The speed of light is ... getting slower

Futuristic quantum computers using photons - light - instead of electrons came closer as two research efforts showed trapped light could store data.

NTT researchers in Japan slowed light down by having it enter a crystal with nano-scale holes inside it. While IBM researchers have diverted light beams into microscopic rings to form a delay line.

Both sets of research showed how light could be used store information in a photonic equivalent of RAM. In the IBM research, the light beams traveled through up to 100 rings or micro-ring resonators. These acted as delay loops and functioned as on-chip optical buffers.

The resonators were built using standard CMOS (complimentary metal oxide semiconductor) production tools. Silicon-on-insulator (SOI) sub-micrometre photonic wire waveguides were used in the rings.

The NTT researchers kept light confined in their crystal's nano-cavities for one nanosecond. The optical pulses were delayed for about 1.45 nanoseconds and their speed slowed down to 50,000 times less than light's speed in a vacuum. The individual cavities were shorter than 10 millionths of a metre.

Last year, a team at Harvard university worked on slowing light down by passing it through very cold structures. Professor Lene Hau believes that ultra-cold atoms termed Bose-Einstein condensates (BECs) can hold the amplitude and phase of a light pulse. An optical processor core could theoretically be built with BEC elements.

The IBM and NTT work points a way forward to storing data in buffers inside such a photonic computer. Work by Professor Lena Hau in 2003 showed that the speed of light could be slowed down to around 15 to 25mph and even to zero - for a few thousandths of a second - by effectively freezing it using a cloud of extremely cold sodium atoms.

This was achieved by "laser-cooling a cigar-shaped cloud of sodium atoms to one-billionth of a degree above absolute zero, the point where scientists believe no further cooling can occur. Using a powerful electromagnet, the researchers suspended the cloud in an ultra-high vacuum chamber, until it formed a frigid, swamp-like goop of atoms."

Such light-freezing technology could also form the basis for memory storage in photonic computers.