The Crystal That Sings in the Cold express
A guitar string rings longer if you don't touch it too much. A resonator, a device that vibrates at a single frequency like a tuning fork, behaves in much the same way. A team of engineers made such a "tuning fork" from aluminum nitride, smaller than a grain of sand, and made it oscillate 16 billion times per second. That's the frequency at which sixth-generation mobile networks will operate.
The main enemy of oscillation is friction: it turns vibrational energy into heat. The less friction, the higher the quality factor — a measure of how many oscillations a resonator completes before it dies down. The scientists cooled the device from room temperature down to −266 °C, measuring the quality factor at each step. It turned out that in the cold, it increased more than fourfold: from 363 to 1589. For comparison, a typical tuning fork has a quality factor of about a thousand, but it oscillates thousands of times slower.
To understand where the losses come from, the engineers built a model that accounts for both internal friction in the material and energy leakage through the supports, as if a string were losing vibration not only to the air but also to the guitar body. The model predicted how the quality factor would change with temperature, and it matched the measurements. Then they tested it on another resonator that oscillates 23 billion times per second, and it matched again.
Such a tool would allow engineers to design filters for quantum computers in advance. In those devices, signals are so weak that any extra noise ruins everything. An aluminum nitride resonator in extreme cold could become that tuning fork that never goes out of tune.
A quality factor of 1589 means the resonator completes fifteen hundred oscillations before its amplitude drops by a factor of one hundred. The human ear perceives the decay of sound as a long ring, but here the "ring" lasts less than a millionth of a second, yet for microchips, that's a very long time.
In Arthur C. Clarke's novel The City and the Stars, computers store memory in crystal lattices that don't lose information even over millions of years. Aluminum nitride resonators are a step toward such reliable quantum memory.