Sound waves come in quantum jumps

In the 1980s, scientists first observed quantized jumps in the energy of ions. In the 2000s, photons' quantum behavior was discovered. Now, researchers at Stanford University in the US have found that phonons, mechanical vibrations responsible for sound, also exhibit quantum behavior. Amir Safavi-Naeini, the team leader, states that the data made the findings evident.

The team's process involves thin films of lithium niobate, which they processed into high-quality devices, a significant improvement since the commercialization of thin-film LiNbO3 a decade ago. This material's piezoelectric properties allow stress, such as from a vibration, to generate a voltage, enabling the coupling of vibrational quanta to superconducting qubits.

The team then worked on extending phonon coherence times from 800 nanoseconds to a millisecond, a task completed after identifying surface defects as the primary source of dissipation. Once the phonon lifetimes were improved, the researchers had to find a way to combine the mechanical and superconducting systems without compromising either.

This was achieved by Takuma Makihara and Erik Szakiel, who spent their PhDs addressing mechanical oscillator issues. The superconducting qubits detect whether an even or odd number of phonons are present in the LiNbO3, effectively performing a parity measurement, which could be useful for quantum error correction. This code, encoding data into phonons in pairs, has already been implemented for microwave photons and proves particularly effective for long-lived particles.

The researchers are collaborating with Michael Roukes at Caltech to explore applications in high-precision measurements of single protein masses and fundamental science questions, such as generating entanglement between mechanical systems through gravitational fields. However, the team acknowledges that their phonon lifetimes need to be significantly longer before achieving these goals.