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 . 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!) .
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?
A couple of posts ago Stephan and Yanick explained what a quantum computer is, how to make it and, most importantly, why we should bother to build one. If you have not read their posts yet I strongly suggest you to do so, since they are two really nice reads. But if I had to shrink them to a handful of words: 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, linking small quantum computers into a huge powerful one (a bit like cloud computing, but much stronger) . 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” . 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 excessively 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 “physically” travel the distance. Of course this does not come for free, we have to pay a price, and that price is entanglement.
Quantum mechanics predicts the possibility of a 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 were always of the same colour, 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 colour 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 . Now we have the tools to prove with experiments that the effect is real , 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, 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?
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 center (nitrogen-vacancy).
N-V centers 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 center as the processor of our node. We can read its state using lasers and manipulate it using microwave pulses. 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 center. 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 center. 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 center emit a photon that is entangled with the electronic spin (like the M&M’s). A second N-V center, in a second diamond, in a different place, does the same. Then, by making the two photon interact we can transfer their entanglement to the two electronic spins, entangling the nodes . This entanglement can then be used for quantum network applications.
We are living the second 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.
 History of the Internet. Wikipedia. https://en.wikipedia.org/wiki/History_of_the_Internet
 Internet of things. Wikipedia. https://en.wikipedia.org/wiki/Internet_of_things
 The quantum internet has arrived (and it hasn’t). D Castelvecchi. Nature 554, 289-292 (2018)
 Beam: the race to make the laser. J Hecht. Optics and photonics news 16.7, 24-29 (2015)
 Action at a distance. Wikipedia. https://en.wikipedia.org/wiki/Action_at_a_distance
 Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres. B Hensen et al. Nature 526, 682–686 (2015)
 Heralded entanglement between solid-state qubits separated by three meters. H Bernien et al. Nature 497, 86 (2013)
By Matteo Pompili, PhD student at the Hanson group at Delft University of Technology, Delft, The Netherlands.