Stacking the deck: Single photons observed at seemingly faster-than-light speeds

Jan 26, 2010
At the boundaries between layers, the photon creates waves interfering with each other, affecting its transit time. Credit: JQI

Researchers at the Joint Quantum Institute (JQI), a collaboration of the National Institute of Standards and Technology and the University of Maryland at College Park, can speed up photons (particles of light) to seemingly faster-than-light speeds through a stack of materials by adding a single, strategically placed layer. This experimental demonstration confirms intriguing quantum-physics predictions that light's transit time through complex multilayered materials need not depend on thickness, as it does for simple materials such as glass, but rather on the order in which the layers are stacked. This is the first published study of this dependence with single photons.

Strictly speaking, light always achieves its maximum speed in a vacuum, or empty space, and slows down appreciably when it travels through a material substance, such as glass or water. The same is true for light traveling through a stack of dielectric materials, which are electrically insulating and can be used to create highly reflective structures that are often used as optical coatings on mirrors or .

In a follow up to earlier experimental measurements (see "A Sub-femtosecond Stop Watch for 'Photon Finish' Races"), the JQI researchers created stacks of approximately 30 dielectric layers, each about 80 nanometers thick, equivalent to about a quarter of a of the light traveling through it. The layers alternated between high (H) and low (L) refractive index material, which cause to bend or reflect by varying amounts. After a single photon hits the boundary between the H and L layers, it has a chance of being reflected or passing through.

When encountering a stack of 30 layers alternating between L and H, the rare photons that completely penetrate the stack pass through in about 12.84 femtoseconds (fs, quadrillionths of a second). Adding a single low-index layer to the end of this stack disproportionately increased the photon transit time by 3.52 fs to about 16.36 fs. (The transit time through this added layer would be only about 0.58 fs, if it depended only upon the layer's thickness and .) On the contrary, adding an extra H layer to a stack of 30 layers alternating between H and L would reduce the transit time to about 5.34 fs, so that individual photons seem to emerge through the 2.6-micron-thick stack at superluminal (faster-than-light) speeds.

A single photon travels through alternating layers of low (blue) and high (green) refractive index material more slowly (top) or quickly (bottom) depending upon the order of the layers. A strategically placed additional layer (bottom) can dramatically reduce photon transit time. Credit: JQI

What the JQI researchers are seeing can be explained by the wave properties of light. In this experiment, the light begins and ends its existence acting as a particle—a . But when one of these photons hits a boundary between the layers of material, it creates waves at each surface, and the traveling light waves interfere with each other just as opposing ocean waves cause a riptide at the beach. With the H and L layers arranged just right, the interfering light waves combine to give rise to transmitted photons that emerge early. No faster than light speed information transfer occurs because, in actuality, it is something of an illusion: only a small proportion of photons make it through the stack, and if all the initial photons were detected, the detectors would record photons over a normal distribution of times.

Explore further: This Slinky lookalike 'hyperlens' helps us see tiny objects

More information: N. Borjemscaia, S.V. Polyakov, P.D. Lett and A. Migdall, Single-photon propagation through dielectric bandgaps, Optics Express, published online Jan. 21, 2010, doi:10.1364/OE.18.002279

Related Stories

Mass weddings -- NIST's new efficient 2-photon source

Apr 12, 2007

For a variety of applications in physics and technology, ranging from quantum information theory to telecommunications, it’s handy to have access to pairs of photons created simultaneously, with a chosen ...

'Dead time' limits quantum cryptography speeds

Sep 28, 2007

Quantum cryptography is potentially the most secure method of sending encrypted information, but does it have a speed limit" According to a new paper by researchers at the National Institute of Standards and Technology and ...

Observing a Photon no Longer a Seek-and-Destroy Mission

Jun 02, 2004

A team of University of Queensland, Australia physicists has devised a sophisticated measurement system for single particles of light, or photons, enabling them to investigate fascinating behaviour in the quantum world. ...

Recommended for you

Gamma ray camera may help with Fukushima decontamination

May 20, 2015

Japanese researchers have significantly improved the performance of a gamma ray-imaging "Compton" camera. The new technology has potential applications in scientific research, medical treatment and environmental ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

3 / 5 (2) Jan 27, 2010
another quantum shenanigan. what i want to know is that if it's possible to alter the temporal distribution of what we observe as the quantum existance of light, can give is us insightt into any other non-standard quantum distributions of what we observe as mass.

---if there are fluctuations in radiation, should there also be fluctiations in mass. or more specifically, in gravity and magnetic field.

not rated yet Jan 27, 2010
Fluctuations in mass ? Sure, due virtual particles. Feynman mapped that can of virtual worms...

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.