Stanford researchers have directly observed quantum jumps of sound in a mechanical resonator for the first time. The findings were published in Science.
Quantum jumps — sudden transitions from one energy state to another — have been part of quantum theory since the early 1900s. Researchers first demonstrated them in trapped ions in 1986, followed by photons in 2007. Sound, however, remained a more difficult target.
A microscopic resonator with an unusually long ring
The mechanical resonator used in the experiment was built with chip fabrication techniques. Its tiny size means that many such resonators could potentially be placed on a single chip to carry out complicated tasks. A critical feature was how long the device could continue vibrating. Acting somewhat like a microscopic tuning fork, the resonator can vibrate for two milliseconds. If a normal-sized tuning fork had the same relative ability to sustain vibrations, it would continue ringing for several hours.
That unusually long "ringdown time" gave the researchers enough time to collect hundreds of measurements. Those repeated readings allowed them to identify the moment when the vibration disappeared, and the sound jumped from an energy state of 1 to 0.
The experiment also required the researchers to solve a long-standing problem in quantum engineering: measuring what is happening inside a quantum system without disrupting the delicate state being measured. The team developed a method for coupling the microscopic mechanical resonator to a superconducting qubit, an electrical circuit capable of storing quantum information that also served as the detector. The qubit repeatedly checks the mechanical resonator during its two milliseconds of vibration, determining whether the phonon - the smallest discrete unit of sound, representing the coordinated motion of many atoms - is in an energy state of 1 or 0.
The researchers see the work as an early but important step toward technologies that use sound as a quantum platform. One potential application is quantum error correction. Quantum computers could eventually solve certain complex problems that are beyond the reach of conventional computers, but their quantum states are extremely fragile. In many quantum computing systems, a quantum jump can signal that an error has occurred. Detecting those jumps has been difficult, so the ability to monitor them in sound could provide an important new tool for identifying and correcting quantum errors.
Researchers are exploring whether the system could be used to detect and identify proteins inside cells. The advance may eventually have applications beyond specialized quantum technologies, for example to smartphones and other electronic devices.