Time shifts with faster-than-light photons

Marie Freebody, marie.freebody@photonics.com

Experiments with faster-than-light photons are highlighting the weird world of quantum tunneling. Researchers at the Joint Quantum Institute (JQI) have boosted single photons to seemingly faster-than-light speeds through a stack of materials by adding a single, strategically placed layer.

“For the first time, we experimentally show a strange effect when an increase of the overall structure length leads to a decrease in traversal time,” said Natalia Borjemscaia, a researcher at JQI, a collaboration of the National Institute of Standards and Technology and the University of Maryland. “Our work offers insight into developing nanostructures with tailored optical properties, particularly with regards to engineering temporal delays and dispersion.”

Natalia Borjemscaia inspects the dielectric stack structure stage. Each sample contains bare regions used for reference and four regions with different combinations of starting and ending dielectric stack layers deposited on top of the substrate.

Intuition would have us believe that light achieves its maximum speed in a vacuum and slows down appreciably when it travels through materials such as glass or water. We would expect the same to be true for light traveling through a stack of dielectric materials, but this is not necessarily so. A quantum object or particle – such as a photon of light – can appear to traverse barriers of one thickness in less time than it would take to traverse a barrier that is less thick.

“We investigate how subtle rearrangements of quarter-wave optical stack layers affect the traversal times of single photons,” Borjemscaia said. “We show that slight rearrangements of layers can significantly change the traversal time – by many times what one would expect from the change in thickness of the structure alone.”

In the experiment, which was published in the February 2010 issue of Optics Express, a pair of photons is generated using parametric down-conversion, a process in which one photon from the pump laser is converted into two identical ones of lower energy with twice the wavelength of the pump.

One photon is sent through one of four sample stacks made with alternating layers of two materials with different refractive indices: high (H) and low (L). The other photon is sent through a calibrated delay line.

The JQI team found that traversal times strongly depend on which layer terminates the structure; in particular, when the group added an H layer to a structure that initially terminated with two L layers (oneon each end), the structure’s thickness increased and the traversal time significantly decreased.

This seemingly superluminal, or faster-than-light, speed can be explained by the wave properties of light. When a photon hits a boundary between the layers of a material, it creates waves at each surface. The waves interfere with each other so that very little light makes it out of the other side of the stack of layers but, provided that the H and L layers are arranged in just the right way, the photons that do make it to the other side emerge early.

This is the experimental setup used to determine propagation delays of photons traversing dielectric stack structures. Courtesy of the Joint Quantum Institute.

Now that Borjemscaia and colleagues have demonstrated the temporal effects with dielectric stacks, which was one system suggested as a model of the quantum barrier, they hope to continue their experiments with a different optical barrier.

“We are actively pursuing measurements of traversal times through another optical ‘analogue’ of the quantum mechanical potential barrier – a narrow gap between two blocks of glass,” Borjemscaia said. “This arrangement is one that better approximates the quantum mechanical barrier and will fill an experimental void in the direct measurement of tunneling times.”