Quantum entanglement, a phenomenon by which two or more particles share correlated properties through some instantaneous link, is tricky business. The quantum-mechanical bond entangling two particles is so delicate, it can be broken by any number of outside perturbations. Try entangling three particles, and the system becomes just that much more vulnerable to interference.
Nevertheless, physicists strive to entangle ever larger systems, with the ultimate goal of harnessing quantum effects in large numbers of particles for computation, communication, or ultraprecise measurements. A paper in the May 14 issue of Science reports progress in that quest, in the form of an experimental setup that entangles five photons. The researchers, from the Weizmann Institute of Science in Rehovot, Israel, coaxed the photons into what is called a NOON state, in which the particles have two possible paths to choose from but collectively follow only one of them.
NOON is shorthand for the two possible states N0 and 0N, which signify that N photons follow one path, while zero photons follow the other. Until a measurement is made, the photons are said to be in a superposition of the two states. For large values of N, the states are cheekily dubbed "high-NOON" states, and five photons is the highest NOON yet.
Experimental physicist Itai Afek, a Weizmann graduate student and study co-author, explains that his group mixed light from two sources at a beam splitter to entangle the photons and separate the two paths. A beam splitter is essentially a mirror that reflects half the incident photons, allowing the other half to pass through unscathed. With properly entangled photons, however, the behavior is strongly correlated—whichever path the photons choose to follow, they do so en masse. "The five photons reach the beam splitter, and either all of them are reflected or all of them are transmitted, so they behave collectively," Afek says.
That correlated action could have benefits beyond clever quantum trickery. "These photons act collectively like one fat photon," Afek says, "and this fat photon has a wavelength that is N times smaller than the wavelength of the light we use." In other words, a five-photon NOON state has a wavelength just one-fifth the size of its entangled photons, which is a boon to precision measurements using optics. "Generally speaking, short wavelengths imply high resolution," Afek adds. One wavelength-dependent measurement approach is interferometry, in which interference between two beams of light can reveal subtle differences in the lengths of the paths the beams have traveled. Experiments are already under way using interferometric arms several kilometers long to try to detect ripples in spacetime known as gravity waves.
Entangled light, with its diminished wavelength, could even be used to etch ever smaller details onto electrical circuits using optical lithography, but that probably will not find its way into desktop computers anytime soon. "I should be honest—it has been discussed a lot, but there are many problems to actually applying it," Afek says. He adds that high-resolution microscopy is likely a more feasible application in the near term.
Although the Weizmann group has generated the largest NOON state yet, entangling five photons is not a record per se. In 2007, another group reported entangling six photons in a different kind of state. But Afek notes that in that work "the number of spatial modes is identical to the number of photons you are measuring"—in other words, there are not two paths the photons can follow, but six. "We're cramming all of the photons into one of two possible situations," Afek adds. "This makes it much more relevant to interferometry, because typically you have two arms. So it's convenient to have all of your photons in those two arms."
The researchers claim that their NOON state scales easily to accommodate more photons—on paper, at least. "As a theoretical scheme it works as well for 100 photons as for five," Afek says. But actually achieving large-scale entanglement in the lab is no picnic. "These fat photons, the fatter they get, the more sensitive they get—they get very touchy," he says. "The bigger they are, the more perfect your setup has to become to observe them."
Afek acknowledges that applications for entangled photons in NOON states appear to lie rather far in the future, but for now his group is content to tinker with one of the finickiest properties of physics. "What we're trying to do is scale up this behavior and see what the difficulties are when the system grows," Afek says. "When you want to get larger and larger states, you really have to meet the high standards that quantum mechanics sets for you."