Stanford team observes quantum jumps of sound in real time
Researchers have directly observed the discrete quantum jumps of phonons in a mechanical resonator for the first time, using a superconducting qubit as a
A Stanford University team has recorded the first real-time observation of quantum jumps in sound. The work, led by physicist Amir Safavi-Naeini and published in Science, demonstrates that the vibrational energy of a microscopic mechanical resonator changes in discrete steps, or jumps, just as light and trapped ions do.
Quantum jumps are sudden transitions between discrete energy levels. Theorized in the early 1900s, they were first shown in trapped ions in 1986 and in photons in 2007. Observing these jumps in sound-whose quantum unit is a phonon, representing the coordinated motion of many atoms-had remained a challenge. "What this study shows will allow us to move forward with developing new quantum technologies with sound," said Safavi-Naeini, an associate professor of applied physics.
The resonator and its ringdown
The key to the breakthrough was a tiny mechanical resonator, fabricated using chipmaking techniques and visible under a microscope. Its critical feature was an exceptionally long resonance, or "ringdown," time of two milliseconds. For scale, if a regular tuning fork had the same quality, it would ring for hours. This long vibration period allowed the team to take hundreds of readings to pinpoint the exact moment the energy state jumped from 1 to 0.
Coupling sound to a quantum detector
A major hurdle in quantum engineering is extracting a signal without disturbing the fragile quantum state. Co-first authors Takuma Makihara and Erik Szakiel developed a method to pair the mechanical resonator with a superconducting qubit, a circuit that stores quantum information. We had to continually develop new processes to make this extremely long-lived, vibrating object and then integrate it with the qubit, which is our little electrical detector-without ruining either subsystem, said Makihara, a recent Stanford doctoral graduate.
The qubit acts as a detector, repeatedly checking during the two-millisecond vibration whether the phonon is in an energy state of 1 or 0. This continuous monitoring enabled the recording of individual quantum jumps as they happened.
A platform for future technologies
This foundational achievement opens several potential avenues. It could aid quantum error correction, a major challenge for quantum computing. In many quantum computing architectures, a quantum jump represents an error; detecting these jumps in real time is a key step toward correcting them.
The sensitive resonator-qubit platform also holds promise for ultra-precise sensing. Safavi-Naeini's team, in collaboration with Michael Roukes' group at Caltech, is already exploring its use to detect and identify proteins within cells.
Everyday technology may benefit as well. Sound is a fundamental component in devices like smartphones. This shows we can have incredibly fine-tuned control of sound, which might mean that devices that use sound as a fundamental technology can get much better, said Szakiel, a current doctoral student. The small size of the resonator also makes it a candidate for packing many onto a single chip to perform complex functions.
