Multiparticle, multidimensional entanglement

A discussion of quantum computation, cryptography, and similar subjects often centers on the quantum bit, the superposition-enhanced version of the classical 0 or 1. Yet there’s no limit to the number of degrees of freedom that can be placed in quantum superposition: Qutrits, ququarts, or any other d-dimensional qudits are possible. Now Manuel Erhard and his colleagues at the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna have created a trio of entangled qutrits, the first demonstration in which more than two particles were entangled in more than two quantum states.

The new research combines two previous efforts of Anton Zeilinger’s group at IQOQI: generating a set of quantum correlations between three or more particles through what’s called Greenberger-Horne-Zeilinger entanglement (see the article by Daniel Greenberger, Michael Horne, and Zeilinger, Physics Today, August 1993, page 22 ); and encoding information into photons’ orbital angular momentum (OAM), which offers a potentially unlimited number of degrees of freedom for each particle (see the article by Miles Padgett, Johannes Courtial, and Les Allen, Physics Today, May 2004, page 35 ). The researchers designed and built a femtosecond light source that produced two pairs of photons, with each pair entangled in the ℓ = 0, 1, and –1 OAM levels. The path to creating the complex entanglement between three of the photons wasn’t obvious—in fact, it required a computer’s help. Using an algorithm named Melvin that essentially plays with various configurations of quantum optics instruments, Erhard and coworkers developed a novel device that consisted of a beamsplitter, nested interferometers, and more. As shown in the diagram, the multiport processed three photons at a time—previous multiphoton-entanglement experiments manipulated only two—and transformed some of the correlations between OAM levels. Simultaneous clicks in four final photon detectors signaled that two of the photons exiting the multiport, B and C, plus photon D, were entangled in three dimensions.

The new technique opens the possibility of conducting more complex tests to rule out local realism alternatives to quantum mechanics. Researchers could also exploit high-dimensional-encoded photons to design quantum systems that carry more information and are less susceptible to eavesdropping. (M. Erhard et al., Nat. Photonics, 2018, doi:10.1038/s41566-018-0257-6 .)