Let There Be Light...In a Vacuum

The quantum mechanical principle which states that a vacuum is not empty space, but instead teeming with particles that flit in and out of existence, has been observed for the first time as photons were coaxed to leave this virtual state and captured as measurable light.

According to the 40-year-old principle, known as the dynamical Casimir effect, if virtual photons are allowed to bounce off a mirror moving at near light speeds, they will become real photons. Chalmers University of Technology scientist Christopher Wilson and co-workers achieved the desired result, with some modifications to the method.

“Since it’s not possible to get a mirror to move fast enough, we’ve developed another method for achieving the same effect,” said researcher Per Delsing.

In the experiments performed by the Chalmers scientists, 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 (referred to as a SQUID) that acts as a mirror. This makes real photons appear — in pairs — in a vacuum. (Image: Philip Krantz, Chalmers)

“Instead of varying the physical distance to a mirror, we've varied the electrical distance to an electrical short circuit that acts as a mirror for microwaves.”

The “mirror” consists of a quantum electronic component referred to as 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.

“The result was that photons appeared in pairs from the vacuum, which we were able to measure in the form of microwave radiation,” Delsing said. “We were also able to establish that the radiation had precisely the same properties that quantum theory says it should have when photons appear in pairs in this way.”

The photons appear because they lack mass, said researcher Göran Johansson, and require very little energy to be excited out of their virtual state. “In principle, one could also create other particles from vacuum, such as electrons or protons, but that would require a lot more energy,” he said.

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 flucuations. Such fluctuations, they said, may have a connection with dark energy, which drives the accelerated expansion of the universe.

The work was published this week in the journal Nature.

For more information, visit: www.chalmers.se