Researchers at the Technische Universität Darmstadt in Germany have managed to stop light for up to one minute inside a crystal and store digitally-encoded information inside it. The technique shatters previous records and could prove very useful in developing faster and highly-efficient quantum and optical computers.

The switch to photonics

Today's technologies use semiconductor-based processors and storage devices to compute and store data electronically. However, with data being increasingly transmitted in the optical regime (e.g. through fiber optics), switching to all-optical components is appearing more and more like an appealing prospect.

Photons are much harder to interact with and manipulate than electrons, but if this hurdle can be cleared, switching from electronics to a photonics ecosystem would bring about many key advantages.

Electronic components dissipate a large portion of energy as heat, whereas in photonics components losses are very limited. Optical storage and computation would also be resistant to radiation, have a much higher transmission bandwidth, and allow for the manipulation of a single photon at a time, which would translate into better performance at a tiny fraction of the energy cost.

"Although classical data storage already offers very high read/write rates, optical data storage has the potential to be much faster," Georg Heinze, who was part of the research team, told Gizmag. "If everything operates in the optical regime there is no need to convert optical pulses in electronic signals and vice versa."

Bringing light to a halt

The information is retained in the crystal for up to one minute (Image: TU Darmstadt)

To store data in a photonics device, the researchers use a technique known as electromagnetically induced transparency (EIT).

EIT consists of firing a laser "control beam" at a crystal which contains ions of the element praseodymium. The laser triggers a quantum reaction in the crystal that has two contemporary effects. First, it makes this normally opaque crystal transparent over a narrow spectrum; and secondly, it changes the refractive index of the crystal dramatically, slowing the incoming light pulse down to a complete halt.

At this point, the researchers fire a second laser beam (containing the information to be stored) at the crystal. At the precise moment in which the laser beam carrying the information is crossing the temporarily transparent crystal, the control beam is turned off, trapping light and information inside the now opaque crystal. The photons are converted into atomic spin excitations (or "spin waves"), which can be stored in the crystal until the control beam is fired again and the spin waves are turned back into light, which finally escapes the crystal.

Scientists have used this technique in previous experiments, but they could only use it to store data for a few millionths of a second. This is because the spin wave is very delicate, subject to energy fluctuations that can corrupt the information it encodes.

The TU Darmstadt team figured out how they could prolong this storage time significantly. Their approach was to develop an algorithm that senses the noise and applies magnetic fields and high-frequency pulses to the crystal to maintain the spin wave in its original state, so that the information is kept safe for as long as possible – up to a minute.

"In this first proof-of-principle experiment we focused on long storage times rather than on transfer rates," says Heinze. "However, we showed that by image storage it is possible to increase the storage capacity of the memory, and this is very important feature for spatially multiplexed quantum memories."

The researchers are now building on this result to try and store light for even longer periods of time (up to a week) in a more energy-efficient way while achieving higher data transfer rates.

In a word of caution, however, Heinze points out that it will probably be several years before this technology is ready for commercial use, and possibly decades before all-photonics devices become the norm in consumer devices.

The team's research appears on the journal Physical Review Letters.

Sources: TU Darmstadt, APS