A quantum microphone is an extremely sensitive device that detects the smallest particles of sound, such as packets of vibrational energy called phonons.
Announced in July 2019 and developed by researchers at Stanford University, the device could lead to the development of smaller quantum computers that are more efficient because they work by manipulating sound rather than light particles (photons). Phonons are much easier to control than light photons and their wavelengths are thousands of times smaller than light particles. Using this method, the researchers believe that they’ve taken an important step in creating a ‘mechanical quantum mechanical’ computer.
The researchers expect the quantum microphone to enable new types of quantum sensors, transducers and storage devices for future quantum machines that could store massive amounts of information in much smaller spaces than by using photons.
To date, it hasn’t been possible to measure phonons because ordinary microphones just aren’t sensitive enough to pick them up.
How the quantum microphone measures sound
A traditional microphone works by detecting when an incoming sound interacts with an internal membrane and the physical displacement is converted into a voltage that can be measured.
However, the phonons are so small that they can't be detected individually because of the Heisenberg Uncertainty Principle, which states that the position of a quantum object can’t be known precisely without changing it. According to researchers, trying to measure the number of phonons with a normal microphone isn’t possible as the act of measuring introduces energy into the system that conceals the exact energy attempting to be measured.
Rather than use acoustic wave measurements, the scientists developed a device that directly measures the energy of phonons using very tiny supercooled nanomechanical resonators. These resonators, which are only visible through an electron microscope, act as mirrors for sound. The quantum microphone captures the photons and measures the vibrations they emit – various energy levels correspond to various numbers of phonons.
In 2012, scientists from Chalmers University of Technology in Sweden demonstrated a device they called a quantum microphone that could pick up very faint sounds. The device was based on a single electron transistor (the electrical current can only flow through this transistor one electron at a time).
The research team at Chalmers studied acoustic waves that propagated over the surface of a crystalline microchip, mimicking the ripples on a lake when someone throws a pebble into it. Even though the wavelength of the sound was just three micrometers, the device, which was even smaller, was able to rapidly sensing the acoustic waves as they passed by.
“The result offers prospects of a new class of quantum hybrid circuits that mix acoustic elements with electrical ones, and may help illuminate new phenomena of quantum physics,” according to the university. The results were published in the journal Nature Physics.