Researchers in Germany have set a new record in a process that could make it possible to not only build super-powerful quantum computers, but to connect them to high-speed, high-capacity quantum networks.
Quantum computers store data based on the quantum state of individual atoms, which is a great way to move data through circuitry within one processor or motherboard. It doesn’t work at distances greater than that; the quantum coherence of atoms is easily disrupted by background noise, on top of the rush-hour-demolition-derby approach of packet-based networking.
Encoding data within the electromagnetic field of a beam of light—using the quantum state of photons rather than atoms—makes it easier to send quantum-encoded data through a fiberoptic network to a target that will decode it.
But that only works if the light doesn’t stop even for a nanosecond, despite the need to be switched, routed, gatewayed and tunneled—as it would have to be for any connection more complicated than a single uninterrupted link between two computers. In other words, making an optical quantum network “packet” travel through networking gear—where it has to be stored momentarily before being sent on its way—requires the ability to stop and store the light (or at least the photons) carrying the data.
That makes it necessary to build a “quantum repeater” that can store light data and emit the photons later with their quantum states intact, according to George Heinze, lead researcher of the team at the University of Darmstadt that finally broke the minute barrier this year—more than a decade after the first experiments that messed with the speed of light specifically to store data.
In 1999, Harvard researchers were able to slow light to just 38 miles per hour by spraying its energy into an array of densely packed, super-cooled atoms in a hard vacuum. In 2001, the same team stopped light altogether without destroying the photons or disturbing their quantum state using one laser to carry the energy, and another to store and release that energy onto a target made of super-cooled sodium atoms.
The photons only stopped for a few thousandths of a second; not nearly long enough for even fast networks. Pushing the storage time up beyond a second makes the process less an exercise in forcing light to hold still than it is finding a way to store the energy of photons coherently in a stable medium, just as storing the energy of electrons in capacitors or batteries is a more practical way to “stop” electricity.
Earlier this year, researchers at the Georgia Institute of Technology stopped a beam of light for 16 seconds, though they admitted that it could only be possible to build quantum information networks spanning continents if they could store light for timespans measured in minutes rather than seconds.
This month, researchers at the University of Darmstadt in Germany broke the minute barrier using a more stable medium—an opaque crystal made temporarily transparent by laser light rather than supercooled gas atoms held in place by electromagnetic fields.
Firing one laser at the crystal made it transparent to some frequencies of light and sent some of its atoms into quantum superposition—the state in which quantum particles appear to exist in more than one state simultaneously.
Using a second laser and a magnetic field, a research team led by George Heinze recorded within the crystal an image made up of three stripes. Then they shut off the first laser, making the crystal opaque again, “trapping” the energy of the light from the other laser inside the crystal until they turned the first laser back on, allowing the information to be read by the second laser.
“We showed you can imprint complex information on your light beam,” Heinze told New Scientist. Storing light data for a full minute stretched the capacity of the crystal in the experiment, but longer times should be possible with different crystals, he added.
As with all quantum experimentation, it’s anybody’s guess when the breakthrough in research might become a practical improvement in computing, but the length of time involved and stability of the medium are extremely promising.