27.09.2018difficulty level - Q

A Quantum Internet made of Diamonds

How cool is that? A Quantum Internet. Made of Diamonds.

by Matteo Pompili

We are constantly connected to Internet. With our computers, our smartphones, our cars, our fridges (mine is not, yet, but you get the idea). In its very first days, the Internet was a very rudimentary, yet revolutionary, connection between computers [1]. It enabled one computer on the network to send messages to any other computer on the network, whether it was directly connected to it (that is, with a cable) or not. Some of the computers on the network acted as routing nodes for the information, so that it could get directed toward the destination. In 1969 there were four nodes on the then-called ARPANET. By 1973 there were ten times as many. In 1981 the number of connected computers was more than 200. Last year the number of devices capable of connecting to Internet was 8.4 billion (with a b!) [2].
Computers on their own are already great, but there is a whole range of applications that, without a network infrastructure, would be inaccessible. Do you see where I am going?

“Internet Map” by Chris Harrison. The brighter the edges, the higher the number of connections.

If you are not new to this blog you will probably already know what a quantum computer is, and in the last post Stephan showed how we could make one. If I had to shrink to a handful of words why we want to make them and how: quantum computers will exploit the weird laws of micro(nano!)scopical objects to solve some problems way faster than any future normal computer. They will do so by encoding the information in quantum systems, which will therefore be quantum information. A Quantum Internet is a network capable of routing this quantum information between quantum computers. We can already foresee some nice application for this quantum network, like establishing an inviolable secure communication link between any two nodes, connecting far-apart telescopes to take ultra-sharp images of stars and galaxies, or linking small quantum computers into a huge powerful one (a bit like cloud computing, but much stronger) [3]. In all likelihood the best way to use these new technologies will come over time, with applications we cannot anticipate. For example, in the early 1960s the invention of the laser was welcomed as “a solution looking for a problem” [4]. Now we have lasers everywhere! I think that the same thing will happen for quantum computers and their internet.

One of the quirks of quantum information is that it cannot be copied. When you try to copy it, you irreversibly destroy the original information. This is, under the hood, what makes quantum communication so secure; but on the other hand, it could also make sharing quantum information really hard! If we had to rely on a single quantum system (a photon for example, a particle of light) to travel undisturbed across the globe, we could as well stop this now.
Fortunately quantum mechanics offers us a solution to this problem: teleportation. It’s not like Star Trek, we can’t teleport people, but we can teleport quantum information. We can transfer the information stored in a single atom in Amsterdam to an electron in London, without reading the atom, without knowing what the information is and, crucially, without making it travel the distance “physically”. Of course this does not come for free; we have to pay a price and that price is entanglement.

“Quantum Teleportation” by xkcd.

Quantum mechanics predicts the possibility of this rather weird phenomenon. If you take two separate quantum objects, say two nanometer-sized M&M’s, and you make them interact in some particular way, you can make them entangled: the two M&M’s lose their individuality and can only be described as one of the parts of a two M&M’s system. Let’s say that our nano-M&M’s can only be of either two colours: red and blue. A non-entangled scenario would be, for example, if the first was red and the second blue. Each M&M’s has its own colour, its own identity. Now, let’s take the two M&M’s (one red and one blue), shuffle them a little a bit, just to lose track of which one is which, and then send them, one to you and one to me. When you observe the colour of your M&M’s, you immediately get to know also the colour of mine! While this is interesting, this is not quantum entanglement. This is called correlation and it is not quantum at all. The two M&M’s had their own colour the whole time, we simply didn’t know what it was.
When you entangle the two M&M’s, they actually lose their own colour! You can make an entangled two-M&M’s system, in which you know that the M&M’s will have different colours when you will observe them, but until then their colour will not be assigned. This effect is so weird that even great scientists believed it was too weird to be true [5]. Now we have the tools to prove with experiments that the effect is indeed real [6], and we can exploit it to build new technologies, such as a quantum internet. The idea is to share entangled objects between the nodes of the network and protect their entanglement from the noisy non-quantum environment in which we live, such that when we need to send a quantum bit of information, we can spend the connection of the entangled objects to teleport the (qu)bit. But how can we share entanglement on this network?

Illustration of the difference between classic correlation and entanglement.

This is where our diamonds finally find a place. Diamonds are crystals made of carbon atoms arranged in a very compact way. Sometimes the crystal may have some defects, like an intruder atom (say nitrogen) or a missing carbon atom (what we call a vacancy). If we are lucky enough, these two defects happen one next to the other. Such a system is called an N-V centre (nitrogen-vacancy).
N-V centres are one of the most promising candidates to act as nodes of a quantum network.
A node is made of three ingredients: a processor to handle information, a memory to temporary store it and a link to the other nodes.

We use the spin of a pair of electrons localized around the N-V centre as the processor of our node. We can read its state using lasers and manipulate it using microwave signals. The information stored in this spin has a short lifetime: the system loses memory of the information we store in it too quickly to use it as a reliable memory. Luckily enough, Nature provides us with a strong quantum memory not far apart from the N-V centre. About 99% of the carbon atoms in nature are C12 which is spin-less; it does not have a spin. Most of the remaining carbons are C13, which has a spin (due to the additional neutron in the nucleus). We can talk to these C13 atoms in the diamond thanks to their spin-spin interaction with our electrons in the N-V centre. Since the C13 spin “feels” the electronic spin, we can manipulate the latter to perform operations on the first.
The last ingredient of the node is the ability to link it to other nodes. We do this by making the N-V centre emit a photon that is entangled with the electronic spin (like the M&M’s). A second N-V centre in a second diamond, in a different place, does the same. Then, by making the two photons interact, we can transfer the entanglement that they have with the electronic spins (photon-electron entanglement), to entanglement between the two electronic spins [7]. This entanglement can then be used for quantum network applications.

Left: Illustration of the lattice of a diamond with an N-V centre jameshedberg.com. Right: Scanning electron microscope image of one our diamonds. In each of these domes there is an NV-centre. The electrical connections are used to deliver microwave pulses and other signals.

We are living a new quantum revolution. We will not be just spectators of quantum mechanics, we will use it as a technology. In a couple of decades everybody will have access to quantum computers connected through a quantum internet, to design drugs, to optimize airports, to play videogames and who knows for what else. Aren’t you excited? I certainly am.


About Matteo Pompili

Matteo Pompili

Matteo is from Rome and he shoots lasers at diamonds. He loves dogs and guitars. And lasers. And dogs.


[1] History of the Internet, Wikipedia.

[2] Internet of things, Wikipedia.

[3] The quantum internet has arrived (and it hasn’t), D Castelvecchi, Nature 554, 289-292 (2018)

[4] Beam: the race to make the laser, J Hecht, Optics and photonics news 16 (7), 24-29 (2015)

[5] Action at a distance, Wikipedia.

[6] Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres, B Hensen et al. Nature 526, 682-686 (2015)

[7] Heralded entanglement between solid-state qubits separated by three meters, H Bernien et al. Nature 497, 86-90 (2013)

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