Stanford researchers have achieved a groundbreaking feat, directly observing quantum jumps of sound within a mechanical resonator – a phenomenon previously confined to theoretical realms and demonstrated only in light and atomic particles. This landmark achievement, detailed in the prestigious journal Science, marks a significant leap forward in quantum physics research, extending a lineage of inquiry that began over a century ago. Quantum jumps, the abrupt transitions of a system from one discrete energy level to another, have been a cornerstone of quantum theory since the early 20th century. While these startling transitions were first experimentally confirmed in trapped ions in 1986 and subsequently in photons, the fundamental quanta of light, in 2007, observing such behavior in sound presented a far greater challenge. The Stanford team, spearheaded by Associate Professor of Applied Physics Amir Safavi-Naeini, has now successfully captured these "phonon jumps" in real-time, opening up exciting new avenues for quantum technology development.

"What this study shows will allow us to move forward with developing new quantum technologies with sound," stated Safavi-Naeini, a leading figure in applied physics within Stanford’s School of Humanities and Sciences. "We have seen that vibrating objects can exhibit quantum behavior, which is the prerequisite for many of the operations needed by quantum computing and sensing." This observation confirms that the seemingly continuous fade of everyday vibrations, like a slowly quieting bell, belies a fundamentally discrete and stepwise process at the quantum level. The quantum equivalent of a photon is a phonon, representing a quantized unit of vibrational energy, a coordinated motion of atoms. At the macroscopic scale, sound waves appear to dissipate energy smoothly, but at the quantum scale, a resonator’s vibrational energy doesn’t diminish gradually; instead, it transitions in distinct, quantized steps, mirroring the behavior previously observed in ions and photons. While prior experiments had hinted at the possibility of sound undergoing these quantum transitions, the Stanford study provides the first direct, real-time tracking of individual phonons as they make these abrupt leaps between energy states.

The key to this breakthrough lies in the ingenious design and properties of the microscopic mechanical resonator employed by the Stanford team. Fabricated using advanced chip fabrication techniques, this minuscule device is designed to be incredibly sensitive and capable of sustaining vibrations for an unusually long duration. Its diminutive size means that vast numbers of these resonators could potentially be integrated onto a single chip, paving the way for complex quantum operations. A crucial aspect of the resonator’s design is its remarkably long "ringdown time." Functioning akin to a microscopic tuning fork, it can maintain its vibrations for a full two milliseconds. To put this into perspective, if a macroscopic tuning fork possessed the same relative ability to sustain its resonance, it would continue to ring for several hours. This extended vibration period was absolutely critical, providing the researchers with a substantial window of approximately two milliseconds to gather hundreds of measurements. These repeated observations allowed them to pinpoint the exact moment when the vibration ceased, signifying the quantum jump of the sound energy from a state of one phonon to zero.

Beyond the resonator itself, a significant hurdle in quantum engineering was overcome: the challenge of measuring the delicate quantum state of the system without inadvertently disturbing or collapsing it. Takuma Makihara and Erik Szakiel, the co-first authors of the study, devised an innovative method to couple the microscopic mechanical resonator to a superconducting qubit. This qubit, an electrical circuit designed to store quantum information, effectively served as the detector in this setup. "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," explained Makihara, a recent Stanford doctoral graduate. The qubit was programmed to repeatedly probe the mechanical resonator throughout its two-millisecond vibration cycle, determining its energy state at each interval – either in a state with one phonon or zero phonons. By performing these measurements with high frequency and precision, the researchers were able to precisely identify the signature of the quantum jump as it occurred.

The implications of this research are far-reaching, extending beyond fundamental physics into the realm of cutting-edge quantum technologies. The researchers view this work as a foundational step towards harnessing sound as a robust quantum platform. One of the most promising applications lies in the field of quantum error correction. Quantum computers, with their potential to tackle problems intractable for classical machines, are hampered by the extreme fragility of their quantum states, making them susceptible to errors before computations are complete. A quantum jump in many quantum computing systems can be an indicator of such an error. The ability to monitor these jumps in sound offers a novel and potentially more efficient method for detecting and rectifying these critical quantum errors. Furthermore, the synergistic combination of the mechanical resonator and the superconducting qubit could evolve into an exceptionally sensitive measurement platform. Safavi-Naeini’s group is actively collaborating with physicist Michael Roukes’ team at Caltech to explore the system’s potential for detecting and identifying individual proteins within living cells, a feat with profound implications for biology and medicine.

The advance also holds promise for applications beyond highly specialized quantum technologies. Sound plays an integral role in countless everyday electronic devices, from smartphones to sophisticated audio equipment. The newfound ability for incredibly precise control over vibrations, as demonstrated by this research, could lead to the development of entirely new generations of these technologies, according to Szakiel, a current doctoral student in Safavi-Naeini’s lab. "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," he remarked, highlighting the potential for enhanced audio fidelity, more efficient transduction, and novel acoustic functionalities.

The research team’s expertise is further bolstered by their affiliations with interdisciplinary centers at Stanford, including Stanford Q-FARM and Bio-X. The collaborative spirit of the project is evident in the extensive list of Stanford co-authors, including David Schuster, the Joan Reinhart Professor and professor of applied physics; Shannon Harvey, a scientist with SLAC National Accelerator Laboratory; Mihir Pendharkar, physical research scientist at the Edward L. Ginzton Laboratory; former applied physics doctoral scholar Rachel Gruenke-Freudenstein; and doctoral scholars Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki, all from the Department of Applied Physics. This pioneering research was made possible through significant funding from a consortium of leading organizations, including Amazon Web Services Inc., the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the U.S. Department of Defense. Notably, both Safavi-Naeini and Schuster hold the distinguished title of Amazon Scholars, underscoring the industrial relevance and potential impact of their quantum research.