Filters distill quantum bitsBy Eric Smalley, Technology Research News
To make quantum computers you need quantum bits, and to make quantum bits you need to entangle pairs of atoms or subatomic particles.
Entangling particles is old hat for physicists these days, and it can be done as simply as shining a laser on the right crystal. But entanglement is a matter of degrees, and one challenge for researchers building quantum computers and quantum cryptographic systems is getting the right amount.
"By and large... you want as much as possible," said Paul Kwiat, a physics professor at the University of Illinois.
Kwiat lead a team of researchers who developed a technique for distilling a collection of partially entangled pairs of photons down to a smaller number of more highly entangled photon pairs.
Two particles can become entangled, or linked, when they are in the quantum mechanical condition of superposition, which is a mixture of all possible states. When one of the entangled particles is measured, it collapses out of superposition into a random state and the other particle immediately collapses into the same state, regardless of the physical distance between them.
The researchers used polarization to distill the photon pairs. Because light is a type of electromagnetic radiation, it contains both electric and magnetic fields. The electric field of light vibrates in a plane perpendicular to the direction of the light wave. The electric field of unpolarized light vibrates in all directions in that plane, while the electric field of polarized light vibrates in only one direction.
The researchers sent the entangled photon pairs through partial polarizers, which partially filter light according to its polarization. "You can think of partial polarizers as a bad pair of sunglasses," said Kwiat.
However, it is inaccurate to consider a collection of entangled photon pairs as having some pairs that are more entangled than others and that the distillation process simply filters out the less entangled pairs, said Kwiat.
"They're all described by the same state, so all of them are... partially entangled," he said. "The net result [of the filtering process is] that you get less out on the other side. What does come through -- what survives this filtering process -- is then in a more highly entangled state," he said.
Ensuring a high degree of entanglement is crucial for some quantum cryptography proposals. "If you're trying to use entangled photons and your system gets... sufficient numbers of errors [due to partial entanglement], you could be leaking out too much information to some eavesdropper and there's no way of knowing that," said Kwiat.
The researchers are developing tools to measure the degree of entanglement, said Kwiat. "We're just now turning to the task of [using] these measures... as a sort of gauge, [an] entangle-meter," he said. "That hasn't really been implemented yet by anyone, but I think that's coming in the next year."
Practical quantum computers are at least two decades away, though quantum cryptographic systems could be developed within a decade, according to many researchers in the field.
Kwiat's research colleagues were Salvador Barraza-Lopez of the National Polytechnic Institute of Mexico, and André Stefanov and Nicolas Gisin of the University of Geneva. Kwiat and Barraza-Lopez were at the Los Alamos National Laboratory when they did the research. The researchers published the work in the February 22, 2001 issue of Nature. The research was funded by the National Security Agency, the Advanced Research and Development Activity (ARDA) and the European Union's Information Society Technologies (IST) Programme.
Timeline: 20 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Experimental entanglement distillation and ‘hidden’ non-locality," Nature, February 22, 2001
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