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' entanglement-detection 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:   2-5 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


November 13/20, 2002

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