**Quantum Internet – it sounds like a typically made-up scientific term used in some movies, which ignores all laws of nature in the process to further the plot. In the case of quantum internet however, this idea seems far from fiction. Scientists are already working on building a quantum internet in multiple countries. In the EU, United States and China, scientists are already busy developing a quantum internet, with the US Department of Energy already publishing the first blueprint of its kind in July of 2020. But what is so special about quantum internet?**

To understand how quantum internet would work, you first need an understanding of how the “classic internet” works. The traditional internet makes use of bits to code its data, which consists solely of zeros and ones. An image for example can then be saved as a long series of zeros and ones. The time it takes to read these bits is still growing progressively. However, the further we progress to faster, smaller circuits, the closer we get to the physical limits of the materials used. The classical laws of physics limit the speed of the traditional internet and that is where quantum internet steps in.

**Qubits**

The concept of quantum internet is based on quantum theory. Instead of using bits, quantum internet uses quantumbits or qubits. These qubits consist of elemental particles such as photons or electrons. Just like with bits, these qubits are used to form a series of zeros and ones. The difference here is that qubits make use of a special property of these elemental particles. According to quantum law, these elemental particles are what is called superposed: they can be a zero and a one simultaneously. The paradox here is that measuring a qubit means that it is assigned a state. This is due to what is called the Observers Effect. After measuring a qubit, it turns into either a zero or a one, just like the classical bits. This phenomenon is called superposition and is an integral part of quantum mechanics. Unfortunately, qubits cannot be used for everyday applications such as WhatsApp, Netflix or e-mails, so the ‘classic’ internet will remain. However, the strange behaviour of qubits allows more opportunities for other interesting applications.

One of the main advantages of quantum internet is the improved security of sending data with qubits. With classical bits, data is secured by encrypting the message and sending a secret key to the recipient. The recipient can then decode the message using this secret key.

**Quantum Key Distribution**

For most communication today an algorithm exists to create secret keys which are difficult, but not impossible, for hackers to break. Here is where qubits present a solution to this problem. We can encrypt the secret key onto a series of qubits and then sent it to the recipient. This process is called Quantum Key Distribution (QKB). When someone intercepts the message and tries to read it, the state of the qubits will be changed since they will have to measure the qubits. Measuring the qubits will cause the state of the qubits to collapse to zero or one. This collapse informs both parties that someone is trying to eavesdrop on them, allowing them to stop their communication.

QKD is still in its very early stages of development. Right now, optic-fibre cables are most commonly used to send qubits in one direction. But qubits can easily get lost or scattered in these cables and struggle to travel long distances, with the current range limited to around hundreds of kilometers. Luckily, there might be another useful property of qubits to use for QKD: entanglement.

When two qubits interact with each other, they can become entangled. The properties of entangled qubits can be dependent of each other. Thus, one change in one of the qubits will translate in a change in the other qubit as well, even if they are physically separated from one another. This allows you to read the state of one qubit by looking at its entangled counterpart, even when they are separated from each other. In theory entanglement could then ‘teleport’ information from one qubit to another without the need of a physical channel between the two qubits.

**Looking forward**

Unfortunately, there are also some engineering challenges to use entanglement on a huge scale. We first need to develop the necessary technology that allows us to entangle qubits and send them to both sender and receiver. Distributing the entangled qubits is also possible with optic-fibre cables, but after a distance of around a hundred kilometers they become separated. Another option to distribute entangled qubits over longer distances is via satellites. Covering the planet with such quantum satellites however is very expensive.

If we can overcome these challenges, quantum internet will be a very useful tool to improve the speed and security of our communication. It will especially be very useful for large organizations like banks and health services for who it is important to safeguard their data. Besides allowing a more secure form of communication, qubits could also be used to solve more complex problems our current bit system is not able to. A classical bit can only store 4 numbers when using just 2 bits (00, 01, 10 and 11), while qubits can store all four numbers simultaneously. When we apply the same reasoning to a series of multiple qubits, the capacity of a quantum computer increases exponentially. Quantum computers can then also be used in other fields to solve complex problems.

Already experiments have succeeded in connecting two endpoints with each other. The next stage is the upscaling of the infrastructure: creating a network of multiple senders and receivers who can exchange data over larger distances. Creating such a network will without a doubt bring along even more obstacles. Quantum internet will most certainly be a long-term project, but the applications that it will bring will undoubtably be a huge benefit to society.