Atomic radio - the first ever musical broadcast

Physicists from the United States created an atomic radio setup in the laboratory, which uses Rydberg atoms illuminated by two pairs of lasers instead of antennas, and conducted the world's first experimental transmission of a stereo music composition using an AM radio wave.

Brief description of the experiment

Five years ago, it was proved that sensitive receivers of electromagnetic radiation can be made on the basis of Rydberg atoms.

Rydberg atoms are excited atoms whose external electrons are at extremely high energy levels, and in this state these atoms react very sharply to weak changes in an external electric field.

In 2019, a more complex experiment was successfully performed, for which a special laboratory setup was created in the form of a container with gas from Rydberg atoms, illuminated by two laser radiation sources with different wavelengths.
 
When radio waves were passed through the capacitance, the absorption spectrum of Rydberg atoms began to shift, and these changes were recorded using lasers. Thus, the installation worked as a receiver of AM radio waves of a certain frequency.
 
As components of the installation, cesium-133 and rubidium-87 atoms were used.

The quality of the signal received with the help of such a setup turned out to be quite good in such an experiment.

A more detailed description of the experiment

As described previously, the Rydberg atom- This is a highly excited atom, the external electron of which has risen to a very high energy level.

As a rule, the main quantum number of this level is n ~ 100. The

properties of the Rydberg atom strongly depend on the number n:

• the lifetime of an atom increases rapidly with increasing n and is proportional to n ^ 3;
• the dipole moment grows as n ^ 2;
• polarizability increases as n ^ 7.

Thus, the stronger the Rydberg atom is excited, the longer it lives and the more sharply it feels an external electric field.

In addition, along with the number n increase:

• radius of a single atom (R ~ n ^ 6);
• characteristic length of the interaction of two atoms (L ~ n ^ 4).

For example , the radius of a hydrogen atom with n = 1000 is approximately 5x10 ^ (- 2) mm, and its lifetime reaches one second.

Theoretically, such properties make it possible to turn Rydberg atoms into sensitive receivers of electromagnetic waves.

Indeed, due to the large dipole moment, such atoms should very well feel the weak changes in the electric field that accompany the electromagnetic wave.

Therefore, if you constantly monitor the state of the atom, for example, highlighting it with a laser, you can restore the amplitude of the wave and the signal that it carries.

Theory is theory, but experiments are needed.

For the first time, the idea of ​​creating such a facility (the first version of a completely simplified atomic radio) was proposed in 2014, at the same time the first experiment was successfully conducted by a group of physicists led by Christopher Holloway, proving theoretical calculations in reality.



→ Link to article

Experiment diagram:



Installation diagram:



Electric field amplitude modeling:



Comparison of calculated and obtained data:





After this first experiment, refinements of the experimental setup began to improve its parameters and expand its capabilities - to obtain additional data, for example, it was possible to measure the phase of the radio wave falling on atomic gas.

→  Link to the article



Experiment Design:





Received data:









And now, after the main elements of atomic radio were discovered, it remains now to assemble a more complex and working installation, with which you can listen to music and radio programs.

And in the new installation they added support for stereo sound, the different channels of which are carried by AM-radio waves with different carrier frequencies.

→ Link to the full article on this experiment.



Scheme of a new setup:



The basis of the new experimental setup is a cavity filled with Rydberg atoms and transmitted through two lasers with different wavelengths.

One of the lasers (“binding”) ensures the coherence of the atoms of the receiver, and the second laser (“probing”) extracts information from it.

Due to the correct tuning of the “binding” laser at rest, the atoms of the receiver are transparent to the “probe” laser.

In this case, transparency is achieved only in a narrow frequency range, so the "probe" laser must be very clean. If a radio wave passes through the receiver, the absorption spectrum of the atoms is shifted, and the laser radiation begins to be absorbed.

The larger the wave amplitude, the greater the loss. Therefore, such a cavity acts as a receiver receiving AM radio waves with a specific carrier frequency.

To achieve the effect of stereo sound, in the experiment, the cavity was filled immediately with two types of Rydberg atoms, each of which independently worked with its own “binding” and “probe” laser.

Cesium-133 and rubidium-87, which received waves with a carrier frequency of 19.623 GHz and 20.644 GHz, were chosen as such atoms.

The signals from the "probe" lasers were fed to a computer and processed using the free program Audacity.

To test the operation of the atomic stereo AM radio, an improvised melody was transferred to it in A-minor, which was performed on two guitars (electric and acoustic with a pickup).

The signals taken from the guitars were sent to amplifiers, converted to an amplitude-modulated form using signal generators and broadcast using two horn antennas.

The acoustic guitar signal was broadcast at a frequency of 19.623 GHz, and the signal of an electric guitar at a frequency of 20.644 GHz.

Both horn antennas were located at a distance of about 15 centimeters from the cavity filled with Rydberg atoms.

Received signal:





The quality of the restored signal turned out to be quite acceptable: despite the small noise, reminiscent of cracking vinyl records, the music was very clear.

Record of the received signal was posted in  open access here .

→ You can listen to it online here.



Thus, physicists in their new work on music were able to show that quantum physics can be not only complex, but also interesting.