The quantization of energy is a fundamental basis of quantum physics and our current world view. It was discovered by the German physicist Max Planck in 1900 when he discovered that atoms can only release or absorb energy in discrete “portions”. In response to an excitation, a quantum jump occurs – the electrons suddenly change their quantum state.
Are mechanical vibrations also quantized?
But while quantum jumps have long been established and proven for atoms, ions and electromagnetic waves, they have so far remained just theory for another area: sound. To our ears, the vibrations caused by fluctuations in air pressure appear continuous: the sound of a struck gong fades away gradually and not step by step. But at the smallest level, the energy of such mechanical vibrations would also have to be quantized – according to the theory.
“Quantum mechanics predicts that the energy of a vibrating object is also divided into discrete packets,” explain Takuma Makihara from Stanford University in California and his colleagues. But so far it has never been possible to prove this experimentally. The reason: In order to observe this quantization, you have to measure the energy state of the mechanical vibrations without influencing it at the same time. However, this was precisely what was impossible for a long time with common measurement methods.
Microresonator and qubit combined
That has now changed. Makihara and his team have developed an experimental setup in which a superconducting quantum bit serves as a measuring aid. In the experiment, a thin film made of lithium niobate stretched between two nanopillars serves as a vibrating measurement object. The crystalline band is structured to act as a microresonator for the vibrations. This is located on a superconducting circuit that generates the measurement qubit.
The qubit is coupled to the microresonator in such a way that it can measure the phonon – the collective, coordinated vibration of the atoms in the crystal. However, the prerequisite for this is that the resonator oscillates long enough and that the qubit remains quantum-physically coupled to it. “We had to constantly evolve our processes to connect the vibrating object to the qubit without ruining either subsystem,” reports Makihara.
Through these optimizations, the physicists were able to extend their measurement time to two milliseconds and during this time measure the energy state of the phonon hundreds of times.
Clear cracks visible
The result: Clearly recognizable jumps could already be seen in the raw data from the measurement qubit. In-depth statistical evaluations confirmed this: “The resulting probability distributions show clear quantum jumps between 1 and 0, similar to those that occur with quantum jumps of photons,” write Makihara and his colleagues. “With a classical oscillator, we would instead see a smooth, gradual shift in the measurement statistics as the energy decays.”
The experiment thus confirms the theoretical predictions that sound and mechanical vibrations are also subject to quantization. “These discontinuous transitions are an impressive manifestation of quantum mechanics in a massive, oscillating object,” the physicists state. They prove that the phenomenon of quantum jumps can also be transferred to mechanical systems.
This finding and the coupling of qubits with microresonators achieved in the experiment could also have very practical uses, as Makihara and his team explain. They see possible applications in quantum sensors, but also in error correction in quantum computers.
Source: Takuma Makihara (Stanford University, USA) et al., Science, 2026; doi: 10.1126/science.aeh7535