World-first: US-designed photon router to help plug qubits into quantum networks

Quantum computers leverage the quantum properties of materials to carry out computations at an unprecedented pace. However, these states can only be reached at ultra-low temperatures, which can be deployed at computing facilities but are impossible to roll out for entire quantum networks. 

Researchers are, therefore, working to leverage existing communication networks to carry quantum information. Photons that can travel through fiber-optic cables anywhere across the globe are the natural go-to to achieve this. 

What is a photon router? 

Researchers at Harvard SEAS have developed a microwave-optical quantum transducer that can work with superconducting microwave qubits, a type of information storage unit used in quantum approaches. 

The device bridges the large energy gap between microwave and optical photons, allowing control of the qubit with a light signal generated even miles away. It is the first to enable control of a superconducting qubit using only light. 

“The realization of these systems is still a ways out, but in order to get there, we need to figure out practical ways to scale and interface with the different components,” said Hana Warner, a graduate student at SEAS who was involved in this work in a press release.

“Optical photons are one of the best ways you can do that, because they’re very good carriers of information, with low loss, and high bandwidth.”

How was it built? 

The router built by the Harvard team looks much like a paper clip and is no bigger than two millimeters. Placed on a chip about two centimeters long, the device consists of two optical resonators linked with a microwave one, allowing the flow of energy between them without the need for bulky microwave cables. 

Built using base material lithium niobate, the device also allows for the readout of the qubit state or convert quantum information into light packets for transfer between quantum computing nodes. 

Rigetti computing provided the superconducting qubit platform for the researchers to work on. At the same time, Harvard’s Center for Nanoscale Systems carried out the fabrication of the chip. 

The breakthrough opens the possibility of developing superconducting quantum processors connected with low-loss, high-powered optical networks. Since superconducting quantum networks are highly scalable, the possibility of a quantum computing world is much closer than before. 

“The next step for our transducer could be reliable generation and distribution of entanglement between microwave qubits using light,” added Marko Loncar, a professor of electrical engineering and applied physics at Harvard SEAS. 

The research findings were published in the journal Nature Physics.