Researchers at the University of Illinois at Urbana-Champaign have constructed a quantum-mechanical state in which the colors of three photons are entangled with each other. The state is a special combination, called a W state, that retains some entanglement even if one of the three photons is lost, which makes it useful for quantum communication. Such entangled states also enable novel quantum applications and tests of fundamental physics.
The uniqueness of this work is that the researchers used color, or the energy of the photons, as the entangling degree of freedom, while previous work used polarization. The energy of a photon cannot be easily changed, which reduces the possibility of errors when the energy-entangled W state is propagating over a long distance. The state was verified for the first time by measuring information about the two-photon sub-systems. Their findings are published in Physical Review Letters.
Written by Mike Koon for Grainger Engineering
Researchers on this project are graduate student Bin Fang and principal investigator Virginia Lorenz, associate professor of physics. Photo by L. Brian Stauffer, University of Illinois at Urbana-ChampaignResearchers at the University of Illinois at Urbana-Champaign have constructed a quantum-mechanical state in which the colors of three photons are entangled with each other. The state is a special combination, called a W state, that retains some entanglement even if one of the three photons is lost, which makes it useful for quantum communication. Such entangled states also enable novel quantum applications and tests of fundamental physics.
The uniqueness of this work is that the researchers used color, or the energy of the photons, as the entangling degree of freedom, while previous work used polarization. The energy of a photon cannot be easily changed, which reduces the possibility of errors when the energy-entangled W state is propagating over a long distance. The state was verified for the first time by measuring information about the two-photon sub-systems. Their findings are published in Physical Review Letters.
“People have created polarization-entangled W states before,” noted Bin Fang, the graduate student on the project. “However, this is the first discrete energy-entangled W state and the first three-photon entangled state created in optical fiber.”
To create the state, the researchers shine a laser into a glass fiber. Through a process called spontaneous four-wave mixing, four laser photons interact with the fiber and are annihilated to create two pairs of photons at different colors (for example, two pairs of red and green photons). These four photons are used to construct the 3-photon W state. One of them is detected to be green, leaving the other three entangled as a W state, which is comprised of all possible iterations of two red photons and a green photon at once.
What makes this research unique is the use of color, or the energy of photons, to create an energy-entangled W state. Image courtesy of Gina Lorenz and Bin Fang, University of Illinois at Urbana-ChampaignThe illustration that the researchers use is that of traffic lights.
“Like three traffic lights that always signal two stops and a go, the photons’ colors always end up being two reds and a green, but the specific combination is not set until we make a measurement – a feature of the quantum mechanical nature of photons,” said Virginia Lorenz, associate professor of physics and the 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 some entanglement, meaning the communication is able to continue.
"Another new aspect of this research is that we found a path to verify the state is the one we aimed for that circumvents a complicated color conversion step, " said Lorenz. "Our theorist collaborators came up with a way to fairly straightforwardly show that the W state exists.”
This work was supported in part by the National Science Foundation. The conclusions presented are those of the researchers and not necessarily those of the funding agencies.
Madeline Stover is a physics doctoral student at the University of Illinois Urbana-Champaign studying atmospheric dynamics applied to forest conservation. She interns as a science writer for Illinois Physics, where she also co-hosts the podcast Emergence along with fellow physics graduate student Mari Cieszynski. When Stover is not doing research or communications, she enjoys hosting her local radio show, singing with her band, and cooking with friends.
Daniel Inafuku graduated from Illinois Physics with a PhD and now works as a science writer. At Illinois, he conducted scientific research in mathematical biology and mathematical physics. In addition to his research interests, Daniel is a science video media creator.
Karmela Padavic-Callaghan, Ph. D. is a science writer and an educator. She teaches college and high school physics and mathematics courses, and her writing has been published in popular science outlets such as WIRED, Scientific American, Physics World, and New Scientist. She earned a Ph. D. in Physics from UIUC in 2019 and currently lives in Brooklyn, NY.
Jamie Hendrickson is a writer and content creator in higher education communications. They earned their M.A. in Russian, East European, and Eurasian Studies from the University of Illinois Urbana-Champaign in 2021. In addition to their communications work, they are a published area studies scholar and Russian-to-English translator.
Garrett R. Williams is an Illinois Physics Ph.D. Candidate and science writer. He has been recognized as the winner of the 2020 APS History of Physics Essay Competition and as a finalist in the 2021 AAAS Science and Human Rights Essay Competition. He was also an invited author in the 2021 #BlackinPhysics Week series published by Physics Today and Physics World.
Karmela Padavic-Callaghan, Ph. D. is a science writer and an educator. She teaches college and high school physics and mathematics courses, and her writing has been published in popular science outlets such as WIRED, Scientific American, Physics World, and New Scientist. She earned a Ph. D. in Physics from UIUC in 2019 and currently lives in Brooklyn, NY.