14 DAY TRIAL // Light pulses that in some ways travel faster than the speed of light can be produced artificially. Recently, scientists at the US National Institutes of Standards and Technology (NIST) have developed a new way of creating such superluminal pulses.
Details of their research appears in a paper entitled “Stimulated generation of superluminal light pulses via four-wave mixing,” which was published in the April 26 online issue of the esteemed scientific journal Physical Review Letters.
The technique developed at NIST is called four-wave mixing, and it works by altering some parts of each individual light pulse. This makes the light move forward faster than it normally would when traveling through a vacuum.
The speed comparison is usually conducted between light waves that travel without being altered, and photon pulses that are partially reshaped and altered. According to the research team, the achievement could be used to significantly improve the timing of communications signals.
Furthermore, the work could provide additional insight into the propagation of quantum correlations. The physicists explain that the new research does not violate Albert Einstein’s theory on general relativity – which states that the speed of light in a vacuum is the fastest achievable in the Universe.
They say that a sort of loophole exists in this theory. By careful tuning of the light source and advanced calculations, it is possible to nudge portions of the light pulses so that they arrive at their destination ahead or behind the main pulse.
“A short burst of light arrives as a sort of (usually) symmetric curve like a bell curve in statistics. The leading edge of that curve can't exceed the speed of light, but the main hump, the peak of the pulse, can be skewed forward or backward, arriving sooner or later than it normally would,” NIST scientists say.
By amplifying the leading edge of a light pulse and cutting off the back end, it is possible to create faster-than-light light. However, this method produces a lot of noise data, while providing only minimal improvements on the speed of light.
The approach the NIST team developed uses 200-nanosecond laser pulses called seeds, which are directed into a heated cell filled with atomic rubidium vapors. This “seed” laser pulse is then amplified and pumped out at a different wavelength.
The vapor then “amplifies the seed pulse and shifts its peak forward so that it becomes superluminal. At the same time, photons from the inserted beams interact with the vapor to generate a second pulse, called the "conjugate" because of its mathematical relationship to the seed,” the team adds.
With four-wave mixing, the NIST investigators produced laser pulses that arrived at their destination a full 50 nanoseconds faster than photons traveling through a vacuum.