The quantum microphone could help develop more efficient quantum computers. A mockup of an array of nanomechanical resonators designed to generate and trap sound particles, or phonons. (Image credit: Standford University)


High-quality microphones may be good at picking up and carrying sound, but they’re not sensitive enough to catch everything, like the smallest unit of sound known as phonons. Recently, researchers at Stanford University developed a “quantum microphone” that can detect phonons. The new device could help develop more efficient quantum computers that operate by manipulating sound instead of light.


Phonons are packets of vibrational energy, which are normally difficult to measure because traditional microphones aren’t sensitive enough to detect them. Regular microphones detect when a sound wave interacts with an internal membrane. This is then converted into a measurable voltage. But this method doesn’t work for detecting photons because, according to the Heisenberg Uncertainty Principle, a quantum object’s position can’t be determined without changing it.


Rather than relying on indirect measurement of sound waves, the team built a device which measures the energy of photons directly using minuscule resonators that act like mirrors for sound. So how exactly does the microphone trap the photons? The device is built with a series of supercooled nanomechanical resonators that are so small they’re only visible through an electron microscope. The resonators are hooked up to a superconducting circuit that has electron pairs that move freely. The circuit creates a quantum bit (qubit) that can exist in two states at once and has a natural frequency. When the mechanical resonators vibrate, they create photons in different states.


Being able to detect phonons could make it easier for devices that encode information using sound energy. It would also allow the storage of massive amounts of data in a small machine. It could also help develop new efficient and compact quantum machines since phonons are easier to manipulate and have wavelengths that are much smaller than light particles, which is what more quantum machines use. 


“Right now, people are using photons to encode these states. We want to use phonons, which brings with it a lot of advantages,” said the lead author of the paper, Amir Safavi-Naeini, an assistant professor of applied physics at Stanford's School of Humanities and Sciences. “Our device is an important step toward making a ‘mechanical quantum mechanical’ computer.”


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