Entangling a Rainbow

28 September 2009—A team of physicists led by Paulo Nussenzveig of the University of São Paulo, in Brazil, has shown for the first time that light of three different wavelengths can be entangled. Entanglement is a quantum mechanics phenomenon that is the key to staggeringly powerful—but still largely theoretical—quantum computers, as well as to unbreakable quantum encryption schemes.

Einstein once called entanglement "spooky action at a distance." Entangled particles are mutually dependent, even when they travel far apart. Quantum theory holds that if you measure one of the photons in an entangled pair, you will also instantly reveal the properties of the other. Entanglement of three light beams has never been demonstrated before. The entanglement produced so far has mostly been between two light beams of the same wavelength, a few atoms of the same element, or molecules of the same chemical. A common way to produce entanglement is to create a pair of photons in such a way that if a measurement shows that one of them is polarized in one direction—say, up—the other will always have a polarization in the other direction—in this case, down.

Nussenzveig and his graduate students Antonio Coelho and Felippe Barbosa shone a green laser at an optical parametric oscillator—a device that converts a laser beam of one frequency to a lower frequency.

The optical parametric oscillator converted photons from the green laser into pairs of infrared photons with slightly different frequencies, which were entangled with one another in a well-known process called parametric down conversion. As these infrared photons bounced back and forth within the oscillator, some of them interacted with the original green laser beam and became entangled with it. Ultimately, three beams of light, at 532.3 nanometers, 1062.1 nm, and 1066.9 nm, were entangled.

Nussenzveig says it was a challenging experiment. "We had to face many difficulties—for instance, the phonon noise, which nobody knew about before." Phonons are vibrations in the atomic lattice of crystals. In optical parametric oscillators like the one used in the experiment, phonon noise is the result of high-frequency photons slamming into the device.

Cooling the apparatus to –23 °C ”substantially reduced” the noise, “allowing us to observe the entanglement," says Nussenzveig.

Olivier Pfister of the University of Virginia, an expert on quantum optics, was impressed with the research and called it "another viable alternative" to standard ”schemes for quantum information and quantum computing."

"Entangled light beams are especially useful for quantum communications," says Nussenzveig, who expects the three-color entanglement to ”find applications in a future quantum Internet in which different pieces of hardware can exchange information by means of entangled light beams.”

Experimental quantum communication networks are already in place. For example, the city of Vienna installed a quantum information network last year that had many different quantum technologies operating together. Nussenzveig’s technique would allow light beams from the different kinds of quantum hardware underpinning the Vienna network to work more efficiently with one another.

The new three-color entanglement might be a useful tool for those working on photon-based quantum computer chips. One such quantum computer recently performed a key code-cracking algorithm for the first time.

But quantum computing is not the next thing on Nussenzveig’s agenda. “Probably the first step will be to demonstrate quantum teleportation,” says Nussenzveig. In quantum teleportation, the quantum state of one particle, its polarization for instance, is transferred to another distant particle without the two ever directly interacting. ”After that, we would like to shift frequencies so that it would be possible to connect our optical parametric oscillator to atoms, for instance. That would enable us to exchange information between matter and light, and to convert [the entangled light's] color so as to propagate more efficiently through optical fibers.”