Short answer: Magnons are collective spin excitations in a magnet, and superconducting circuits can now couple to them at the single-quantum level. A 2016 paper in Comptes Rendus Physique reports strong coupling of a magnon mode in a ferromagnetic insulator sphere to a microwave cavity and to a superconducting qubit, and a 2020 Science paper reports detecting a single magnon with a quantum efficiency of up to 0.71. These are laboratory proof-of-principle experiments.
What quantum magnonics is
The 2020 abstract defines quantum magnonics as the aim of controlling and reading out quanta of collective spin excitations in magnetically ordered systems. It says this offers opportunities for the study of magnetism and for quantum technologies, and that hybrid systems built on superconducting circuits make it possible to engineer quantum sensors that use different degrees of freedom.
2016: a magnon coupled to a cavity and a qubit
The 2016 abstract applies microwave quantum optics techniques to collective spin excitations in a macroscopic sphere of a ferromagnetic insulator. The authors demonstrate, in the single-magnon limit, strong coupling between a magnetostatic mode in the sphere and a microwave cavity mode. They then introduce a superconducting qubit in the cavity and couple it to the magnon excitation through virtual photon exchange. They observe a magnon-vacuum-induced Rabi splitting, and say the hybrid system makes it possible to generate and characterize non-classical quantum states of magnons. The generation of such states is their stated prospect, since the abstract reports the coupling and the splitting.
2020: detecting one magnon
In the Science paper, the authors use a superconducting qubit as a quantum sensor to detect a single magnon in a millimeter-sized ferrimagnetic crystal. They report a quantum efficiency of up to 0.71. The detection relies on entanglement between a magnetostatic mode and the qubit, followed by a single-shot measurement of the qubit state. They call it a proof-of-principle experiment that establishes the single-photon detector counterpart for magnonics. A quantum efficiency of up to 0.71 means the detector registered most but not all single magnons under the best conditions they report.
A third source
An arXiv preprint, number 1410.3781, was listed as a source, but its abstract was not readable here, so it is not described.
How the two results relate
The first paper shows that a qubit and a magnon mode can be coupled through a cavity, and observes the resulting splitting. The second uses that kind of coupling for detection, reading out the magnon number with a single-shot qubit measurement. The 2020 abstract measures efficiency, the 2016 abstract reports a coupling signature, so the two are steps in one line of work and are not directly comparable numbers.
Limits
Both experiments use a macroscopic or millimeter-scale magnetic crystal and a superconducting qubit in a cavity, at the single-quantum level. Neither abstract reports a quantum memory or a gate between magnons, and the generation of non-classical magnon states in 2016 is described as a possibility.
FAQ
What is a magnon?
A quantum of collective spin excitation in a magnetically ordered material.
What was coupled to what?
A magnetostatic mode in a ferromagnetic sphere to a microwave cavity and a superconducting qubit, in 2016.
How efficient was the single-magnon detector?
Up to 0.71 quantum efficiency, per the 2020 Science abstract.
How was detection done?
By entangling a magnetostatic mode with the qubit and measuring the qubit in a single shot.
Is this a quantum computer?
No. It is a hybrid-system proof of principle for sensing magnons.
All field notes