Researchers create the first state w entangled three-photon color

Researchers at the University of Illinois at Urbana-Champaign have constructed a quantum state in which three-photon colors are entangled. The state is a special combination, called W state, which retains some entanglement even if one of the three photons is lost, which makes it useful for quantum communication. Such entangled states also allow new quantum applications and fundamental physics tests.

The peculiarity of this work is that the researchers used the color, or photon energy, as an entangling degree of freedom, whereas previous work used polarization. The energy of a photon can not be easily changed, which reduces the risk of error when the W entangled state of energy is spreading over a long distance. The state was verified for the first time by measuring information on two-photon subsystems. Their results are published in Letters of physical examination.

"People had already created W states mixed with polarization," noted Bin Fang, the graduate student of the project. "However, it is the first W state with entanglement of discrete energy and the first three-photon entanglement state created in an optical fiber."

To create this state, researchers shine a laser in a fiberglass. Through a process called spontaneous four-wave mixing, four laser photons interact with the fiber and are annihilated to create two pairs of photons of different colors (for example, two pairs of red and green photons). These four photons are used to build the 3-photon W state. One of them is detected as being green, leaving the other three entangled in a W state, which includes all possible iterations of two red photons and one green photon at a time.

The illustration used by researchers is that of traffic lights.

"Like three traffic lights that always signal two stops and one stop, the colors of the photons always end up being two reds and one green, but the specific combination is only defined once we have performed a measure, characteristic of the quantum nature of photons., "said Virginia Lorenz, associate professor of physics and principal investigator.

Compared to other types of three-particle entanglement, the W state is useful for quantum communication in that, if one of the photons is lost, the other two retain an entanglement which means that communication can continue.

"Another new aspect of this research is that we have found a way to check the status, that is the one we were aiming for, that bypasses a complicated color conversion step," Lorenz said. "Our theoretical collaborators have come up with a way to show quite simply that the W state exists."