Scalability to the quantum cloud achieved as quantum light source becomes fully integrated on-chip.

An international team of researchers from Leibniz University Hannover (Germany), University of Twente (Netherlands), and startup QuiX Quantum has presented a fully integrated entangled quantum light source for the first time on a chip. The results of the study have been published in the journal Nature photonics.

“Our breakthrough allowed us to shrink the size of the source by a factor of more than 1,000, allowing for the possibility of reproducibility, stability over a longer period, scalability, and possibly mass production. All of these properties are required for real-world applications such as quantum processors,” says Prof. Dr. Michael Koese, Head of the Institute of Photonics. , and a board member of the PhoenixD Excellence Group at Leibniz University, Hannover.

Quantum bits (qubits) are the building blocks of quantum computers and the quantum internet. Quantum light sources generate quanta of light (photons) that can be used as quantum bits. On-chip photonics has become a leading platform for photonic quantum state manipulation because it is compact, powerful, and allows many elements to be accommodated and arranged on a single chip. Here, light is directed onto the chip through highly compact structures, which are used to build optical quantum computing systems. These are already available today through the cloud. They are widely implemented, and can solve tasks that are inaccessible to traditional computers due to their limited computing capabilities. This superiority is referred to as a quantitative advantage.

“Until now, quantum light sources required external, external, and large-scale laser systems, which limited their use in this field. However, we have overcome these challenges with a new chip design and by exploiting different integrated platforms,” ​​says Hatem Mahmudlu. Ph.D. A student on the Kues team. Their new development, an electrically coupled, laser-integrated photonic quantum light source, fits perfectly on a chip and can emit frequency-entangled qubit states.

“Qubits are very susceptible to noise. The chip must be driven by a laser field, completely noise-free, and requires an on-chip filter. Previously, integrating the laser, filter and cavity on the same chip was a major challenge,” says Dr. Raktim Haldar, Humboldt Fellow in the Koise group. There was no unique material that was effective in building these various components.

The key was a “hybrid technology” that combines a laser made of indium phosphide, a filter, and a cavity made of silicon nitride and brings them together in a single chip. On-chip, in a spontaneous nonlinear process, two photons are generated from a laser field. Each photon simultaneously spans a range of colors, which is called “superposition”, and the colors of both photons are correlated, that is, the photons are entangled and can store quantum information. “We are achieving remarkable efficiencies and state qualities required for application in quantum computers or the quantum internet,” says Koise.

“Now we can combine lasers with other components on a chip so that the entire quantum source is smaller than a single euro coin. Our tiny device can be seen as a step towards a quantum advantage on a chip that contains photons. Unlike Google, which currently uses ultracold qubits in cryogenic systems, it can achieving the quantum advantage using such photonic systems on a chip even at room temperature,” says Haldar.

The scientists also expect their discovery to help lower production costs for applications. “We can imagine that our quantum light source will soon become an essential component of programmable optical quantum processors,” says Koise.

Prof. Dr. Michael Koss is Chair of the Institute of Photonics and a Board Member of the PhoenixD Excellence Group: Photonics, Optics, and Engineering – Innovation across Disciplines at Leibniz University Hannover, Germany.