Pushing the speed limits of quantum memory

It feels like quantum computers have barely been invented, and scientists are already testing how extensible the current technology is. A paper published in Nature Photonics this week describes how researchers are beginning to push the bandwidth limits of quantum memory. Using photon pulses and cesium vapor has provided bandwidths on par with broadband connections, rates 100 times those of other quantum memory systems currently being tested. However, the system's efficiency is still very low, and advances will have to be made in other fields before it can be improved.

Since many quantum computing implementations operate on photons, a quantum memory that doesn’t involve converting photons into other media, like electrical pulses, would be ideal. Unfortunately, current photon-based media suffers from problems with storage time, retrieval efficiency, and bandwidth. The paper tackles the last issue, as current quantum systems are limited to a data rate of a few megahertz at most.

To create more bandwidth, scientists used a bunch of atoms instead of one or two. A cloud of hot cesium vapor served as the storage medium. Once the vapor had been prepared in its ground state, scientists sent a "write" pulse into the vapor along with a signal pulse that contained the information to be stored. The vapor converted these two pulses into a collective atomic excitation, called a spin wave. To get the information back out, scientists sent a "read" pulse that converted the spin wave back into a signal that was read by a detector.

With this method, scientists found that the bandwidth of the signals they could process was a bit better than 1GHz. This transmission was actually limited by the output detector, not the signal itself—theoretically, this style of storage should be capable of even larger bandwidths. The system also had excellent coherence, holding up well when subjected to interference.

However, the setup didn’t retain the information that well. While scientists found that they could increase the retention by sending more energetic read and write pulses, the total efficiency of the system topped out at 15 percent: the cesium vapor was able to store 30 percent of the incident signals, and they could only retrieve half of what was stored.

Researchers speculate that the overall efficiency of the process was poor because the signal pulse may not be optimal. Changing any of a number of its attributes, such as the shape and frequency, could lead to much higher storage and retrieval rates. However, shaping pulses with frequencies less than a nanosecond is itself a developing technology.

While the experiment did push a few boundaries, it seemed to hit limits already established by other technologies—limits that will have to be pushed themselves before much more progress can be made. The researchers noted that the applications of this particular kind of storage are broad, and might work with cold gases and solid-state systems in addition to the hot vapor they used. However it will eventually be accomplished, the baton pass from quantum memory conception to development seems to be underway.

Source: ars technica

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