A team of researchers from Austria, Canada, and Finland used the spatial structure of light to experimentally implement so-called high-dimensional quantum gates. In the ongoing transition from classical to quantum information science, one driving force will be the increasing ability of scientists to generate, manipulate, and measure groups of individual quantum systems in order to harness them as information carriers. The most common unit of quantum information is a two-valued bit (a qubit). The processing of qubits relies on a few fundamental operations called quantum gates. Photonic quantum information can be manipulated by the controlled transformation of the light’s spatial structure, which is made visible here by using a strong laser light. Courtesy of Markus Hiekkamäki/Robert Fickler. Quantum information can also be encoded in larger digits as information carriers that can take more values than only 0 and 1. “These so-called high-dimensional quantum states show advantages such as increased information capacity, strengthened noise resistance, and improved communication security,” Marcus Huber, group leader at the Institute for Quantum Optics and Quantum Information, said. To harness the benefits of high-dimensional states, the team used the spatial structure of photons, which are single quantum systems of light, to encode the high-dimensional information they were seeking to manipulate. By shaping the spatial structure of the photon by means of carefully designed computer-generated holograms, they were able to achieve high-quality transformations while maintaining maximal flexibility and versatility. The researchers experimentally implemented several high-dimensional quantum gates for up to five-dimensional states encoded in the full-field mode structure of photons. They demonstrated near-perfect “unitarity” by means of quantum process tomography, showing a process purity of 99%. They further demonstrated the benefit of the two independent spatial degrees of freedom, that is, azimuthal and radial, and implemented a two-qubit controlled-NOT quantum operation on a single photon. “We hope that this experiment might trigger new pathways in the field of optical quantum communication and computation,” professor Robert Fickler, group leader at Tampere University, said. Similar to their binary counterparts, high-dimensional quantum gates could act as future building blocks for more advanced high-dimensional quantum information technologies. The team included researchers from the Institute for Quantum Optics and Quantum Information (IQOQI), Tampere University, and the University of Ottawa. The research was published in Optica, a publication of The Optical Society (OSA) (www.doi.org/10.1364/OPTICA.375875).