An international team of researchers from Leibniz University Hannover, the University of Twente and the start-up company QuiX Quantum has presented an entangled quantum light source fully inte­grated for the first time on a chip. “Our breakthrough allowed us to shrink the source size by a factor of more than 1000, allowing repro­ducibility, stability over a longer time, scaling, and potentially mass-production. All these charac­teristics are required for real-world applications such as quantum processors,” says Michael Kues, head of the Institute of Photonics at Leibniz University Hannover.

Quantum light sources generate photons that can be used as quantum bits. On-chip photonics has become a leading platform for processing optical quantum states as it is compact, robust, and allows to accommo­date and arrange many elements on a single chip. Here, light is directed on the chip through extremely compact structures, which are used to build photonic quantum computing systems. These are already accessible today through the cloud. Scalably imple­mented, they could solve tasks that are inaccessible to conven­tional computers due to their limited computing capa­cities. This superiority is referred to as quantum advantage.

“Until now, quantum light sources required external, off-chip and bulky laser systems, which limited their use in the field. However, we overcome these challenges through a novel chip design and by exploiting different integrated platforms,” says Hatam Mahmudlu, a Ph.D. student in Kues' team. Their new develop­ment, an elec­trically-excited, laser-integrated photonic quantum light source, fits entirely on a chip and can emit frequency-entangled qubit states.

“Qubits are very susceptible to noise. The chip must be driven by the laser field, completely free from noise, requiring an on-chip filter. Previously, it was a major challenge to integrate laser, filter, and a cavity on the same chip as there was no unique material that was efficient to build these different compo­nents,” says Raktim Haldar, a Humboldt fellow in Kues’ group. The key was the hybrid technology that sticks the laser made of indium phosphide, a filter, and a cavity made of silicon nitride and brings them together into a single chip. On the chip, in a spontaneous nonlinear process, two photons are created from a laser field. Each photon spans a range of colors simultaneously and the colors of both photons are correlated, i.e., the photons are entangled and can store quantum infor­mation. “We achieve remarkable efficiencies and state qualities required for appli­cation in quantum computers or the quantum internet,” says Kues.

“Now we can integrate the laser with other compo­nents on a chip so that the whole quantum source is smaller than a one-euro coin. Our tiny device could be considered a step towards quantum advantage on a chip with photons. Unlike Google, which currently uses super-cold qubits in cryogenic systems, the quantum advantage could be achieved with such photonic systems on a chip even at room tempera­ture,” says Haldar. The scientists also expect their discovery to help lower the production costs of applications. “We can imagine that our quantum light source will soon be a funda­mental component of pro­grammable photonic quantum processors,” says Kues. (Source: LUH)

Reference: H. Mahmudlu et al.: Fully on-chip photonic turnkey quantum source for entangled qubit/qudit state generation, Nat. Phot., online 17 April 2023; DOI: 10.1038/s41566-023-01193-1

Link: Cluster of Excellence PhoenixD, Leibniz University Hannover, Hannover, Germany