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Record number of photons lassoed into a quantum limbo 

Physicists entangle five particles that exist in two states

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12:04pm, May 15, 2010
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Researchers have combined two of the quantum world’s funkiest properties to set a new record — inextricably linking, or entangling, five light particles possessing the weird feature of “superposition.”

Superposition is a condition in which a light particle, or photon, exists in both of two possible states at once. The new work, described by Israeli researchers in the May 14 Science, paves the way for physicists to better control particles that are both superposed and linked by quantum entanglement. Potential applications include better microscopes or other instruments that involve light, or even future quantum measurement devices.

“It is actually a major step and a great idea,” says Christoph Wildfeuer, a physicist at Louisiana State University in Baton Rouge who was not involved in the work.

Superposition exists only in the quantum world and is perhaps best known from the example of Schrödinger’s cat. Physicist Erwin Schrödinger proposed a thought experiment in which a cat sits in a closed box along with a radioactive substance that might, or might not, decay and break open a bottle of poison gas. Until the box is opened and its contents are observed, the cat exists in a superposition: it is both alive and dead at the same time.

Similarly, the photons streaming through the new experiment exist in multiple states at once. Imagine, for instance, photons approaching a crossroads, where they could either turn left or continue straight ahead. Superposed photons do neither; instead, they do both at once, making their true location undefined.

The arrangement is known generally as a NOON state because the equation describing it looks like the word NOON embedded in a sea of mathematical notation. When more than two photons are involved, it’s called a “high-NOON” state.

Entanglement is a second bizarre feature of the quantum world, in which particles can become linked to other particles in fundamental ways. Changing the spin direction of one entangled particle, for instance, will instantaneously change the spin direction of its entangled partner, even if the two are physically separated.

Previous experiments have entangled up to three photons in NOON states, and up to six in non-NOON states. But five is a record for NOON states.

In 2007, a different research team published theoretical work suggesting a way to entangle NOON photons by sending laser light through a laboratory beam splitter. Following up on that idea, the Israeli researchers sent a stream of ordinary photons into the beam splitter, along with a second stream converted to have twice the number of photons with half the energy. After passing through the beam splitter, both streams acquired NOON states.

The researchers then recombined the two streams, and the resulting pattern of overlapping light confirmed that the photons were entangled. “It’s as if [the photons] had a meeting together and decided what to do,” says Itai Afek, a graduate student at the Weizmann Institute of Science in Rehovot, Israel, and coauthor of the Science paper.

Turning the lab observation into practical applications will take some time, however. Afek thinks the first use might be in sharpening images taken by light microscopes. Other instruments that split and then recombine light, such as interferometers used to hunt for ripples in spacetime, might also benefit, says Wildfeuer.

The key will be to get more and more NOON photons to entangle. In theory, the Weizmann approach should easily scale up to work for greater numbers of photons. But only a small fraction of the photons put in at the start survive to be measured at the end. To entangle lots of photons, the researchers need to fine-tune their equipment to have as many photons as possible survive, says Afek.

Eventually, such studies could bridge the gap between the quantum and nonquantum worlds, says physicist Holger Hofmann of Hiroshima University in Japan, who led the 2007 theoretical work. The new study, he says, “is an encouraging sign that the quantum features of multiphoton states may be more accessible and intuitive than many of the more abstract discussions indicate.”

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