We are immersed in a mixture of electromagnetic waves with very different properties. However, in order to be able to capture signals from different parts of their spectrum (and thus convert radiation into an electrical signal), we need receivers with different structures and parameters.

For example, waves with subatomic wavelengths are detected by radioactivity sensors. Picometre waves require X-ray sensors (e.g. X-ray film). For visible light, i.e. waves with wavelengths of 380-780 nanometres - human eyes are excellent sensors. Submillimetre waves (ThZ frequencies) are recorded by infrared sensors. And microwaves (GHz) with 'pocket' lengths (3mm-30 cm) are received by mobile phones. Radio antennas, in turn, register meter-long radio waves (MHz), and waves with a length of hundreds of meters can be received, for example, by huge receiving masts or satellite dishes. However, a big challenge is to build a receiver that will be compatible with radiation of a wide range of frequencies.

Michał Parniak-Niedojadło, PhD, a researcher at the Faculty of Physics of the University of Warsaw and the Centre for Quantum Optical Technologies of the University of Warsaw, estimates that for now, if we wanted to use traditional antennas to receive very different waves with lengths from submillimeters to hundreds of meters (from THz to MHz), we would probably need hundreds of different sensors. In the meantime, his team is building an antenna that will be capable of receiving waves of almost any frequency from this wide range. The largest waves received by the same receiver will therefore be millions of times larger than the smallest ones.

Such a compact universal wave receiver is an important proposition for the space sector, where each additional gram of devices launched into space means high costs.

That is why the European Space Agency (ESA) ordered such a prototype Rydberg radio receiver from Parniak. The project is worth 600 thousand euros. ESA plans to use such antennas to study the atmosphere and natural radiation, including radiation reflected from the planet. The agency also wants to have a tool to detect whether companies or institutions are actually using the frequency bands officially assigned to them in radio communication. The antennas can also be used as extremely precise thermometers - devices measuring temperature even in conditions close to absolute zero.

These extraordinary possibilities - unattainable for traditional antenna technologies - are provided by quantum physics, in a version friendly for immediate application.

The scientists plan to use Rydberg atoms. This are rubidium atoms, in which electrons are excited by laser to such high energies that the electron jumps to a higher orbital - it moves its orbit away from the nucleus of the atom from 5s even to level 48 or 120. In this way, the entire atom reaches a diameter of up to 10 microns. It becomes thicker than a spider's thread and about as large as a blood cell.

If such an atom finds itself in an electromagnetic field of a specific frequency, it enters into resonance with it. This phenomenon can be recorded using photonic methods proposed by the Polish researchers. By modulating the signal from the laser and the orbit in which the electron is located, it is possible to 'tune' the large atom to receive signals of a given frequency - from about 1 megahertz to 1 terahertz.

Rydberg receivers designed by the Polish team operate at room temperature and are quite compact. The receiver itself is tiny - it is a cube with a side of a few mm, but its activation and reading require precise lasers, which are currently one meter in size.

'Our antennas are so small that they can detect the wave without disturbing it. No one can see that they are being watched', Parniak says. He adds that these antennas should be resistant to traditional attacks - there are no electronics in the receiver itself, so it cannot be burned remotely - for example, by sending an excessively strong radio signal.

According to Parniak, telecommunications companies may also be interested in Rydberg antennas, which can be used in a new type of receiver to detect and amplify even very weak radiation signals.

Potential applications also concern quantum computers. A Rydberg radio receiver could one day be used to build the equivalent of a network card in a quantum computer.

'Quantum computers are still a thing of the future. But quantum sensors - they will soon find applications', Parniak believes.

He adds that quantum effects have long been used in the space industry, in devices sent into space - not only lasers, but also, for example, atomic clocks, which are needed to increase GPS accuracy.

Ludwika Tomala (PAP)

PAP - Science in Poland

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