Compact Disks make Comeback: Memory could Exceed Petabytes (Patreon)

[This is a transcript with links to references.]

What do we want? Better memory.

When do we want it? Want what?

Joking aside, I have a two Terabyte hard disk and used to think that’s a lot. Then I bought a new video camera and it’s clogged my disk in no time. And yes there’s cloud storage but you know, it’s not actually in the clouds. They still store it on some thing down on earth. And besides that, it’d take me forever to upload.

So I was thrilled to see this recent new paper in Nature about a new optical storage method that could bring disk memory into the Petabyte range. Petabytes. That’s a thousand Terabytes. Let’s have a look.

The new technology is an old technology, it’s optical storage. Some of you may even be old enough to remember optical storage. We used to carry music around on those things called compact disks and let me just say you had to walk very carefully if you didn’t want to listen to the same sentence a thousand times.

Later we put movies on these things called DVDs.

We have a neighbour who’s hung her old DVDs up on the balcony to scare off the pigeons. True story.

These compact disks were usually made of some kind of plastic with a coating, which gave them their shiny appearance. They worked by using a laser to burn the information into the coating, and then read it out again. Actually, a pretty nice idea, quite straight forward.

Why did compact disks die? They were outcompeted by hard drives that can store information packed much more densely. The information density on compact disks or any optical storage is ultimately limited by the frequency of the laser light, for visible light that’s a few hundred nanometres. In modern flash drives the information is stored in little magnetizable cells that are just a few atoms in size and can be as small as 10 nanometers or so. They can store much more data in the same space, hence the death of compact disks.

But maybe they’ll soon make a comeback. Because in this new work a group of researchers from China figured out how to write data in multiple layers on a disk with lasers and read them out again.

They say this could work up to hundreds of layers, which would be a *huge increase in memory capacity. They say that a single disk of this type could store as much as a Petabit. Yes a bit isn’t the same as a byte, but it’s still an impressive improvement. They also envision that in data centres the disks could further be stacked into arrays that could each hold exabits. Sounds good but just exactly what did they do?

This new work is really a combination of several advances. Partly they made it work because they have a synthetic material that can be modified in a very targeted way by changing the way its polymers are linked. They can write at a certain depth in the material by focusing the laser on one particular spot.

They actually use two lasers, one that initiates a change of the molecular property of the disk, the second one that terminates it, leaving behind an altered region. This region can later be read out by causing it to emit light with yet another laser and that can then be detected. So, to write, you locally change the property of the material with lasers. To read, you cause it to emit light.

The other advancement that they make use of is that this two laser method also allows them to encode structures below the wave-length of the lasers. This also makes it possible to pack the data more densely.

I guess it also helps that they are working with femtosecond that prevents them from melting the entire disk to a pool of goo before they’re done working on it.

Writing to and reading data from  the disk actually works very nicely, as you can see in this video. What you see here is a scan through the layers. They have alternatingly encoded the acronyms of their institute and the university and you can see that they are pretty clearly readable.The distance between two layers is just 1 micrometre. So you could pack 1000 of them into a millimetre.

That said, there is some way to go from there to commercialization. First and most importantly when it comes to storing data it’s not just the density of the storage that matters, it also matters how quickly you can write the data and and read it out. They didn’t say anything about that, but I assume since this is a prototype it’s fairly slow. Then there’s the question how much energy that takes, and also, where do you get a femtosecond laser from.

Then again, you know, this entire area of technology is currently evolving so quickly, these challenges might be possible to overcome given a few years’ time.  So who knows maybe compact disks will make a comeback in the not too far future. And inevitably the day will come when we’ll all record our entire life in 8K, I can’t wait for it.