The German quantum computer will soon be inaugurated. Here comes an excursion into the world of the smallest. A guest post.
1. What is the difference between a classical bit and a quantum bit?
EThe classic bit is an object that can take on the two values ”0″ or “1”. There are many different physical realizations for bits. Usually it is an electronic component such as a capacitor or transistor, but it is also possible to imagine a wire that carries current or not, a lamp that lights up or not, or a clock hand that can be horizontal or vertical. Classic bits have two central properties: First, the value of a bit is clearly defined at every point in time during the calculation. It can be measured and the measurement does not change the value of the bit. Second, the value on one bit does not change automatically when the value on another bit changes. It takes a period of time (albeit often a very short) before the change in one bit can affect another bit – at least the time it takes for the light to overcome the space between the two bits, because the Information about the change in the first bit must first arrive at the other bit.
A quantum bit, in turn, is an object that can have the two values “0” or “1” or something in between. There are many different approaches to how quantum bits can be physically realized: With light particles (photons), trapped ions or atoms, or superconducting elements. But you can also imagine the pointer of a clock, which can be horizontal or vertical or somewhere in between. Quantum bits do not have the two properties of conventional bits mentioned above. Because the laws of quantum physics apply to them and not those of classical mechanics, which means in concrete terms: First, the measurement process changes quantum bits. If a quantum bit is measured, it delivers the value “0” or “1”, but never anything in between. The measurement result is random. In the imagination with the clock hand, the probability for the measured value “0” is greater, the more horizontally the pointer was before the measurement.