Physicists create quantum microphone to count sound particles

Physicists from Stanford have created a device they call a "quantum microphone" that is sensitive enough to measure the individual particles of sound called phonons. The scientists say that the new device will allow new types of quantum sensors, transducers, and storage devices for future quantum machines.

A phonon is something first proposed by Einstein in 1907 and is the name for a packet of vibrational energy emitted by jittery atoms. The packets manifest as sound or heat depending on their frequencies. Phonons have vibrational energies that are restricted to discrete values.

The energy of mechanical systems can be represented by different "Fock" states as 0, 1, 2, and so on based on the number of phonons it generates. The team at Stanford have created the world's most sensitive microphone that uses quantum principals to pick up the whispers of atoms. It does this by measuring the Fock state and thus the number of phonons.

Scientists know that a 1 Fock state has a single phonon, 2 Fock state has two, and so on. The quantum microphone consists of a supercooled nanomechanical resonator so small it's only visible through an electron microscope. The resonators are coupled to a superconducting circuit that contains electron pairs that move without resistance.

The circuit forms a quantum bit or qubit that can exist in two states at once and has a natural frequency. The defect the resonators introduce forms periodic structures that act like mirrors for sound and trap phonons in the middle of structures. The team says that mastering the ability to generate and detect phonons precisely could pave the way for new mechanical devices that can store and retrieve information encoded as particles of sound. The systems could also seamlessly convert between optical and mechanical signals.