University of Illinois researchers at Urbana-Champaign have constructed a quantum mechanical state in which the colors of three photons are interwoven. The state is a special combination called the W state, which retains some entanglement even if one of the three photons is lost, making it useful for quantum communication. Such entangled states also allow new quantum applications and fundamental physics tests.
The uniqueness of this work is that the researchers used color or the energy of the photons as entanglement degrees of freedom, whereas in earlier work polarization was used. The energy of a photon can not be easily changed, reducing the possibility of errors as the energy-involved W state propagates over a long distance. The condition was verified for the first time by measuring information about the two-photon subsystems. Their results were published in Physical Review Letters .
"Humans have previously created polarization-bound W states," noted Bin Fang, the project's doctoral student. "However, this is the first discrete energy entangled W state and the first three photon entangled state generated in an optical fiber."
To create the state, the researchers beam a laser into a fiberglass. Through a process called spontaneous four-wave mixing, four laser photons interact with the fiber and are annihilated to create two photon pairs of different colors (for example, two pairs of red and green photons). These four photons are used to build up the 3-photon W state. One of them is recognized as green, the other three as W state, which includes all possible iterations of two red photons and one green photon at once.
The illustration used by the researchers is that of a traffic light.
"Like three traffic lights that always signal two stops and one go, the colors of the photons are always two reds and one green, but the specific combination will not be determined until we take a measurement – a feature of quantum mechanical 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 because if one of the photons is lost, the two others remain entangled, meaning that the communication can continue.
"Another new aspect of this research is that we have found a way to verify the state is the one we are targeting to bypass a complicated color conversion step," Lorenz said. "Our theoretical collaborators have found a way to pretty simply show that the W state exists."
Correlation of entangled photons according to radial position and momenta
See, for example, Fang et al., Three-photon discrete-energy entangled W state in an optical fiber, Physical Review Letters (2019). DOI: 10.1103 / PhysRevLett.123.070508
Researchers create first color-entangled three-photon W state (2019, September 23)
retrieved on September 23, 2019
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