Researchers are able to couple the spins of a magnetic material with the photons of a microwave cavity by using a static magnetic field. In the last years, the resulting hybrid systems were the focus of intense research, which is important for quantum technological applications, for studying dissipative systems, and for searching Axions. Our research falls into the last category.

Axions are hypothetical particles which could form the Dark Matter halo of our galaxy. According to the theory, a bunch of spins should react to the interaction with an Axion cloud as they would to a tiny magnetic field, therefore by precisely sensing the magnetisation of a sample one can say something about the presence of Dark Matter. This is the working principle of the ferromagnetic haloscope. Interestingly, a way to perform such measurement is to use an hybrid system of photons and magnons (quanta of spin excitations); ours is composed as follows. An Yittrium Iron Garnet (YIG) sphere provides the spins to couple to a copper cavity. We turn on the electromagnet to match the electron spin resonance with the cavity mode, and combine light and magnetisation. By doing so, a precise magnetic measurement can be performed using regular microwave detectors.

This haloscope is essentially a resonant antenna sensitive to a Dark Matter signal. The Axion mass determines the signal frequency, but unfortunately it is unknown, forcing Axion hunters to tune their instruments over a broad range. Here we come to our work, which wants to answer the question: how much can we tune our haloscope just exploiting the static magnetic field? Changing the magnetic field is a fairly simple way to vary the frequency of the detector, but if the spin and cavity resonances are too distant, the signal gets lost.

An infrared laser is shined on the YIG sphere to selectively excite the spins and create magnons. Since the sample is strongly coupled to the cavity, these quasiparticles are quickly converted into cavity photons, allowing us to detect them. The collected microwave signals are shown in the figure with different tones of blue, representing the variations of the applied magnetic field.

The model used to predict the tunable band of this system was validated by this result, and can now be applied to any other ferromagnetic haloscope out there. The magnon-to-photon transduction frequency obtainable changing the magnetic field, which we baptised "dynamical bandwidth", can be used for Axion searches, but may also be interesting for quantum technological applications.

If you are interested in this work, please check our paper published in Communications Physics at the following link: https://www.nature.com/articles/s42005-020-00435-w.

Cover: the picosecond infrared laser used in this work.