Four-Wave Mixing Generates Superluminal Pulses

A novel four-wave mixing technique that restructures parts of light pulses to travel faster than the speed of light could improve the timing of communications signals and help examine the propagation of quantum correlations.

Einstein’s theory of relativity states that light passing within a vacuum represents the universal speed of light. A short burst of light emerges as a type of symmetric curve. The curve’s leading edge cannot surpass the speed of light, but the peak of the pulse can be altered forward and backward.

In four-wave mixing, researchers send “seed” pulses of laser light into a heated cell containing atomic rubidium vapor along with a separate “pump” beam at a different frequency. The vapor amplifies the seed pulse and shifts its peak forward, making it superluminal. At the same time, photons from the inserted beams interact with the vapor to generate a second pulse called the “conjugate.” Its peak, too, can travel faster or slower, depending on how the laser is tuned and on the conditions inside the gain medium. (Image: NIST)

Recent experiments demonstrated that by increasing the leading edge of the pulse and by cutting the back end, “uninformed” faster-than-light pulses with increased noise are generated. However, four-wave mixing produces less noisy, cleaner and more rapid pulses by rearranging or re-phasing the pulse-generating lightwaves.

In the four-wave mixing technique developed by scientists at the National Institute of Standards and Technology (NIST), laser light “seed” pulses up to 200-ns long are introduced into a heated cell containing atomic rubidium vapor and a separate pump beam at a different frequency from the seed pulses. The seed pulse is amplified by the vapor, shifting its peak forward so that it becomes superluminal. The photons from the inserted beam interact with the vapor to generate a second pulse, called the “conjugate” because of its mathematical relationship to the seed. The speed of the peak is based on the conditions inside of the laser and how it is tuned.

The NIST experiment yielded pulse peaks that arrived 50 ns faster than light traveling through a vacuum.

The team is now looking to use its method to study quantum discord, which mathematically defines the quantum information between two correlated systems such as the conjugate and seed pulses. The group hopes to determine how useful this light could be to transmit and process quantum information.

For more information, visit: www.nist.gov