Scientists from Argentina and Germany create an innovative sound laser

Based on particles in a quantum state termed “Bose-Einstein condensate”, physicists from Argentina and Germany created for the first time a novel kind of sound laser, or “saser”. The experimental development represents a scientific breakthrough, and opens the way to applications in quantum computation, telecommunications, and health.

Publication date: 09/11/2020

Bose-Einstein condensates (BECs) were first theoretically proposed by Satyendra Nath Bose y Albert Einstein almost a century ago. In 1995, Eric Cornell and Carl Wieman were able to generate for the first time in a laboratory this type of quantum state, so-called the “5th state of matter”, using Rubidium atoms at very low temperatures.

Now, a team of scientists from Argentina and Germany demonstrated for the first time a sound laser operating at the technologically relevant “super-high-frequency” range, which is based on the coupling between Bose-Einstein condensates of another type or particles, the polaritons.

This type of sound laser, or “saser”, represents a scientific breakthrough relevant to the description of the quantum world. And it also could be applied in different fields, particularly quantum technologies, telecommunications, and biomedical imaging.

Polaritons are particles of the boson type that result from the strong and indivisible coupling between photons (the quantum particles of light) and an oscillation of the electron charge in atoms. They can be generated by exciting with a laser certain resonant devices. Under the appropriate stimulus, millions of them can form a Bose-Einstein condensate in which they respond in a synchronized way, like a “large atom” all with the same energy level and phase.

The new development consist on a hybrid system that combines different tools of quantum physics to generate coherent hypersound by the interaction between light and the polaritons in the BEC.

The experiments were performed at 5 degrees Kelvin (approximately 270 degrees Celsius below zero) at the Photonics and Optorelectronics Laboratory of the Comisión Nacional de Energía Atómica of Argentina (CNEA, in its Spanish acronym), as part of a collaboration with scientists from the Paul Drude Institut in Berlin.

The main result reported is the demonstration of a “saser” of very high frequency that is emitting coherent hypersound, inaudible to humans. The scientific paper that reports this work has been published in Nature Communications.

The physicists from Argentina are students and professors from Instituto Balseiro (it depends of CNEA and Universidad Nacional de Cuyo, UNCUYO), researchers from the Consejo Nacional de Investigaciones Científicas y Técnicas of Argentina (CONICET) and from the Instituto de Nanociencia y Nanotecnología (CNEA-CONICET). The German team belongs to the Paul-Drude-Institut in Berlin. The collaboration is financed by the Science Ministry from Argentina, and the German DFG.

Why do the authors talk of a hybrid system? Alejandro Fainstein, head of the team at Centro Atómico Bariloche, in Bariloche city, comments that the novelty of the work is that they were able to demonstrate the efficiency of a novel approach. This was done by creating a device that consists of coupled resonant microcavities that combine polaritons in a BEC, something that is the subject of study of the field of cavity quantum electrodynamics (CQED), with concepts of the field of cavity optomechanics. In this hybrid resonators both sound and light waves become amplified.

The working principle of a laser is based on quantum phenomena: the stimulated emission of monochromatic and coherent “packages” of light, or photons. That is the origin of the name, “Light Amplification by Stimulated Emission of Radiation”.

For the case of the new sound laser, the scientists were able to generate the same kind of phenomenon, but involving mechanical vibrations, that is hypersound, and using as the excitation source a laser.

“The generated sound is of a very high frequency, 20GHz (Gigahertz), that is, a million times larger than the highest frequency that can be perceived by humans,” says Fainstein, explaining that our audible limit is around 20kHz (kilohertz).

“In our hybrid system we use a continuum laser that enters a resonant cavity and interacts with atoms forming the so-called polaritons. Inside the cavity light travels part of the time as photons, and another part of the time is captured by the atoms inducing charge oscillations,” explains Fainstein. On increasing the laser excitation power, these polaritons colapse into a Bose-Einstein condensate, which implies the synchronization of their emission of photons. In this way, millions of polaritons behave like a single atom, “singing” harmonically in unison.

“This leads to the emission and reabsorption of light simultaneously and very intensely. It is this synchronized Bose-Einstein condensed state that impinges on the mirrors of the cavity, generating the coherent mechanical vibrations,” explains Fainstein. The result is a saser, which is constituted by hundreds of thousands of “phonons”, the quanta of acoustical vibrations.

It turns out that these vibrations back-act on the cavity, that is, on the Bose-Einstein condensate, by changing the size of the optical resonator (the cavity “breathes” due to the mechanical oscillations). “In this way everything oscillates synchronously: the polariton BEC, and the mechanical vibrations,” says Fainstein. The consequence of this complex dance that involves light, the electrons in the atoms, and the mechanical vibrations, is a very efficient way to convert a beam of light, into the coherent emission of sound.

How was the hypersound detected? “The polariton BEC generates the coherent mechanical vibrations, and the latter in turn affect the BEC by inducing the oscillation of the optical cavity that contains the BEC. By modulating the polariton condensate, the coherent sound changes in a characteristic way the light spectra emitted by the BEC. We measured this characteristic spectrum, which contains spectral lines of different color when the coherent sound is present. From the amplitude of these spectral side-bands we were able to estimate the intensity of the emitted sound”, explains Fainstein.

Dr. Christian Schmiegelow, researcher at the Quantum Foundations and Information Group of the Physics Department in Buenos Aires University, comments on the work published by the Argentina and German teams: “They demonstrated that they can interconnect quantum polariton systems in mechanical cavities in a coherent way. This is a fundamental step in the use of these systems for diverse applications and quantum technologies”.

Schmiegelow, adds that on working with quantum systems the first difficulty always lies in isolating the system from its environment. The second difficulty is that of coupling, in a controlled way, several of these systems. “This second step, that is solved in this work by our colleagues from Instituto Balseiro and Paul-Drude-Institut, is typically even more difficult than the first, because it implies the design and ingenious control of many variables,” Schmiegelow says.

Next challenges include the more detailed understanding of the quantum physics behind the observed phenomena, and the optimization of the new device for technological applications. A beginning, rather than an end.

By Communications and Press Office of Instituto Balseiro

Link paper: https://www.nature.com/articles/s41467-020-18358-z

Esta obra está bajo una Licencia Creative Commons Atribución 3.0 Unported.