A quantum internet provides radically new internet technologies that allow us to solve tasks that are impossible to accomplish on the classical internet. As with any radically new technology, we can not yet foresee all applications of a quantum internet, but it already has quite a number of exciting ones. For example, it allows us to do absolutely secure communication, secure identification, position verification, secure dedicated computing, and many others.
On a quantum internet, we don’t send classical bits, 0’s and 1’s, but we will transmit qubits. But otherwise, the basic elements of a quantum internet do not look so different from a classical one. The first element of a quantum internet is what we call an end node. An end node is basically your computer or laptop or phone that is attached to the internet and that you use in order to run applications. So you need the end node in order to use the quantum internet.
The name suggests, on a quantum internet we will not use normal laptops, cell phones, or computers, but instead, we will use quantum computers. These quantum computers actually don’t need to be very complicated. It turns out that most applications of a quantum internet only require these end-node quantum computers to be very simple and have less than 10 qubits. In fact, for most applications, they only need to have one qubit. The reason why we typically do not need many qubits is that a quantum internet draws its power from quantum entanglement.
Already one qubit at each endpoint is sufficient to have entanglement. In contrast to a quantum computer we always need more qubits than can be simulated on a classical computer in order to do something new and interesting. The next element of a quantum internet is that, similar to a classical internet, we have all kinds of elements that allow us to maximize the use of existing infrastructure. On a classical internet, not every computer on the internet has a direct fiber connection to every other computer on the internet.
Instead, fibers run through central points where there are switches that direct the bits in the right direction. If you want to build a quantum internet, then similar to a classical internet, you for example want switches that are capable of switching single qubits. Now ideally we would like to send qubits over very long distances; from any point on earth to any other point on earth. In order to achieve this, we will need something that is capable of sending qubits over long distances.
This requires a very special form of repeater called a “quantum repeater”. A quantum repeater works very differently than the classical repeater. In a separate article, you will learn all about quantum repeaters. When realizing a quantum internet, then just like on the classical internet, we will also need some control traffic. Basically, next to quantum communication, we will also use classical communication, for example, to direct the qubits to the right destination in the network.
This is what a quantum internet looks like. Now, I have already mentioned that a quantum internet allows us to solve tasks that are impossible to accomplish on the classical internet. Now the question is: what makes a quantum internet, or what makes the transmission of qubits so much more powerful than what we have today? Qubits have very special features. For example, they cannot be copied, making them ideal for security applications. Two qubits can also be in a very special state: namely an entangled state.
An entangled state between two qubits is the essence of the power of a quantum internet. In order to understand entanglement why entanglement is so useful, it is sufficient to understand two very fundamental properties of entanglement. So let me explain these two properties of entanglement and why they give power to a quantum internet. The first feature of entanglement is that it allows maximum coordination. So what does this mean? Two qubits can be entangled even at very long distances.
For example, I can have a qubit in Delft, which is entangled with a qubit very far away, for example in China. Now if I make a measurement on my qubit here in Delft and a friend of mine would make the same measurement in China, then it will turn out that we will always get the same outcome. You can think of a measurement as asking a question to a qubit. For example, I might ask the qubit: “Are you pointing left or are you pointing right?” Maximum coordination means that if I see the outcome left in Delft.
Then immediately/instantaneously, if my friend in China makes the same measurements qubit will also be pointing to the left. And if I see it pointing to the right then also in China it will be pointing to the right, even if this answer is not determined ahead of time. In fact, randomly we will get left-left or right-right, but the point is that the outcomes will always be the same. And the amazing thing about entanglement is that this is true for any measurement or any question we might ask.
If I were to ask the qubit: “Qubit, are you red or blue?” Then we would have always observed maximum coordination: red-red or blue-blue but never anything else. So the first feature of entanglements maximum coordination and it is this feature that makes entanglement so suitable for tasks that require synchronization or coordination. The second feature of entanglement is that it is inherently private. Because of course, you might be wondering given that qubits are so powerful allowing this instantaneous maximum coordination.
Wouldn’t it be great if many qubits could be entangled? Now it turns out that only 2 qubits can be maximally entangled with each other. So entanglement is inherently private. If I have a qubit here in Delft and the qubit that it’s entangled with is somewhere in China, then you can think of this entanglement as a private connection that nothing else can have part of. It is not possible for any other qubit anywhere, to have any share of this entanglement between the Delft qubit and the qubit in China.