Text: Nina Beier / Picture: IBM

Our computers are getting faster and faster. And: They are also becoming smaller. Nowadays, almost everyone carries their own small computer in their trouser pocket. Today’s smartphones weigh only a few hundred grams and are significantly more powerful than supercomputers weighing tons about 50 years ago.

This development towards ever smaller and faster computers can be explained by the * Moore’s Law* which the physicist and Intel co-founder Gordon Moore formulated in 1965: The number of computing components of a computer – the transistors – doubles every two years, and so does the computing power of computers. Since the law was formulated, it has predicted the development of computer chips surprisingly well. But in the meantime we have reached a physical limit, because the size of the transistors now corresponds to that of a few atoms. It is at these sizes that the laws of quantum physics begin to play a role – and this brings with it both new challenges and opportunities. The key word here is quantum computing.

## The Qubit

A classical computer calculates with bits that can take the value 1 or 0. The arithmetic building blocks of quantum computers are called qubits (quantum bits) – because they follow the rules of quantum mechanics. The values these qubits can take on are much more complex than just 1 or 0, as is the case with a normal computer – and this is precisely where the strength of quantum computers lies.

## Superposition – the “superpower” of qubits

The reason for this is the principle of ** superposition**. Because of this, a qubit can not only be in state 1 or 0 like a normal bit, but also in any combination of these two. The advantage: The qubits can represent not only one but many values simultaneously. Therefore, in one step, a calculation can be performed on several values simultaneously (this is called

*). In addition, different qubits can be*

**Quantum Parallelism***…that is, they are linked. The more qubits one has, the better the computing power of the quantum computer.*

**entangled**But it’s not quite so simple with these quantum states. The superposition of these different states only works as long as they are well shielded from the environment. A major challenge in the development of a quantum computer is therefore already to maintain these parallel states during the computing operations. However, at the end of a calculation, one also wants to have a result available. In order to obtain this information, one must necessarily interact with the superimposed states (in the form of a measurement). During this process, the superposition dissolves and only one single state can be registered at the end. In order to obtain results about the many different states, a calculation must be performed several times.

## Clever algorithms solve special problems

Does this mean that the “superpower” of the quantum parallelism is nullified? Not quite. The quantum nature of qubits can be harnessed after all. Based on the quantum physical properties, algorithms can be developed that give the quantum computer an advantage over classical computers. They make it possible that a result sought is more likely to occur, and thus faster and more frequently, than other results.

Such a thing becomes interesting, for example, if you want to search a large database. It’s kind of like looking for a needle in a haystack. A classical computer does this: It checks every component of the haystack, every straw, until it hits the needle. That can take a while. A quantum computer is equipped with an additional capability through a special algorithm (which is tuned to the problem). This gives it a kind of feeling for where to find the needle.

With the right algorithm for certain problems, a quantum computer can therefore be quite superior to classical computers. However, the development of these different computers is still in full swing. Companies like IBM, D-Wave and Google have been competing for years to develop commercial quantum computers. A few products have already made it onto the market, but they are usually only accessible via the cloud. These are the first steps, and these quantum computers still consist of only a handful of qubits. We can be curious to see how this development continues.

## Do you want to learn more about quantum computers?

Later this week, the blog will feature an interview with the science journalist Ulrich Eberl, who gave me his assessment of quantum computers.