This image shows a module composed of superconducting qubits that can be used to directionally emit microwave photons. (Image: Krantz NanoArt)

MIT researchers have developed a quantum computing architecture that aims to enable extensible, high-fidelity communication between superconducting quantum processors.

The work, published in Nature Physics, demonstrates step one — the deterministic emission of single photons, information carriers, in a user-specified direction. The method ensures quantum information flows in the correct direction more than 96 percent of the time. Linking several of these modules enables a larger network of interconnected quantum processors.

“Quantum interconnects are a crucial step toward modular implementations of larger-scale machines built from smaller individual components,” said Bharath Kannan, PhD, co-lead author of the research paper. “The ability to communicate between smaller subsystems will enable a modular architecture for quantum processors, and this may be a simpler way of scaling to larger system sizes compared to the brute-force approach of using a single large and complicated chip.”

A quantum network links processing nodes using photons that travel through special interconnects known as waveguides, which can either be unidirectional, moving a photon only left or right, or bidirectional.

“We can get rid of these lossy components if we have a waveguide that can support propagation in both the left and right directions, and a means to choose the direction at will. This ‘directional transmission’ is what we demonstrated, and it is the first step toward bidirectional communication with much higher fidelities,” said Kannan.

To accomplish this, the researchers built a module comprising four qubits. The technique involves preparing the two qubits in an entangled state of single excitation — a Bell state. This quantum-mechanical state comprises two aspects: the left qubit being excited and the right qubit being excited. Both aspects exist simultaneously, but which qubit is excited at a given time is unknown.

“The photon has a certain frequency, a certain energy, and you can prepare a module to receive it by tuning it to the same frequency,” said co-lead author Aziza Almanakly, Graduate student at MIT. “If they are not at the same frequency, then the photon will just pass by. It’s analogous to tuning a radio to a particular station. If we choose the right radio frequency, we’ll pick up the music transmitted at that frequency.”

“The work demonstrates an on-demand quantum emitter, in which the interference of the emitted photon from an entangled state defines the direction, beautifully manifesting the power of waveguide quantum electrodynamics,” said Yasunobu Nakamura, Director of the RIKEN Center for Quantum Computing. “It can be used as a fully programmable quantum node that can emit/absorb/pass/store quantum information on a quantum network and as an interface for a bus connecting multiple quantum computer chips.”

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