Giant artificial atoms in superconducting circuits in three experiments

Short answer: A giant atom is an artificial atom that couples to a field at several separated points, so its size is no longer small compared with the wavelength and the usual dipole approximation fails. Three experiments with superconducting qubits report giant-atom behaviour with surface acoustic waves in 2019, with a microwave waveguide in 2020, and with a transmon on a transmission line in 2021. These are laboratory devices.

What is a giant atom?

The 2020 Nature abstract explains that models of light-matter interaction typically use the dipole approximation, in which atoms are point-like compared with the wavelength of the modes they interact with. When the ratio of atom size to mode wavelength increases, the approximation breaks down, and the atom is called a giant atom. The authors say that until then, experimental studies in solid-state devices were limited to superconducting qubits coupled to short-wavelength surface acoustic waves, probing the atom at a single frequency.

What did the acoustic experiment show?

The 2019 Nature Physics abstract reports an experimental demonstration of the giant-atom regime where an artificial atom interacts with acoustic fields whose wavelength is several orders of magnitude smaller than the atom. A superconducting qubit is coupled to surface acoustic waves at two points separated by about 100 wavelengths, which the authors compare to controlling an atom's radiation by attaching it to an antenna. The slow speed of sound gives a significant internal time delay for the field to cross the atom, producing non-Markovian dynamics. They demonstrate the non-Markovian character in the frequency spectrum and as non-exponential relaxation in the time domain.

What did the waveguide experiment show?

The 2020 Nature paper uses a different architecture: small atoms coupled to a waveguide at multiple, well-separated discrete locations. The authors say this allows tunable atom-waveguide couplings with large on-off ratios and a coupling spectrum that can be engineered by device design. They also demonstrate decoherence-free interactions between multiple giant atoms, mediated by the quasi-continuous spectrum of modes in the waveguide, an effect they say small atoms cannot achieve. The abstract available to us breaks off after saying these features let qubits in this architecture switch, so the remaining claims are not quoted.

What did the 2021 transmon experiment show?

The 2021 Physical Review A paper notes that light-matter interactions have traditionally been engineered by coupling natural atoms and ions to cavity fields, where the emitter is approximated as a dipole, and that superconducting circuits now allow giant atoms. The authors explore a giant artificial atom formed from a transmon qubit coupled to propagating microwaves at multiple points along an open transmission line. The coupling lets the qubit's radiation field interfere with itself. They observe strong frequency-dependent couplings of the qubit energy levels to the electromagnetic modes of the line, and show that, with in situ tuning of the qubit energy levels, they can change the relative coupling rates of multiple qubit transitions by more than an order of magnitude. The rest of that abstract was cut off in the page text.

How to read this

The three experiments use different media, sound in 2019 and microwaves in 2020 and 2021, so the physics is related but not identical. Non-Markovian means the field's memory over the atom's size matters. This post is based on abstracts, and full texts were not checked. It makes no claim about products or computing advantage.

FAQ

Why is it called giant?

Its coupling points are separated by distances comparable to or larger than the wavelength, per the 2020 abstract.

What did the acoustic device show?

Non-Markovian dynamics, with the qubit coupled at two points about 100 wavelengths apart.

What is a decoherence-free interaction here?

An interaction between giant atoms mediated through waveguide modes that the authors say small atoms cannot achieve.

How much could couplings be tuned?

By more than an order of magnitude in the 2021 transmon device.