GOTHENBURG, Sweden – The quantum mechanical principle that says a vacuum is not empty space but full of particles that fluctuate in and out of existence, has been observed for the first time as photons were coaxed to leave this virtual state and be captured as measurable light.
The 40-year-old principle, known as the dynamical Casimir effect (DCE), states that if virtual photons are allowed to bounce off a mirror moving at near light speeds, they will become real photons. Scientists at Chalmers University of Technology have achieved this effect – with some modifications to the method. Instead of varying the physical distance to a mirror, the scientists altered the electrical distance to an electrical short circuit that acts as a mirror for microwaves.
The “mirror” consists of a quantum electronic component called a Squid (superconducting quantum interference device), which is extremely sensitive to magnetic fields. By changing the direction of the magnetic field several billions of times a second, the scientists made the mirror vibrate at a speed of up to 25 percent of the speed of light. By transferring some of its kinetic energy to the virtual photons, the mirror helps them materialize.
Virtual photons bounce off a “mirror” that vibrates at near light speed. The round mirror in the picture is a symbol, and under that is the quantum electronic component that acts as a mirror. This makes real photons appear – in pairs – in a vacuum. Courtesy of Philip Krantz, Chalmers University of Technology.
This resulted in the photons appearing as pairs within the vacuum, and the pairs were measured in the form of microwave radiation. The scientists were able to establish that the radiation had the same properties quantum theory predicts for photons that appear in pairs in this way. The photons appeared because they lack mass and require very little energy to be excited out of their virtual state. This observation could also, in principle, create other particles from a vacuum, including protons and electrons, but it would require more energy to do so.
While the scientists think the photons might prove useful for quantum information and the development of quantum computers, the main value of the experiment is that it increases their understanding of basic physical concepts, such as vacuum fluctuations. Such fluctuations, they said, may have a connection with dark energy, which drives the accelerated expansion of the universe.
“Besides the DCE itself, this is one of the first experimental demonstrations of nonadiabatic (very fast) dynamics of the electromagnetic field, which is a potentially broader and more general field, which could find some applications,” said Christopher Wilson, a Chalmers scientist.
“The DCE and related effects are also relevant to understanding some effects in the cosmology of the early universe, black holes, etcetera. This could point the way for some ‘tabletop’ experiments that could simulate these more exotic systems.”
The work appeared in the Nov. 17, 2011, issue of Nature (doi: 10.1038/nature10561).