Abstract
We demonstrate a superconducting artificial atom with strong unidirectional coupling to a microwave photonic waveguide. Our artificial atom is realized by coupling a transmon qubit to the waveguide at two spatially separated points with time-modulated interactions. Direction-sensitive interference arising from the parametric couplings in our scheme results in a nonreciprocal response, where we measure a forward/backward ratio of spontaneous emission exceeding 100. We verify the quantum nonlinear behavior of this artificial chiral atom by measuring the resonance fluorescence spectrum under a strong resonant drive and observing well-resolved Mollow triplets. Further, we demonstrate chirality for the second transition energy of the artificial atom and control it with a pulse sequence to realize a qubit-state-dependent nonreciprocal phase on itinerant photons. Our demonstration puts forth a superconducting hardware platform for the scalable realization of several key functionalities pursued within the paradigm of chiral quantum optics, including quantum networks with all-to-all connectivity, driven-dissipative stabilization of many-body entanglement, and the generation of complex nonclassical states of light.
- Received 21 December 2022
- Revised 12 April 2023
- Accepted 26 April 2023
DOI:https://doi.org/10.1103/PhysRevX.13.021039
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Viewpoint
ViewpointArtificial Atoms Go Chiral
Published 26 June 2023
A device’s selective interaction with left- and right-propagating modes could pave the way for directional information flow in quantum computing based on superconducting circuits.
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Popular Summary
Waveguide quantum electrodynamics (QED) explores the interactions between quantum emitters through propagating photons within 1D systems. In this framework, chiral emitter photon interfaces, where photons are emitted directionally into the waveguide, are pursued for quantum information routing and investigating many-body dynamics in certain quantum systems. While superconducting quantum circuits have proved to be an effective platform for studying waveguide QED at microwave frequencies, developing an efficient and scalable chiral interface in this platform has remained elusive. Here, we present such an interface in the form of a chiral artificial atom: a superconducting qubit operating in the “giant atom” regime, where light-matter interaction occurs at a pair of points separated by a quarter wavelength.
We control the atom’s emission phase via parametric time-modulated interactions, resulting in interference that leads to unidirectional atom-waveguide coupling. Using this approach, we demonstrate near-perfect chiral emission, with the propagation direction controlled by simple tuning of the relative phase of the parametric couplings. Furthermore, this method relies on a single qubit as the source of emission. Therefore, it is robust against decoherence and, under strong drives, manifests quantum nonlinearity—that is, the device can absorb or emit just one photon at a time, which makes it able to create quantum states and send them in a specific direction. We also show that the qubit’s second transition energy can display directional behavior, which can be used for implementing entangling gates between traveling-wave photons and stationary qubits in future experiments.
Our approach offers a chip-scale, low-loss alternative for realizing nonreciprocal interactions with superconducting quantum circuits, which so far rely on lossy and bulky devices based on ferromagnetic materials. This platform can be used for applications such as routing quantum information between nodes of a network with complex qubit connectivity, as well as for experimental studies of many-body physics with chiral atom arrays.
