Quantum light source goes completely ‘on-chip’

18 Apr 2023

Brings scalability to quantum cloud, say developers at Leibniz-Hannover, Twente, and QuiX Quantum.

An international team of researchers from Leibniz University Hannover, Germany, the University of Twente, The Netherlands, and start-up company QuiX Quantum has developed an entangled quantum light source fully integrated on a chip – which they say is a first.

Their achievement is described in Nature Photonics.

“Our breakthrough enabled us to shrink the source size by a factor of more than 1,000, allowing reproducibility, stability over a longer time, scaling, and potentially mass-production,” said Prof. Dr. Michael Kues, head of the Institute of Photonics, and board member of the Cluster of Excellence PhoenixD at Leibniz University Hannover. “All these characteristics are required for real-world applications such as quantum processors.”

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.

In such devices, 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.

‘Novel chip design’

“Until now, quantum light sources had 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,” said Hatam Mahmudlu, a Ph.D. student in Kues’.

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,” said Dr. Raktim Haldar, a Humboldt fellow in Kues’ group. “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.”

The key was the hybrid technology that combines 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, which is called “superposition” and the colors of both photons are correlated, meaning 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,” said Kues.