The computing units of quantum computers are called quantum bits, or qubits for short. They are the counterpart to the bits in conventional computers. However, a computer bit can only have the value one or zero; it’s like an on/off switch. A qubit can have both values simultaneously. This is because the information carriers are atoms, electrons, or other objects from the microcosm, and these obey the complex rules of quantum physics.

Two phenomena of quantum physics are used for quantum computers: superposition and entanglement. Yet these quantum states are very fragile. They must be protected from external sources of disturbance such as heat or radiation; otherwise, they will break down within a short time. This leads to errors in calculations.

Quantum objects have the properties of waves, which means that they can overlap themselves. Physicists call this state “being in superposition”. As a result, quantum objects can have an unambiguous state or multiple states simultaneously. A qubit can therefore have the values zero and one at the same time, but also any values between zero and one.

Quantum objects can also become entangled, meaning that they interact with each other. If one qubit is influenced, the same effect will be produced on its entangled partner. In this way, a larger overall system – a huge information carrier – can be formed from many entangled qubits. However, the more qubits are connected, the more fragile their common quantum state becomes.

In order to perform a calculation with many qubits, they must be in superposition and entangled – in other words, in a quantum coherent state. By coupling the two phenomena, quantum computers are able to perform many computational operations simultaneously with each switching operation.

These qubits are generated from currents that flow without resistance in circuits made of superconducting materials. This is currently regarded as the leading approach in the field, and is the one chosen by Google and IBM, for example. However, it is still unclear whether these qubits can be used to develop computers with thousands of qubits.

This qubit type consists of multiple semiconducting layers. Due to the special layer structure, a plane exists on which electrons move back and forth without being able to leave the plane. Quantum chips with semiconductor qubits are particularly closely related to the chips used in microelectronics today. The extensive experience that has already been accumulated in microchip production could help to develop quantum chips with a very large number of semiconductor qubits.

In ion traps, individual ions are strung together in a vacuum by means of electromagnetic fields – like a string of pearls.

This qubit type consists of two quantum materials. They are considered to be significantly more stable than semiconductor or superconducting qubits, which are sensitive and therefore prone to errors. However, research into these qubits is still in its infancy. Recently, Jülich researchers were the first to succeed in integrating a topological insulator into a conventional superconducting qubit.