Radio waves have been cooled to close to their quantum ground state, a process that removes the noise in radio detection, allowing even the faintest signals to stand out. The achievement could advance many areas of research where small temperature fluctuations impede progress, including the detection of dark matter, a necessary step to explaining the composition of 85 percent universe’s mass.
Radio stations may aspire to be cool, but for most people the idea of the radio waves themselves having a temperature feels like a category error. However, the antenna with which we detect radio waves are made up of atoms that, like everything else in the universe, move around randomly, introducing noise in the system. Since the movements reflect the antenna’s temperature physicists refer to affected radio signals as hot.
A nearby powerful radio signal can overwhelm any heat in the system, so we can listen to our favorite music with clarity. However, when trying to tap into radio waves at the edge of our capacity to hear, cooling the antenna, and thus the waves themselves, is vital to avoid being overwhelmed with static. A team from Delft University of Technology have announced in Science Advances they have brought radio waves to near the coolest temperatures possible.
For ordinary cooling purposes an object can be bathed in a very cold medium, such as liquid nitrogen, that draws heat away. When even colder temperatures are required, such as to remove heat that might overpower signals from an MRI machine, liquid helium is used instead, bringing temperatures down to just 2.2 degrees above absolute zero.
Even that is far too warm some research purposes, however. Optical, infrared and gigahertz frequency telescopes have used quantum techniques to cool electromagnetic radiation, but these have failed for frequencies in the hundreds of megahertz, coincidentally those used by FM radio stations.
It’s this part of the spectrum Professor Gary Steele and co-authors are seeking to cool. They used an adaptation of the laser cooling techniques that recently set a world record of 38 picoKelvins (38 trillionths of a degree above absolute zero). The adapted process is known as photon pressure coupling and uses the radiation pressure of photons at one frequency to draw heat from those elsewhere on the electromagnetic spectrum. Steele and co-authors coupled two superconducting circuits and transferred heat from one to the other to achieve their stunning cold radio waves.
“The dominant noise left over in the circuit is only due to quantum fluctuations, the noise that comes from the strange quantum jumps predicted by quantum mechanics,” Steele said in a statement.
The general idea of photo pressure coupling has been around for several years, but the paper claims the authors achieved coupling strength around 10 times better than anything reported previously. This pushed the radio signals to record low temperatures, but the team describe ways in which they expect to go lower still, taking effective temperatures “far below the physical temperature of any bath.” These could allow radio detection far more sensitive than anything achieved so far, which could prove invaluable for dark matter detection or certain quantum computing operations.