Everyone is talking about quantum computers. Do you sometimes feel like you’ve missed the boat and no longer dare to ask how a quantum computer actually works and what it’s supposed to be good for? Then my blog series “FAQ: Quantum Computer” is for you! Many news articles on quantum computing do not (or no longer) go into sufficient detail about the new quantum machines, which quickly leads to misunderstandings and confusion.

I have gone into the details of the “miracle machines” in three articles. Here you will find an overview of the questions I have tried to answer – including a short version of the answer.

## From Bit to Qubit

*All about the basic building blocks of quantum computing and what distinguishes them from the bits in classical computers.*

### What are qubits?

Quantum bits, or qubits for short, are bits built from quantum systems. While bits switch back and forth between the two options 0 and 1, qubits are only in one of the states 0 and 1 with a certain probability. Put casually, qubits can be in the states 0 and 1 at the same time thanks to their quantum superpower called *superposition*.

### How to translate quantum to classical information?

To be able to read out the information stored in qubits, we need a translator from the quantum language into our own. Because even if a qubit is in a superposition of 0 and 1, then a detector can click or not – it cannot click a bit. A measurement affects a qubit so strongly that afterwards it is in one of the two states 0 or 1, depending on whether the detector clicks or not. The superposition is broken in the process.

### What ARE qubits? What are they made of?

Qubits are quantum systems. There are many approaches of how to build a qubit: atoms, ions (charged atoms), superconducting qubits (tiny little circuits), impure diamonds, photons (particles of light) and a few more. All platforms have their advantages and disadvantages and all are part of current research.

### Is one qubit better than a bit?

No. Because the superposition is broken when it is read out, a qubit is reduced to one bit after the measurement and is therefore not one bit better than a bit.

## From Qubit to Quantum Computer

*All about what you need for a quantum computer and what makes it so powerful.*

### What makes a quantum computer so powerful?

*or:* What is quantum parallelism?

The state of a qubit can depend on the state of another qubit, unlike normal bits. This property is called *entanglement *and is the basis for quantum computers. This interdependence means that *N* qubits can be in *2 ^{N}* different states – simultaneously. If you change the state of a single qubit, all the other qubits of the quantum computer notice this – this is what we call

*quantum parallelism*.

### Can a quantum computer solve every problem in one step?

No. Quantum parallelism gives the impression that one could superimpose different initial situations and calculate them simultaneously. This generally leads to a highly entangled final state that breaks when measured. So we have to make sure that the qubits at the end of a calculation are in a unique state that we can read out without losing information.

### What do we need for a quantum computer?

Physicist *David DiVincenzo* has formulated five rules that a quantum computer must fulfil. These are:

- You need a collection of qubits, i.e. quantum systems consisting of two easily separable states.
- It must be possible to bring each qubit into a fixed initial state.
- The qubits must be stable long enough to perform the operations.
- We must be able to perform all the necessary computational operations on the qubits.
- We must be able to read out the qubits.

### Why is it so hard to build a quantum computer?

There are many difficulties, all related to the fact that quantum systems are super fragile and break quickly. The biggest problems are *stability *(qubits must not decay before the calculation is over), *control *(we must be able to control the evolution of qubits) and *scalability *(we must be able to assemble many qubits into a quantum computer and address them all individually and in pairs) of qubits.

## From classical to quantum computer

*All about the differences and similarities between classical and quantum computers*

### Do quantum computers work the same way as classical computers?

Theoretically, we can use quantum computers (almost) in the same way as normal computers. But that would be stupid because quantum computers are much more error-prone and harder to build. Quantum computers can only outperform classical computers if we have a suitable “recipe” that solves a problem more cleverly using quantum power. We call these recipes *quantum algorithms*.

### How do quantum algorithms work?

Quantum algorithms are generally based on constructive and destructive interference. We want to superimpose matter waves (for example electrons) in such a way that the wrong solutions average away and only the correct solution remains in the end. One example is the *Grover algorithm*, which shows how quantum computers can be used to solve search problems more quickly. *Shor’s algorithm* is able to decompose prime numbers into their factors more quickly.

### What can quantum computers do better than normal computers?

Roughly speaking, quantum computers are superior to normal ones whenever the solution could be found by mere guesswork. More specifically, quantum computers can solve those problems faster for which we have quantum algorithms. One thing that quantum computers can do, but that normal computers are not capable of, is generating perfectly random numbers.

### What can normal computers do better than quantum computers?

Everything else. Without quantum algorithms, quantum computers are no better than normal ones, usually even worse. One thing that quantum computers actually cannot do is copy. According to the no-cloning theorem, it is impossible to copy arbitrary quantum states. However, it is precisely this property that quantum cryptography makes use of.

### How many qubits does a quantum computer have?

Current devices from Google and IBM, for example, have around 70 noisy qubits. However, to solve hard problems, such as modern encryption, you need 4000 perfect qubits.

### Don’t quantum computers already exist?

Yes, but not the miracle machines that everyone wants. Adiabatic quantum computers from D-Wave, for example, are based on a different principle and are suitable for solving optimisation problems instead of executing quantum algorithms. Other devices, such as the quantum computers from IBM and Google, are still very error-prone. This is what we call the NISQ era, for *Noisy Intermediate Scale Quantum*, because although the number of qubits is already quite good, they are still too noisy. Lastly, there are cloud-based quantum computers, which also fall into the NISQ realm, but have been made publicly available by companies over the internet.

### When will there be home quantum computers?

Home quantum computers are not the goal of current research because we would have no use for them at home. We are still in the research phase and are far from even industrial production. Maybe they will never exist for the home, but maybe it will become common to access quantum computers via the cloud.

*In this article I talked about the quantum computer, which is just one of many quantum technologies. You might also be interested in the article on the second quantum revolution. Do you like what you read? Then subscribe to my blog and never miss a new post, or, if you like, you can buy me a coffee here!*