Scientists try out a new type of "acoustic" amplifier

According to the report of the American Physicists Organization Network on September 8 (Beijing time), this year, on the occasion of celebrating the 50th anniversary of the birth of lasers, scientists are working on a new type of coherent sound beam amplifier that uses sound instead of light. Scientists have recently demonstrated that in an ultra-cold atomic gas, phonons can also be excited in the same direction, similar to photon stimulated emission, so this device is also known as the "sonicator."

Phonon excitation theory was first proposed by the Max Planck Institute and the California Institute of Technology in 2009. It is still in a relatively new research field. The theory is that the phonon is the smallest independent unit of vibrational energy and can also produce a highly coherent acoustic beam, like a photon, by excitation, especially high-frequency ultrasound. They described for the first time that a magnesium ion was frozen in an electromagnetic trap to a temperature of about 1/1000 Kelvin, and that it could generate excited ions of a single ion. However, there is a difference between the stimulated amplification of a single phonon and one photon. The phonon frequency is determined by the frequency of single-atom vibration rather than the collective vibration.

In the new study, JT Mandenka and the cooperative team of the Lisbon Institute of Technology in Portugal extended the concept of single ion phonon excitation to a large atomic whole. To do this, they demonstrated ultracold atomic gas-integrated phonon excitation. Compared to the single ion case, the phonon frequency here is determined by the internal vibration of the gaseous atoms, and the photon frequency is determined by the vibration inside the cavity.

Regardless of whether coherent electromagnetic waves or coherent sound waves, the biggest difficulty comes from the choice of system, frequency range, and so on. Mandenka said that the difficulty in this study is to mimic the mechanism by which light waves are stimulated and amplified, but instead of photons, phonons are produced. That is, by precisely controlling the ultra-cold atomic system, it can completely emit coherent phonons according to the laser emission mechanism.

The new method confines the gas to magneto-optic traps and generates excited phonons through three physical processes. First, a red detuned laser cools the atomic gas to an ultra-cold temperature; then a blue detuned light vibrates the ultracold precursor gas, generating a beam of invisible light, finally causing the atom to form a phonon coherent emission, and then decays to Low level status. The researchers pointed out that the resulting sound waves can be connected to the outside world mechanically or electromagnetically, and the system only provides a source of coherent emission.

Regarding the naming of phonons, scientists first used the word "saser" in the name of "laser," which means that the sound is amplified and emitted. But Mandenka believes that the use of "phaser" is more accurate. It emphasizes the quantum properties of the phonons rather than the sound, and also implies that the emission process is similar to photon-stimulated emission.

One possible use of a highly coherent ultrasound beam is to greatly improve the resolution of the image in terms of X-ray tomography. Mandenka said: "Laser was only developed as an invention that couldn't solve any problems. Therefore, for the screaming, what we are worried about now is only basic scientific issues, not application problems."