International team achieves breakthrough: Quantum light source integrated on chip for quantum computing applications

A groundbreaking achievement has been made by an international collaboration between Leibniz University Hannover in Germany, the University of Twente in the Netherlands, and QuiX Quantum, a startup company. They have successfully developed a fully integrated quantum light source on a chip, marking a significant milestone in the field. Their findings have been published in the esteemed journal Nature .

This remarkable breakthrough has enabled the researchers to reduce the size of the light source by more than 1,000 times. This reduction in size brings numerous benefits, including enhanced reproducibility, increased stability over extended periods, scalability, and the potential for mass production. These characteristics are crucial for the practical implementation of quantum processors, which are key to realizing quantum computers and the .

Quantum bits, or qubits, serve as the fundamental units of information in quantum computers and the quantum internet. Quantum light sources generate individual particles of light, called photons, which can be employed as qubits. On-chip photonics has emerged as a leading platform for manipulating optical quantum states due to its compactness, resilience, and capacity to accommodate and arrange multiple components on a single chip. These photonic systems are already accessible through cloud-based services. If implemented on a larger scale, they could tackle computational problems that are currently beyond the capabilities of classical computers. This superiority in computational power is commonly known as quantum advantage.

The whole quantum light source fits on a chip smaller than a one-euro coin. The researchers reduced the size of the light source by a factor of more than 1,000 by using a novel “hybrid technology” that combines a laser made of indium phosphide and a filter made of silicon nitride on a single chip. The new light source is efficient and stable and can find applications to drive quantum computers or the quantum internet. Credit: Institute of Photonics/LUH

“We have successfully addressed the long-standing challenges associated with quantum light sources by developing a groundbreaking chip design,” explains Hatam Mahmudlu, a Ph.D. student in the team led by Kues. Their latest innovation is an integrated, electrically-excited photonic quantum light source that incorporates a laser directly onto the chip. This eliminates the need for external, bulky laser systems, allowing for greater practicality and wider applicability.

One of the main concerns with qubits is their sensitivity to noise. To ensure a noise-free operation, the chip requires an on-chip filter to drive the laser field. In the past, integrating a laser, filter, and cavity on the same chip presented significant challenges due to the lack of a suitable material that could efficiently accommodate these diverse components. Dr. Raktim Haldar, a Humboldt fellow in Kues's group, highlights the importance of an on-chip filter to eliminate noise and explains the difficulties encountered during integration.

The breakthrough came with the introduction of “hybrid technology,” which combines different materials on a single chip. The laser component utilizes indium phosphide, while the filter and cavity are made of silicon nitride. Through a spontaneous nonlinear process on the chip, two photons are generated from the laser field. Each photon simultaneously spans a range of colors, demonstrating a known as “superposition.” Furthermore, the colors of the two photons are correlated, resulting in entangled photons capable of storing quantum information. Kues emphasizes that their development achieves exceptional efficiencies and state qualities required for quantum computers or the quantum internet.

In summary, the team's achievement lies in the integration of an electrically-excited, laser-integrated photonic quantum light source on a single chip. This novel chip design overcomes previous limitations and enables the emission of frequency-entangled qubit states. The breakthrough is made possible by hybrid technology, which combines indium phosphide for the laser, silicon nitride for the filter and cavity, allowing for efficient integration and improved functionality. The resulting entangled photons exhibit remarkable qualities suitable for application in quantum computers and the quantum internet.

They developed the new integrated quantum light source (from left): Prof. Dr Michael Kues, head of the Institute of Photonics and board member of the Cluster of Excellence PhoenixD at Leibniz University Hannover, with doctoral student Hatam Mahmudlu and Humboldt fellow Dr. Raktim Haldar. Credit: Sonja Smalian/PhoenixD

“Our breakthrough allows for the integration of the laser and other crucial components onto a compact chip, making the entire quantum source smaller than a one-euro coin,” states Haldar, emphasizing the remarkable miniaturization achieved. This advancement represents a significant step towards achieving quantum advantage with photonic systems on a chip, unlike Google's current use of super-cold qubits in cryogenic setups. Remarkably, the potential for quantum advantage could be realized even at room temperature using these photonic chip systems.

Furthermore, the researchers anticipate that their discovery will have a positive impact on reducing production costs for various applications. Kues envisions their quantum light source becoming a fundamental component of programmable photonic quantum processors in the near future.

Prof. Dr. Michael Kues, the head of the Institute of Photonics and a board member of the Cluster of Excellence PhoenixD: Photonics, Optics, and —Innovation across Disciplines at Leibniz University Hannover, Germany, leads the accomplished team behind this groundbreaking development.

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