How do quantum computers actually work?

Quantum computers and classical computers work in very different ways. The latter use binary codes as the basis for processing information: the smallest unit is a bit, which can assume the state 1 (on or true) or the state 0 (off or false). Quantum computers, on the other hand, do not work with binary yes or no logic, but with a system in which a yes AND a no is also possible. The basis and smallest unit of information here is a qubit, which can assume the entire spectrum of values between 0 and 1 – and at the same time.

Superposition and entanglement

This is possible because quantum computers work on the basis of quantum physical states. Two essential quantum mechanical basic concepts are used here – the so-called superposition and the entanglement of states. The principle of superposition allows qubits to be in a superposition of states, which can thus represent all possible solutions to a problem. Entanglement, in turn, ensures that quantum computers can work much more efficiently than classical computers: When a single qubit is manipulated, the state of all other qubits is also changed by the entanglement. Instead of having to execute all the computing steps one after the other, quantum computers can therefore execute all the computing steps in parallel, which enables a massive increase in speed.

How does this work exactly?

In classical computers, logical operations such as the functions “AND”, “OR” or “NOT” are implemented in so-called logic gates, which are physically implemented in transistors. The desired information is passed through the component as an electrical signal. Quantum computers also have such logic switches, but the practical implementation is much more complicated and not comparable to that of classical computers: The manipulation of tiny physical particles with lasers or magnetic fields requires highly complex technological platforms. Depending on their design, quantum computers require, for example, sources and detectors for individual photons, vacuum and cryogenic systems or ion traps.

 
Reading results is also much more difficult than with classical computers – because every reading of a result destroys – according to the laws of quantum mechanics – the superposition state of the qubit. It is therefore essential to read out the information only when a calculation has been completed.

Quantum computers are also not superior to classical computers in all other areas. Because they react very sensitively to external disturbances, it is often necessary to carry out elaborate error corrections or to discard results. As long as such computers are not completely sealed off from interference, it is therefore also difficult to carry out extensive computing tasks with them.

What are the possible applications for quantum computers?

Quantum computers are particularly well suited for optimisation tasks and searching in disordered data sets, for example. A quantum computer finds the optimum for a problem in just one run, whereas classical computers first have to calculate all the possibilities one by one and only at the end can the optimum solution be determined by comparing the results. Conversely, a quantum computer will probably never replace a pocket calculator – because the simple addition or multiplication of numbers is much easier with a device that is less susceptible to interference.

Applications for quantum computers can be found in the field of cyber security, for example. The use of high-performance quantum computers harbours an enormous risk potential for conventional encryption systems. Conversely, quantum mechanics enables tap-proof cryptographic systems that cannot be undermined even by the use of quantum computers. In addition to use in optimising logistical problems or finding new chemical compounds, applications in biology or optimisation tasks in the financial sector are also possible. In the meantime, there are already projects dealing with the question of how humanoid robots can be made more human through the use of quantum computers.