Method
measures quantum quirk
By
Eric Smalley,
Technology Research News
Quantum entanglement, which Einstein once
dismissed as impossible, is a physical resource that could transform information
processing. It is key to producing phenomenally powerful quantum computers,
and is the critical component of the most secure form of quantum cryptography.
Until now, however, researchers have had no way to measure entanglement
directly, but have had to rely on indirect measurements or mathematical
estimates.
Researchers from the Technical University of Gdansk in Poland and the
University of Cambridge in England have come up with a scheme for measuring
entanglement that could give scientists the means to judge the purity
of the primary resource used in quantum information processing.
The scheme could mark the beginning of quantum metrology  the science
of quantum measurement, said Artur Ekert, a professor of quantum physics
at the University of Cambridge. "Efficient tests for quantum entanglement
will be important in all applications where quantum entanglement is used,"
he said.
Entanglement links physical properties, such as polarization or momentum,
of two or more atoms or subatomic particles. It is part of numerous schemes
for secure communication, precise frequency standards, atomic clocks and
clock synchronization.
When an atom or subatomic particle is isolated from its environment, it
enters into the weird state of superposition, meaning it is in some mixture
of all possible states. For example, a photon can be polarized in one
of two opposite directions. In superposition, however, the photon is polarized
in some mixture of both directions at the same time.
When two or more particles in superposition come into contact with each
other, they can become entangled. A common example is photons that have
their polarizations entangled. When one of the photons is knocked out
of superposition to become, say, vertically polarized, the other photon
leaves superposition at the same instant and also becomes vertically polarized,
regardless of the distance between them.
Existing methods of checking for entanglement involve either indirect
measurements, which are inefficient and leave many entangled states undetected,
or a mathematical estimation, Ekert said.
The researchers' method is similar to the mathematical approach, but works
on the particles directly rather than on a mathematical representation
of them. "We have managed to find a physical operation that mimics the
mathematical one," said Ekert.
Quantum operations alter particles that are used as quantum bits, or qubits,
to represent the 1s and 0s of computing in quantum information systems.
One way to carry out a quantum operation is to use a laser beam to rotate
an atom held in a magnetic trap so that its orientation flips from a position
representing a 1 to a position representing a 0. The basic logic of quantum
computing is made up of many series of these quantum operations.
The researchers' entanglementdetection method could be included in several
proposed architectures for quantum computers, including ion traps, which
hold individual atoms in magnetic fields, and quantum dots, which trap
individual electrons in microscopic specks of semiconductor material,
according to Ekert.
The research is excellent; it is an original idea about how to detect
entanglement in an efficient way, said Vlatko Vedral, a lecturer of physics
at Imperial College and the University of Oxford in England. "One of the
most fundamental issues in quantum information theory is whether two systems
are entangled or not," he said. Scientists have had a good theoretical
understanding of how to detect entanglement, but these methods are not
practical in the physical world because they involve physical impossibilities
like reversing time, he said.
The researchers have come up with a practical method of testing for entanglement,
said Vedral. The basic idea is to mix a bit of noise into the operation
so there will always be a physically possible result, he said. "It turns
out that this mixing can be performed in an efficient way," he added.
Entanglement is crucial for quantum communications, said Vedral. "Some
forms of quantum cryptography depend critically on the presence of entanglement
and cannot be implemented without it," he said.
It's not yet clear how useful being able to measure entanglement will
be for quantum computing because researchers do not know if there is a
direct link between amount of entanglement and the speed of quantum computers,
Vedral said. "Everything indicates that entanglement is an important ingredient,
but how much of it is enough to be clearly better than any classical computer
remains an open question," he said.
The method could be used in practical applications in two to five years,
said Ekert. It is likely to be used first in quantum cryptography and
frequency standards, he said.
Ekert's research colleague was Pawe Horodecki of the Technical University
of Gdansk in Poland. They published the research in the September 16,
2002 issue of Physical Review Letters. The research was funded by the
Polish Committee for Scientific Research, the European Commission, Elsag
SpA, the Engineering and Physical Sciences Research Council and the Royal
Society of London.
Timeline: 25 years, 20 years
Funding: Government, Corporate
TRN Categories: Physics; Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Method for Direct Detection
of Quantum Entanglement," Physical Review Letters, September 16, 2002
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November 13/20, 2002
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