Grapes double sensor magnetic power in an epic quantum breakthrough

In interesting research, insights from ordinary supermarket grapes led researchers to boost quantum sensor performance.

The study reveals that grape pairs generate localized magnetic field hotspots for microwaves, aiding compact and cost-effective quantum sensor development.

The Macquarie University team’s work in Sydney builds on viral videos of grapes producing plasma, glowing charged particles, in microwave ovens.

“While previous studies looked at the electrical fields causing the plasma effect, we showed that grape pairs can also enhance magnetic fields, which are crucial for quantum sensing applications,” said Ali Fawaz, a quantum physics PhD candidate at Macquarie University and lead author, in a statement.

Magnetic field enhancement

Grapes are a popular fruit with numerous health benefits. Since sparks were first observed between two grape pieces in a microwave oven in 1994, they have become key to studying an intriguing physics problem.

Research shows that grape pairs, or similar water-based structures, act as microwave resonators, trapping electric fields due to their shape and high permittivity. Sparks occur when plasma forms from metallic ions in the grapes. According to the team, the phenomenon has inspired the exploration of technical applications requiring strong microwave field enhancement.

Microwave resonators, used in technologies like satellites, masers, and quantum systems, confine fields to small areas. In quantum applications, they drive systems like spin qubits via magnetic fields.

In the new work, grape pairs enhance magnetic fields to efficiently drive nitrogen-vacancy center spins in nanodiamonds, potentially enabling compact quantum technologies. The Macquarie team looked at magnetic field effects that are important for quantum applications, whereas earlier research concentrated on electric fields.

Specialized nano-diamonds with nitrogen-vacancy centers—atomic-scale flaws that function as quantum sensors—were employed by the team. These flaws, which are among the numerous flaws that give diamonds their color, have the ability to sense magnetic fields and act like tiny magnets.

“Pure diamonds are colorless, but when certain atoms replace the carbon atoms, they can form so-called ‘defect’ centers with optical properties. The nitrogen-vacancy centers in the nanodiamonds we used in this study act like tiny magnets that we can use for quantum sensing,” said Sarath Raman Nair, a lecturer in quantum technology at Macquarie University and the study co-author, in a statement.

Grapes enhance sensors

For the study, the team notes that on the end of a thin glass fiber, they positioned their quantum sensor—a diamond with unique atoms—between two grapes. These atoms may glow red if they shine green laser light through the fiber. The red glow’s brilliance demonstrated the intensity of the microwave field surrounding the grapes.

Researchers demonstrated that adding grapes to microwave setups doubles the strength of the magnetic field. The findings pave the way for exploring alternative microwave resonator designs, potentially enabling smaller and more efficient quantum sensing devices.

According to the team, the size and shape of the grapes were critical, with experiments relying on grapes approximately 27 millimeters long to focus microwave energy at the correct frequency for diamond quantum sensors.

Traditionally, sapphire is used in quantum sensing, but the team hypothesized that water could perform better. Grapes, being mostly water-encased in a thin skin, provided an ideal model to test this innovative approach.

“Water is actually better than sapphire at concentrating microwave energy, but it’s also less stable and loses more energy in the process. That’s our key challenge to solve,” said Fawaz, in a statement.

The researchers are now exploring more reliable materials that utilize water’s unique properties, aiming to advance efficient sensing devices.

The details of the team’s research were published in the journal Physical Review Applied.