Quantum Breakthrough: Tiny Optical Device Slows Light

The speed of Light, denoted as ‘c’ in physics equations like Einstein’s famous E=mc², is approximately 186,282 miles per second, which equals over 6 trillion miles traveled in one year (i.e., one light year). It represents the limiting speed at which “information” may by transmitted. Being able to manipulate this speed may hold the key to new sensing and imaging possibilities.

In the past, physicists were able to slow light’s speed down to .1 mile per second — but only in an extremely cold temperature environment (just above absolute zero). Light was thus “frozen” for a few billionths of a second.

But this latest breakthrough attains an important benchmark: quantum engineers  haven now successfully slowed down light — at room temperature — to 155 miles per second, which is also the slowest speed for light ever achieved on a chip.

The effect was produced on a small computer chip (smaller than a nickel coin) and exploiting a form of atomic quantum state control called optical quantum interference. This brings physicists ever closer to controlling the fundamental interface of light and matter.

Using a clever arrangement of “hot” (excited) rubidium (Rb) atoms, a weak laser light was blocked from penetrating the Rb “capillary”, or channel. When a second such laser was sent through the channel, the configuration of Rb atoms was altered (their electrons “dropped down” an energy level) and became 44% more “transparent” — an effect known as electromagnetically induced transparency (EIT). However, the altered Rb atoms still exerted influence on the laser light; acting like atomic “speed bumps”, the Rb atoms caused the light beam to slow down (i.e., compress, “slinky”-like) by a factor of 1200 (see diagram, below).

A diagram of the chip, showing the channel
through which light travels to hit a capillary full of
Rubidium atoms. The capillary connects the two chambers
where the Rubidium atoms are stored. (Credit: Brigham
Young University).

This slowing of light is seven-fold slower than previous speeds achieved with photonic crystal waveguides. The engineers believe that even slower speeds will be possible using weaker laser beams (note: the laser used here was just slightly more energetic that a typical laser pointer).

The tiny light-slowing chip, smaller than
a coin, contains two chambers where Rubidium atoms
are stored. (Credit: Brigham Young University).

The breakthrough may open the door to new photonic and computational applications. Lead electrical engineer Holger Schmidt of the University of California, Santa Cruz, quoted in a recent Nature article*, described the experiments as “fantastic and inspiring”. Right now, Schmidt sees no practical applications.

I recently contacted Holger Schmidt and asked him about future applications of this technology.

PS: What specific innovations/applications for this can you foresee?

HS: Ultrasensitive sensors (e.g. optical or magnetic field sensors), atomic clocks, nonlinear optical devices (e.g. switches) that operate at ultralow energies on the few photon level.

*For more information (and cool graphs), check out the Nature Photonics paper abstract: Slow light on a chip via atomic quantum state control, by Wu et al.

Top Photo: Brews ohare; cc – by – sa 3.0

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