A pair of grapes doubled the magnetic field, paving the way for better quantum sensors

The Internet is full of videos of grapes literally lighting up in the microwave. When the microwave is turned on, a berry cut into two halves begins to spark and soon flares up brightly, demonstrating effects that are dangerous at first glance. It would seem like sheer entertainment, but a thoughtful experiment showed that there is interesting physics behind the phenomenon, which can give impetus to the development of quantum detectors.

Image source: Fawaz, Nair, Volz

Amateur experiments with burning grapes began for the first time in 1994. They were all the same – the grapes were cut into two halves so that they remained connected by a thin skin. Later it turned out that this was not necessary. It is enough for halves or whole grapes to remain nearby. Moreover, gooseberries, large blackberries and hydrogel beads showed a similar effect in the microwave.

In all cases, the physics was approximately the same. The density of grapes, for example, turned out to be optimal so that cell rupture occurs, followed by ionization of molecules and their rupture. The cellular fluid itself is an electrolyte – it contains ions to which ions formed under the influence of microwave radiation have been added. The grapes began to emit plasma, which ignited in the radiation stream.

During another experiment in 2019, it turned out that grapes do not necessarily have to be physically connected. The effect occurs if they are nearby. In the new work, scientists set up a more subtle experiment – they measured the strength of the electromagnetic field near grapes and without them. For this purpose, an artificial nanodiamond with nitrogen defects in the crystal lattice was produced. The defects reacted to the light of a green laser, and by the intensity of their glow, it was possible to determine the intensity of the microwave field near this sensor.

The nanodiamond was placed on a waveguide through which a green laser pulse was transmitted. A pair of grapes were placed above the nanodiamond. Measurements showed that in the presence of grapes, the field showed twice the strength than without them. This is explained by the fact that in the case of the optimal size of the berries (about 27 mm in length), the pair creates a “hot spot” between one berry and the other, amplifying the applied radiation and increasing the field strength near the point.

Using the discovered effect, it is possible to create a whole range of highly sensitive sensors for space operating in the microwave range, including the search for hypothetical dark matter particles. The discovery will also help advance quantum computing, where microwave radiation and fields play a key role. But first it is necessary to narrow the scope of the experiment to determine more accurate parameters of the elements of future sensors. The experiment was quite crude and does not outline the boundaries of what is possible.