Picture of research.
Two superconducting qubits acting as giant artificial atoms by coupling to a waveguide at multiple points. These “atoms” are protected from energy losses into the waveguide, yet still interact with each other through the waveguide. Illustration: Philip Krantz, Krantz NanoArt

Giant atoms merge quantum processing and communication

Researchers at Chalmers University of Technology in Sweden and MIT in the US, among others, have demonstrated a new quantum-computing architecture that makes it possible to both perform quantum computations and communicate quantum information between distant parts of the quantum processor, all with low losses. The results were recently published in the renowned scientific journal Nature.
Picture of Anton Frisk Kockum."We showed that quantum bits can communicate through a waveguide without the quantum information being lost", says Anton Frisk Kockum (to the right), researcher at the Applied Quantum Physics Laboratory at the Department of Microtechnology and Nanoscience – MC2, at Chalmers, and one of the authors of the article.

A challenge for scaling up quantum computers is to enable communication between quantum bits (qubits) that are far apart. Coupling qubits to a long waveguide is usually detrimental, since it provides a channel through which quantum information can leak out. The solution the researchers found was to use “giant atoms”, a new regime of light-matter interactions.

“Natural atoms are usually much smaller than the wavelength of the light they interact with. However, an experiment in the group of Professor Per Delsing at Chalmers in 2014 showed that an artificial atom made from superconducting circuits can connect to a waveguide at multiple points spaced wavelengths apart. When calculating how two such giant atoms would behave, we found that interference effects due to emission from the multiple coupling points could prevent the atoms from decaying into the waveguide, but still allow them to talk to each other via the waveguide. This was now demonstrated in the experiment carried out at MIT”, explains Anton Frisk Kockum.

The researchers used the interference effects of the giant atoms to demonstrate both that the individual atoms could be protected from losing quantum information into the waveguide and that the two atoms could be entangled, with 94% fidelity, through their protected interaction via the waveguide.

This is the first time that anyone has even reported a number for the fidelity of a two-qubit operation with qubits strongly coupled to a waveguide, since the fidelity for such an operation would be low if the qubits were not giant. The ability to perform high-fidelity quantum-computing operations on qubits coupled to a waveguide creates exciting new opportunities.
“It is now possible to prepare a complex quantum state in the qubits, and then quickly adjust the interference effect in the giant atoms to turn on the coupling to the waveguide and emit this quantum state as photons that can travel a long distance”, says Anton Frisk Kockum.

The study is a collaboration between scientists from Chalmers (the theoretical part), MIT, and the research institution RIKEN in Japan. From Chalmers, Anton Frisk Kockum contributed.

The work was partly supported by the Knut and Alice Wallenberg Foundation and The Swedish Research Council. The experiments were performed at the Research Laboratory for Electronics at MIT.

Photo of Anton Frisk Kockum: Michael Nystås
Illustration: Philip Krantz, Krantz NanoArt

Anton Frisk Kockum, Researcher, Applied Quantum Physics Laboratory, Department of Microtechnology and Nanoscience – MC2, Chalmers University of Technology, anton.frisk.kockum@chalmers.se

Read the article in Nature >>>
Waveguide quantum electrodynamics with superconducting giant artificial atoms

Read more about the research project >>>

Further reading >>>
Propagating phonons coupled to an artificial atom. Gustafsson et al., Science 346, 207 (2014)
Decoherence-Free Interaction between Giant Atoms in Waveguide Quantum Electrodynamics. Kockum et al., Physical Review Letters 120, 140404 (2018)

Press release from MIT >>>

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