New form of quantum entanglement achieved with three "twisted" photons

Researchers from the Austrian Academy of Sciences, Vienna University and UAB have succeeded, for the first time, in entangling three photons in a new asymmetric pattern. The research, with applications in quantum cryptography, has been published in Nature Photonics, and it sets a new record in entanglement dimensions.

01/03/2016

Researchers at the Institute of Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Universitat Autònoma de Barcelona have achieved a new milestone in quantum physics: they were able to entangle three particles of light in a high-dimensional quantum property related to the "twist" of their wavefront structure. Just like Schrödinger's famous cat that is simultaneously dead and alive, all previous demonstrations of multi-particle entanglement have been with quantum objects in two discrete levels, or dimensions. The twisted photons used in the Vienna experiment have no such limit to their dimensionality, and can simultaneously exist in three or more quantum states. The three-photon entangled state created by the Vienna group breaks this previous record of dimensionality, and brings to light a new form of asymmetric entanglement that has not been observed before. The results of these experiments were published this week in Nature Photonics.

Quantum-entangled ice dancers

Entanglement is a counter-intuitive property of quantum physics that has long puzzled scientists and philosophers alike. Entangled quanta of light seem to exert an influence on each other, irrespective of how much distance there is between them. Consider for example a metaphorical quantum ice dancer, who has the uncanny ability to pirouette both clockwise and counter-clockwise simultaneously. A pair of entangled ice-dancers whirling away from each other would then have perfectly correlated directions of rotation—if the first dancer twirls clockwise then so does her partner, even if skating in ice rinks on two different continents. "The entangled photons in our experiment can be illustrated by not two, but three such ice dancers, dancing a perfectly synchronized quantum mechanical ballet," explains Mehul Malik, the first author of the paper. "Their dance is also a bit more complex, with two of the dancers performing yet another correlated movement in addition to pirouetting. We are the first to actually create this type of asymmetric quantum entanglement in the lab."

From fundamentals to applications: Layered quantum cryptography

The scientists created their three-photon entangled state by using yet another quantum mechanical trick: they combined two pairs of high-dimensionally entangled photons in such a manner that it became impossible to ascertain where a particular photon came from. Besides serving as a test bed for studying many fundamental concepts in quantum mechanics, multi-photon entangled states such as these have applications ranging from quantum computing to quantum encryption. Along these lines, the authors of this study have proposed a new type of quantum cryptographic protocol, using their state, that allows different layers of information to be shared asymmetrically among multiple parties with unconditional security. Many technical challenges remain before such a quantum communication protocol becomes a practical reality. However, given the rapid progress in quantum technologies today, it is only a matter of time before this type of entanglement finds a place in the quantum networks of the future. “The experiment opens the door for a future quantum Internet with more than two partners and it allows them to communicate more than one bit per photon,” says Anton Zeilinger.

Taking part in the research project were Mehul Malik, Manuel Erhard, Mario Krenn, Robert Fickler and Anton Zeilinger of the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Science, and Marcus Huber of the Quantum Information and Phenomena Group within the UAB Department of Physics: a theoretical physicist who invented the techniques needed to analyse the experiment. This research was supported by the European Commission, the European Research Council (ERC) and the Austrian Science Fund (FWF).

Original article:

Multi-Photon Entanglement in High Dimensions Mehul Malik, Manuel Erhard, Marcus Huber, Mario Krenn, Robert Fickler, Anton Zeilinger. Nature Photonics, http://dx.doi.org/10.1038/nphoton.2016.12.