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One of the enabling technologies for a quantum internet is the ability to store and retrieve quantum information in a reliable and repeatable way. 

One of the more promising ways to do this involves photons and tiny clouds of rubidium gas. Rubidium atoms have an interesting property in that a magnetic field causes their electronic energy levels to split, creating a multitude of new levels. Switching the field off, returns the atoms to their normal state.

So one way to store photons, and the quantum information they carry, is to send them into a cloud of rubidium atoms and switch on the magnetic field.  If the photons have a wavelength that is absorbed by the new electronic levels in the gas, they become trapped within it. 

As long as the field remains on, that is. Switch the field off and the atoms are forced to emit the photons allowing the information they hold to be retrieved.

That immediately suggests a way of building a quantum memory.

Indeed various teams have spent the last few years testing this technique and others related to it. The results have been impressive. They can store not just single photons but entire images which they send into the gas by placing an image mask over the beam. 

The storage lasts for tens of microseconds and the images can be retrieved with accuracies approaching 90 per cent. (The storage duration is limited by the movement of the atoms in the gas which blurs the images over time.)

Today, Quentin Glorieux and pals at the National Institute of Standards and Technology in Maryland go a step further. These guys have used exactly this technique to store two images at the same time. That’s clearly a very short movie but the important point is that it’s a proof-of-principle demonstration of the technique. 

The images are the letter T and the letter N and the sequence of pictures above shows the images being released from the gas, as recorded by a high speed camera in 100 nanosecond frames. “We have demonstrated that multiple images can be stored and retrieved at different times, allowing the storage of a short movie in an atomic memory,” say Glorieux and co. 

Interestingly, the images are released on a “last in, first out” basis, so this movie is running backwards.

That’s an impressive feat. Until now, sequences of images have only ever been stored at the same time in solid state media, such as holographic memories.These seem to have impressive potential as quantum memory devices.  

But it looks as if rubidium gas clouds will give holograms a run for their money in this race. 

Ref: Temporally Multiplexed Storage of Images in a Gradient Echo Memory

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