Refined Waveguide Quantum Electrodynamic Architecture Spurs Qubit Photon Generation

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CAMBRIDGE, Mass., Oct. 14, 2020 — By connecting superconducting quantum bits to a microwave transmission line, MIT researchers demonstrated how qubits can generate, on demand, photons needed to support communication between quantum processors. The demonstration is a step toward reliably achieving the interconnections that would enable a modular quantum computing system capable of performing at speeds exponentially faster than classical computers.

Superconducting qubits are unable to support interactivity beyond that occurring locally, with nearby and neighboring (co-located) qubits. By inserting microwave waveguides to function as the basis for quantum interconnects, quantum information can travel from one location to another. The microwave transmission line, or waveguide, drives that communication, as the excitations contained within the qubits generate photon pairs that emit into the waveguide and then travel to two distant processing nodes. The photons are entangled, acting as a singular system, and distributing entanglement throughout a quantum network at a highly efficient rate.

The new waveguide quantum electrodynamics architecture that generated the photons showed that qubits can function as quantum emitters for the waveguide. The researchers further demonstrated that quantum interference between the photons emitted into the waveguide generated entangled, itinerant photons that traveled in opposite directions. Those photons and their motion can be used for long-distance communication between quantum processors.

Entangled pairs of photons are generated by and propagate away from qubits placed along a waveguide. Courtesy of Sampson Wilcox
Entangled pairs of photons are generated by and propagate away from qubits placed along a waveguide. Courtesy of Sampson Wilcox.
The architecture derived from previous work by current MIT research team members Bharath Kannan and William Oliver that introduced a superconducting qubits-based waveguide quantum electrodynamics architecture. That work achieved low-error quantum computation and the sharing of quantum information between processors by adjusting the frequency of qubits to tune the strength of the qubit-waveguide interaction so that fragile qubits are shielded from the effects of decoherence caused by the waveguides and allowed to perform qubit operations. The demonstration featured the researchers then readjusting the qubit frequency so that the qubits could release quantum information into the waveguide in a photonic form.

In performing computations, classical computers rely on wires to route information back and forth through a processor. In a quantum computer, information itself is quantum mechanical, as well as extremely fragile, leading to the need for strategies that simultaneously process and communicate information.

Where spontaneous parametric down-conversion and photodetectors can generate entangled photons in an optical system, that entanglement is generally random. That randomness detracts from modularity and the entanglement’s ability to support the on-demand communication of quantum information within a distributed system.

The researchers have yet to perform the communication between processors, showing instead how generated photons are both useful in application for quantum communication and in interconnection protocols.

The work was published in Science Advances (www.doi:10.1126/sciadv.abb8780).

Published: October 2020
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