Entangled quantum light source: fully integrated on chip

A team of researchers from Leibniz University Hannover (Germany), the University of Twente (Netherlands), and the startup company QuiX Quantum has presented an entangled quantum light source fully integrated on a chip. Professor Michael Kues from Leibniz University Hannover said the researchers were able 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 necessary for real-world applications, such as quantum processors.

Quantum bits (qubits) are the basic building blocks of quantum computers and the quantum internet. Quantum light sources generate light quanta (photons) that can be used as quantum bits. On-chip photonics is a platform for processing optical quantum states, as it is compact and robust, and can 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 through the cloud. Scalably implemented, they could solve tasks that are inaccessible to conventional computers due to their limited computing capacities. This is referred to as quantum advantage.

Hatam Mahmudlu, a PhD student in Kues’ team, said that 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,” Mahmudlu said.

The new development, an electrically-excited, laser-integrated photonic quantum light source, fits on a chip and can emit frequency-entangled qubit states. Dr Raktim Haldar, a Humboldt fellow in Kues’ group, said qubits are susceptible to noise; as a result, 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,” Haldar said.

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 colours simultaneously, which is called ‘superposition’, and the colours of both photons are correlated (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,” Kues said.

Haldar said that the researchers can now integrate the laser with other components on a chip so that the whole quantum source is small. The 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,” Haldar said.

The discovery could help lower the costs of applications; Kues said that the researchers’ quantum light source could soon be a fundamental component of programmable photonic quantum processors. The research findings were published in the journal Nature Photonics.

Image caption: Artistic illustration of the chip-integrated quantum light source for the generation of entangled photons. Image credit: Raktim Haldar/Michael Kues.