The digital world of classical computers – as we know it today – can basically be represented by nothing other than a sequence of binary states, ones and zeros, on and off. The state of these so-called bits is always unique, the light is always on or off. A bit can never have two states at the same time.
Quantum computers are also based on the states “1” and “0”, but these are encoded with quantum mechanical properties of atoms, electrons or even photons. These are then called QuBits (quantum bits). Now in quantum mechanics a QuBit can take not only absolute values of one and zero, but also mixed forms in the two-dimensional space. This superposition is called superposition.
However, these QuBits are also extremely sensitive information states, which have to be preserved for a certain period of time with a currently high technical effort. At the present time (spring 2021), this means maintaining a quantity of up to 100 QuBits over a time span of seconds or minutes.
The influence of external magnetic or electric fields or e.g. high temperatures or interactions with other QuBits (crosstalk) cause QuBit errors, which can only be corrected to a very limited extent.
Now there are different possibilities to generate and preserve QuBits by physical-technical means. QuBits can be operated with superconducting materials, be stabilized in ion traps by electromagnetic fields, or be generated e.g. by orthogonal polarization of photons, by manipulation of the atomic nucleus spin and semiconductor structures.
Latest advances in building quantum computers:
- Google has publicly unveiled the Google Sycamore processor with 53 QuBits in 2019.
- A quantum computer from IBM-Q is currently being built in Stuttgart and will operate with 20 QuBits; IBM is also currently testing a chip with 50 superconducting QuBits.
- In the EU project AQTION, a quantum computer is being developed at the University of Innsbruck that is based on the storage of individual charged atoms and their manipulation with laser light. A processor is currently being used that supports up to 50 QuBits.
- In a cooperation with Google, FZ Jülich is working on a 50-100 QuBit quantum computer.
P. Kaufmann, S. Naegele-Jackson, II. Quantenrevolution – die Welt der Qubits: DFN-Mitteilungen Ausgabe 99 Juni 20/21 Seite 22