A team at the Technical University of Denmark (DTU) has developed what it calls a “complete platform” for an optical quantum computer. The platform is universal and scalable, the team said, and takes place at room temperature. The technology is compatible with standard fiber optic networks.

The demonstration of a so-called universal gate set — and the implementation of several operations by means thereof — is what the team said constitutes an optical quantum computing advancement. Gates serve to perform operations on one or more qubits, and to implement an algorithm given that qubits are carriers of information.

“Our demonstration of a universal set of gates is absolutely crucial. It means that any arbitrary algorithm can be realized on our platform given the right inputs, namely optical qubits,” said Mikkel Vilsbøll Larsen, lead author of a paper describing the advancement. “The computer is fully programmable.”

The scalability of the optical quantum computing platform to thousands of qubits is crucial to the ability of the technology to dramatically increase processing power relative to standard transistor-based computers. Such an increase supports applications in the pharmaceutical industry, the transport sector, and materials development for carbon capture and storage.

The complete platform for an optical quantum computer supports a quantum device that is universal measurement-based, and uses optical fiber to eliminate the need for cooling with large cryostats. Image Courtesy of DTU. 

 “Theoretically, there’s no difference between whether a quantum computer is based on superconducting or optical qubits. But there’s a decisive practical difference,” said senior researcher Jonas Neergaard-Nielsen. “Superconducting quantum computers are limited to the number of qubits fabricated on the specific processor chip. In our system, we’re constantly creating new ones and entangling them quantum mechanically with those we are performing calculations on. This means that our platform is easily scalable.

“In addition, we don’t need to cool everything down in large cryostats. Instead, we can do it all at room temperature in optical fibers. The fact that the system is based on optical fibers also means that it can be connected directly to a future quantum internet, without difficult intermediaries.”

According to the researchers, the group passed the scaling milestone in 2019. In an article, the researchers reported that it produced the basic structure for a measurement-based optical quantum computer — a so-called two-dimensional cluster state with over 30,000 entangled light states. Earlier this year, they developed and patented a full theoretical framework for how their technology can also embrace error correction, in the long term.

“The long-term goal is a quantum computer that can solve relevant problems and fulfill the potential we’re all striving toward,” Ulrik Andersen, head of bigQ, said. Andersen has supervised the research program at bigQ, which is a research center that is part of DTU Physics.

“We know what it takes to place our current technology on an optical chip and introduce error correction, and we have the relevant international collaborations in place,” Andersen said. “The same applies to the corporate sector, where companies are eager to develop use cases with us.”

The research was published in Nature Physics (www.doi.org/10.1038/s41567-021-01296-y).

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