Quantum light sources run completely on-chip, bringing scalability to the quantum cloud

April 17, 2023

(Nanowerk News) An international team of researchers from Leibniz University Hannover (Germany), University of Twente (Netherlands), and startup QuiX Quantum have presented for the first time a fully integrated entangled quantum light source on a chip (Nature Photonics, “Fully on-chip photonic turnkey quantum source for entangled qubit/qudit state generation”).

“Our breakthrough allows us to shrink source size by a factor of over 1000, enabling reproducibility, stability over longer periods, scalability and potential for mass production. All of these characteristics are necessary for real-world applications such as quantum processors,” said Prof. Dr. Michael Kues, head of the Institute of Photonics, and member of the board of the Cluster of Excellence PhoenixD at Leibniz University Hannover. Artistic illustration of a chip-integrated quantum light source for entangled photon generation. (Image: Raktim Haldar, Michael Kues)

Quantum bits (qubits) are the basic building blocks of quantum computers and the quantum internet. Quantum light sources produce light quanta (photons) that can be used as quantum bits. On-chip photonics has become the leading platform for optical quantum state processing because it is compact, powerful, and makes it possible to accommodate and organize multiple elements on a single chip. Here, light is directed onto the chip via a very compact structure, which is used to build photonic quantum computing systems. It is already accessible today via the cloud. Implemented in a scalable manner, they can accomplish tasks that are inaccessible to conventional computers due to their limited computing capacity. This advantage is called a quantum advantage.

“Until now, quantum light sources required external, off-chip and bulky laser systems, which limited their use in the field. However, we are addressing this challenge through novel chip designs and by exploiting multiple integrated platforms,” ​​said Hatam Mahmudlu, a Ph.D. . students on the Cookies team. Their new development, a laser-integrated, electrically excited photonic quantum light source, fits completely 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 of noise, requiring an on-chip filter. Previously, integrating lasers, filters and cavities on the same chip was a huge challenge because there was no unique efficient material to manufacture these different components,” said Dr. Raktim Haldar, a Humboldt researcher in the Kues group.

The key is ‘hybrid technology’ which attaches a laser made of indium phosphide, a filter and a cavity made of silicon nitride and fuses them into a single chip. On the chip, in a spontaneous nonlinear process, two photons are created from the laser field. Each photon covers a range of colors simultaneously, which is called a ‘superposition’, and the colors of the two photons are correlated, that is, the photons are entangled and can store quantum information.

“We achieved the extraordinary efficiency and quality required for applications in quantum computers or the quantum internet,” said Kues.

“Now we can integrate the laser with other components on a chip so that the entire quantum source is smaller than a one euro coin. Our tiny device can be considered a step towards quantum superiority on a chip with photons. Unlike Google, which currently uses supercooled qubits in cryogenic systems, quantum superiority can be achieved with such a photonic system on a chip even at room temperature,” said Haldar.

The scientists also expect their invention to help lower the cost of producing apps.

“We can envision that our quantum light source will soon become a fundamental component of programmable photonic quantum processors,” said Kues.

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