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Waveguide quantum electrodynamics with superconducting artificial giant atoms

Abstract

Models of light–matter interactions in quantum electrodynamics typically invoke the dipole approximation1,2, in which atoms are treated as point-like objects when compared to the wavelength of the electromagnetic modes with which they interact. However, when the ratio between the size of the atom and the mode wavelength is increased, the dipole approximation no longer holds and the atom is referred to as a ‘giant atom’2,3. So far, experimental studies with solid-state devices in the giant-atom regime have been limited to superconducting qubits that couple to short-wavelength surface acoustic waves4,5,6,7,8,9,10, probing the properties of the atom at only a single frequency. Here we use an alternative architecture that realizes a giant atom by coupling small atoms to a waveguide at multiple, but well separated, discrete locations. This system enables tunable atom–waveguide couplings with large on–off ratios3 and a coupling spectrum that can be engineered by the design of the device. We also demonstrate decoherence-free interactions between multiple giant atoms that are mediated by the quasi-continuous spectrum of modes in the waveguide—an effect that is not achievable using small atoms11. These features allow qubits in this architecture to switch between protected and emissive configurations in situ while retaining qubit–qubit interactions, opening up possibilities for high-fidelity quantum simulations and non-classical itinerant photon generation12,13.

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Fig. 1: Giant atoms with superconducting qubits.
Fig. 2: Tunable coupling for a single giant atom.
Fig. 3: Decoherence-free interactions between two giant atoms.
Fig. 4: Entangling qubits in waveguide QED with engineered giant-atom geometries.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request and with the permission of the US Government sponsors who funded the work.

Code availability

The code used for the analyses is available from the corresponding author upon reasonable request and with the permission of the US Government sponsors who funded the work.

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Acknowledgements

We thank Y. Sung and A. Greene for valuable discussions. This research was funded in part by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract no. DE-AC02-05-CH11231 within the High-Coherence Multilayer Superconducting Structures for Large Scale Qubit Integration and Photonic Transduction programme (QISLBNL); and by the Department of Defense via MIT Lincoln Laboratory under US Air Force contract no. FA8721-05-C-0002. B.K. acknowledges support from the National Defense Science and Engineering Graduate Fellowship programme. M.K. acknowledges support from the Carlsberg Foundation during a portion of this work. A.F.K. acknowledges support from the Swedish Research Council (grant no. 2019-03696), and from the Knut and Alice Wallenberg Foundation through the Wallenberg Centre for Quantum Technology (WACQT). F.N. acknowledges support from the Army Research Office (grant no. W911NF-18-1-0358), the Japan Science and Technology Agency (via the Q-LEAP programme, and CREST grant no. JPMJCR1676), the JSPS KAKENHI (grant no. JP20H00134), the Foundational Questions Institute, and the NTT PHI Laboratory. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements of the US Government.

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Contributions

B.K., D.L.C., A.F.K., F.N., S.G. and W.D.O. conceived and designed the experiment. B.K. and D.L.C. designed the devices. B.K. and M.J.R. conducted the measurements, and B.K., M.J.R., A.F.K., J.B. and M.K. analysed the data. D.K.K., A.M., B.M.N. and J.L.Y. performed sample fabrication. B.K. and M.J.R. wrote the manuscript. P.K., A.V. and R.W. assisted with the experimental setup. T.P.O., S.G. and W.D.O. supervised the project. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Bharath Kannan or William D. Oliver.

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The authors declare no competing interests.

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Peer review information Nature thanks Michael Hartmann, Wolfgang Pfaff and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Experimental setup.

A schematic diagram of the experimental setup used to obtain the data presented in the main text.

Extended Data Fig. 2 Device C.

a, A schematic diagram of a giant-atom device with two qubits coupled to the waveguide at three points. The ratios ϕ1/ϕ2, ϕ1/ϕ3 and ϕ2/ϕ3 are fixed in hardware. b, A false-colour optical micrograph image of the device in the configuration shown in a. Each qubit (yellow) has a readout resonator (red) and flux line (green) for independent readout and flux control. The central waveguide (blue) is terminated to 50 Ω. Airbridges are placed every 80 μm along the waveguide to tie the ground planes together and prevent slotline modes.

Extended Data Table 1 Device parameters

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Kannan, B., Ruckriegel, M.J., Campbell, D.L. et al. Waveguide quantum electrodynamics with superconducting artificial giant atoms. Nature 583, 775–779 (2020). https://doi.org/10.1038/s41586-020-2529-9

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