”

Read "Proposal for inverting the quantum cloning of
photons."

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