How do quantum computers work? - Easily explained

Nowadays, scientists are continuously working on quantum computers. IBM recently launched its first quantum computer. We explain how these work here.

Quantum computers: That is what is called qubits

So-called qubits are used in a quantum computer.

• Normal bits on the computer can only take two different values: 0 and 1, or "on" and "off". However, a qubit can be in an intermediate state of zero and one for a certain period of time, the so-called coherence time.
• In this state, scientists speak of a superposition . Through a measurement, the qubit then changes to one of the two clearly defined states, so that the measurement result can be saved in a classic bit. In technical terms, the loss of superposition is called decoherence .
• In the laboratory, such qubits are made from ions or superconducting loops, so-called SQUIDs .
• When working with ions, an unexcited ion corresponds to state 0 and an excited one to state 1. An atom with the lowest possible energy is said to be unexcited. However, if you add energy to an atom, it is excited because outer electrons reach higher energy levels. The ions can be excited with a laser.

Quantum registers - you need to know that

Several qubits are required to solve arithmetic operations. One speaks of a so-called quantum register. The information is then distributed to all qubits of a register.

• Such a quantum register usually consists of 14 ions that are stored along an axis at a distance of a few micrometers. It is important that these qubits are easy to manipulate, but are also immune to interference.
• This means that the qubits must remain in their respective states for as long as possible until the arithmetic operation has been carried out. Decoherence, i.e. falling back into a classic state, must be delayed as long as possible.
• Logical operators are used to manipulate the states, which are already used in computer science. In quantum computers, these operators are called quantum gates . These are decisive for the duration of the irradiation and for the wavelength of the light.
• The simplest operation is negation, called NOT . The state of a qubit is simply flipped over or negated. In the binary system, 0 would become 1 and vice versa. This flipping happens very quickly and very often in succession and follows the algorithm of the program.
• To determine the initial state of a quantum lattice, it is irradiated with laser pulses. The length of the irradiation can determine the probability with which an atom is in the excited state.
• After about ten microseconds of irradiation, an ion that is initially not excited is in the excited state. However, if this atom is irradiated for only half as long, it will be in that intermediate state, since it is 50 percent more likely to be in the ground state and 50 percent more likely to be in the excited state.
• To read the result after executing the algorithm, another laser pulse with a different wavelength is fired at the ions. Fluorescence indicates whether they are excited or not. The computer can then determine the correct values.

Quantum computers: state of the art today

At the electronics fair in Las Vegas, IBM presented its first market-ready quantum computer this year.

• Compared to previous models, the IBM Q Systems One already calculates with 20 qubits, which is a yardstick for a correctly functioning quantum computer. According to IBM, it has managed to keep 20 qubits in the prepared state for 75 microseconds.
• A quantum computer with 50 qubits should be able to put any classic supercomputer in its pocket.
• The IBM Q Systems One - a glass box with a length and width of two and a half meters should not be offered for sale. Instead, selected users can access it from the cloud and perform calculations.

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In the next practical tip, we will show you how to correctly convert binary and hexadecimal numbers.