Quantum microphone hears the world’s lowest sound
The world's lowest sound has so far been a theoretical phenomenon, but now scientists have created a microphone that can perceive the smallest particles of the sound: the phonons. The audio particles will be used in surgery and computers in the future.
The new knowledge of phonons may perhaps be used to develop new forms of ultrasound treatments
The smallest part of the sound transforms sound into crackles
When sound moves, it happens as vibrational energy that can move in waves through the atoms of a material. The atoms are usually arranged in a lattice, but the wave interferes with the lattice structure so that the atoms begin to vibrate together with the wave.
The vibrational energy comes in small packages called phonons - the smallest element of sound. The sounds you hear in everyday life consist of many more phonons than can be counted.
Normally, therefore, phonons are only used to describe vibrational energies that are so small that they are impossible for humans to hear.
If you could still hear the small vibrations, you would still not experience a continuous sound.
The phonons come only one at a time and you can't add half or quarter phonons. The result is a crackling rather than a smooth sound.
Every time you hear a sound, there is vibration in the air. For example, if you play music on your stereo, the speaker will trigger vibrations in the air.
These vibrations propagate like sound waves and eventually hit your eardrum as music. When you lower the volume, the vibrations become weaker until the volume becomes so low that you can no longer hear the music.
However, individual sound particles, called phonons that make up the vibrations, are still active. Everything we normally call sound, even a button pin that falls into the floor, produces so many phonons that they are impossible to count.
The energy from a phonon is so low that it would take around a quadrillion phonons to keep a lamp lit for a second – the equivalent of about a million times as many grains of sand as are found on the entire earth.
With so many phonons in even very weak sounds, it is difficult to imagine that individual phonons can be isolated. Scientists at Stanford University, USA, have now succeeded.
The researchers have developed a microphone that can measure individual phonons and thus perceive very low sounds.
The breakthrough can, among other things, be used to send information between supercomputers faster than with today’s technology-based on photons – the particles that make up light waves.
Good vibrations reveal phonons
In 1907, Albert Einstein used vibration to describe how solid materials work.
However, vibrations were not associated with sound until 1932 when Soviet physicist Igor Tamm discovered that sound consisted of vibration. At the same time, the physicist introduced the word phonon.
Because phonons are so small, they cannot be measured directly, but in return, they obey the principles of quantum mechanics: The energy of the vibrations is limited to certain states – so-called quantum states.
Researchers have now used this knowledge to develop a quantum microphone that can perceive phonons. Quantum state acts as a step.
When standing on a staircase you can stand on a certain step, but you cannot stand between two steps. In the same way, physicists use the phonons to count steps.
In quantum mechanics, these steps are called Fock states. A vibration can be in a 1-phonon state, 2-phonon state, and so on, but not between two states.
The energy of the phock state can be measured and since it can be translated directly to the number of phonons, one can find out the exact number of phonons.
Microscopic drum for noise
The problem when measuring phonons is that the energy is so low. In a normal microphone, the sound waves hit a diaphragm which in turn converts the pressure into a measurable electrical voltage.
However, phonons cannot be measured like this because their interaction with the membrane disturbs the measurement and hides the phonon’s own energy.
Instead of trying to measure individual phonons, the researchers have therefore found a way to measure the total vibrational energy of the sound waves.
If they can describe the total vibrational energy accurately enough, they can simultaneously find the number of phonons.
The first challenge is to create the microscopic sound to be measured. The researchers had to develop a drum set so small that its parts could only be seen through an electron microscope.
The “drum beater” is a so-called transmon quantum bit: An advanced electronic component from a quantum computer, the state of which is determined by an electrical charge.
The quantum bit sends signals to a drum stick. It consists of electrons moving in an electric orbit.
When the electrons pass through a so-called resonator, which corresponds to the drum skin, “the drum beaker” switches on, which emits phonons.
However, the resonator does not behave like a normal drum skin. It holds the phonons so that the skin continues to vibrate. This vibration is captured by the transmon quantum bit.
By measuring the quantum bit, the researchers can thus say exactly how many phonons are caught in the drum skin.
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Microscopic microphone captures sound particles. To find the phonons, the smallest sound particles, the researchers built a microphone that can both emit and perceive vibrations. The phonons were then calculated using vibrational energy.
Phonons are isolated and captured in a resonator. When the electrons pass through a so-called resonator, it produces phonons that are then trapped and vibrate inside the resonator. The resonator is surrounded by so-called phononic crystal that can control the direction of vibration. It should ensure that the phonons are captured by the transmon quantum bit.
Microscopic sound is produced and measured. The vibrations of the phonons affect the electrical charge – the energy – in the transmon quantum bit.
The number of phonons is determined. Based on the energy of the transmon quantum bit, researchers can now read how many phonons have been created in the resonator. The number can be read with curves whose height reveals the energy of the phonons.
Dive into the experiment with the microscopic quantum microphone
The researchers’ experiments with the quantum microphone were published in the journal Nature.
The scientific article can be found right here .
Phonons outperform the light
One of the areas where phonons may play a role is in the supercomputers of the future, which are based on quantum mechanics. Today, computers count with bits corresponding to the numbers 1 and 0.
However, quantum computers are based on so-called quantum bits that follow the laws of quantum mechanics. Quantum bits are not binary and can have many different states between 1 and 0.
This allows for new ways of doing calculations. If you want to run a calculation on a quantum computer, the quantum bits must first be coded to a specific state.
On a regular computer, this is equivalent to setting the bit to 1 or 0, but since quantum bits are more complex and can contain much more information, coding is also more difficult.
Today, light particles – called photons – are used to encode quantum bits, but phonons have several advantages over photons.
For example, phonons have a shorter wavelength than photons – up to several hundred times shorter than laser light. This means that the coding can take up much less space.
This means that quantum computers, which can be quite large today, can be made much smaller with the help of the phonons.
Medical science sharpens the ears
The new measurement of phonons should not only be used in future quantum computers. The researchers also hope that the experiment with the small drum kit can have an impact on the future of medical science.
Just as you can make laser light with precise control over the wavelength of photons, physicists believe that you can also construct laser sound.
So far, the construction of such a laser has been hampered by the fact that it is extremely difficult to emit phonons with the same energy because there is so little difference between the phonons’ energy.
The technology behind the quantum microphone is a step in the right direction. If researchers succeed in building a phonon laser, it will have advantages over a traditional laser for the same reason that phonons have advantages in a quantum computer – the shorter wavelength of sound.
This can be used in medical science for more accurate ultrasound measurements and perhaps also in the long term for precision surgery.
The dream vision is that the phonons, for example, can be used to operate cancer in very difficult places in the body.
In short: In the future, music may prove to have healing powers.