A quantum network is very similar to the kind of network, i.e. the internet, we use to send and receive digital information, from sharing Facebook posts to uploading videos to TikTok. Like the most famous network of all time, the internet, quantum networks are composed of interconnected computers.

How are quantum networks different from the internet?

Digital information sent via the vast global internet network, connecting billions of computers, is processed and transmitted only as zeros (0) and ones (1), known as classical computer bits. Quantum networks, however, use quantum bits (qubits). Qubits carry information in an entirely different way compared to classical bits because of their quirky quantum nature.

Quirky concept: Superposition

Unlike classical bits which can only be either 0 or 1, qubits can be in multiple states at the same time – a principle known as superposition. This can be thought of in terms of a coin: if tails is 0 and heads is 1, then a spinning coin offering limitless possible answers in between 0 and 1 is exhibiting superposition.

Quirky concept: Entanglement

Another feature that sets qubits apart from classical bits, is entanglement. Simply put, entanglement can be thought of as a strong connection between two qubits. If you measure the state of one, you can determine the state of the other, even if they are far apart, e.g., one in New York and one in Delft. 

Quirky concept: Quantum measurement

Qubits are also special in the sense that measuring them changes their state. While this property has its benefits, such as exposing eavesdropping attempts, it also requires very special hardware to enable qubit handling.

Handling and transmitting qubits: components of a quantum network

These distinct quirky concepts mean that most of the physical elements composing a quantum network need to be fundamentally different from the conventional hardware used for the current classical internet. A quantum network is made up of the following main elements:

• Physical connections: standard telecom fibres (fibre-optic channels) can be used. These channels are, however, inherently very lossy – so transmitted qubits can cover only short distances before their quantum information is lost. A quantum channel covering large distances, therefore, requires quantum repeaters.
• Quantum repeaters: these repeat and improve the quality of the signal. A quantum repeater is necessary for establishing long-distance communications on a quantum network. Quantum signals cannot be amplified using classical repeaters and amplifiers, since any qubit manipulation (measurement) changes the qubit’s state, and thus the information it is carrying. Researchers at QuTech are working on developing quantum repeaters that employ quantum teleportation and so-called entanglement swapping, to repeat and amplify the signal without changing the quantum information.
• End nodes: these are quantum computers (processors) for manipulating quantum information, for example preparing and measuring the qubits, establishing entanglement, applying quantum logic gates, and performing error correction.

The quantum network infrastructure also includes switches and conversion elements that allow qubits to be transmitted over existing telecom cables.

A quantum internet will additionally require specialised software to control hardware, manage quantum-internet traffic, and run applications on connected quantum computers.

What will quantum networks do?

A quantum internet promises improvements that are impossible to achieve with current technology – all the way from security, healthcare, agriculture, water, and food supply through to energy, sustainability, mobility, and logistics.