How does parallelism work in a quantum computer

How does a quantum computer work?

Even if scientists around the world are working towards this goal, quantum computers outside of the laboratory situation will only exist a few decades in the future. However, seventy to eighty years ago, the situation with conventional computers was similar. However, a quantum computer will not establish itself as a universal computer in the future either. Only with special problems could the use of quantum mechanical effects make sense and lead to faster solutions.

To build a quantum computer, you first need computing and storage units. These so-called qubits are the quantum mechanical counterpart to the bits of conventional computers. Bits can have exactly one of two possible states, which is either zero or one in the binary system. The qubit, on the other hand, can be in an intermediate state of zero and one for a certain period of time, the so-called coherence time. This state is also called superposition. A measurement then changes the qubit into one of the two clearly defined states, so that the measurement result can be saved in a "classic" bit. This loss of superposition is called decoherence.

Ions in the Paul trap

In the laboratory, researchers produce qubits from ions or superconducting loops, so-called SQUIDs, among other things. Rainer Blatt from the University of Innsbruck and his colleagues decided to experiment with ions. They hold the positively charged ions - i.e. atoms that are missing an electron - trapped in so-called Paul traps using electrical fields.

In this type of qubit, a non-excited ion corresponds to the state zero and an excited ion corresponds to the state one. An atom with the lowest possible energy is called not excited. Excited means that the particle has been supplied with energy and the outermost electron has reached a higher energy level as a result. With lasers, Blatt and his team can now bring the ions into an excited, non-excited state or a superposition state in between. In order to realize a quantum computer, however, further requirements have to be created. To solve arithmetic problems, you need several qubits, a so-called quantum register. The information is then distributed to all qubits of a register.

String of pearls made from qubits

In Rainer Blatt's laboratory, a register consists of 14 ions that are stored a few micrometers apart along an axis. According to the researcher, they are lined up “like a string of pearls”. One requirement of qubits is that they are easy to manipulate. At the same time, however, they should also be insensitive to interference.

Luminous ion chains

Meeting these two conflicting requirements is difficult, but necessary. Because in order to be able to exploit the quantum mechanical effects, the qubits must remain in the specified intermediate states until the arithmetic operation has been carried out. This means that the researchers have to delay decoherence, i.e. falling back into a classic state, for as long as possible.

In order to describe the manipulation of the states, the physicists use logical operators, as they are also used by computer scientists for the elementary operations of a classic computer. These operators are called quantum gates in quantum computers. Applied to qubits, they change the information stored there. In the case of ion traps, quantum gates describe the way in which an ion is manipulated by a laser - i.e. the duration of irradiation and the wavelength of the light.

Similar to classical computer science, a few basic logical operations are sufficient to carry out any arithmetic process on the corresponding quantum computer. The simplest operation is the negation "NOT". The status of a storage unit is simply "flipped" or negated. That means in the binary system the zero would become one and vice versa.

Principle of the qubit

A computation process consists of a sequence of these basic operations and runs as follows in a classic computer: The bits lie next to one another as registers and are each in a fixed initial state of zero or one. Then the computer flips over the individual bits and thereby changes their stored value from zero to one or vice versa. This folding happens very quickly, very often one after the other and works according to a fixed scheme, the algorithm of the started computer program. So that this can be carried out by the computer, the code, which is written in a programming language such as Java or C ++, has to be translated into machine code that only consists of a binary code made up of zeros and ones. At the end of the process, the result of the calculation is in the register as a binary number and can thus be read out.

Calculating with quantum gates

In the quantum mechanical system, a calculation process works according to the same principle: First, the initial state of the quantum register must be determined. Rainer Blatt and his team do this with laser pulses that they shoot at the chain of ions. Through the length of the irradiation, you can determine the probability with which the ion can then be measured in the excited or non-excited state. These probabilities are mathematically described by wave functions that are assigned to the individual qubits.

How long the physicists from Innsbruck have to irradiate an ion in order to achieve the desired state results from the necessary irradiation time for the simplest basic operation “NOT”: After about ten microseconds, an initially non-excited ion is in the excited state. The researchers used the simple negation "NOT" and "flipped" the qubit from zero to one. If you only irradiate the ion for half the time, i.e. five microseconds, it is then exactly in the quantum mechanical intermediate state in which it is still in the ground state with a probability of 50 percent and already in the excited state with a probability of 50 percent is located.

Isolate the quantum system

In contrast to classical computer science, the calculation result is not clearly defined and is only generated during the measurement via a so-called non-local interaction. This happens through the entanglement of the qubits, another phenomenon of quantum mechanics. If a qubit of the register is manipulated with lasers, for example a measurement is carried out on it, the state of the entire register now changes instantaneously - i.e. without any loss of time. For the calculation process it is important to preserve the quantum mechanical states of the system - i.e. the superposition of the individual qubits and their entanglement with one another.

Construction of the laser apparatus

The researchers working with Rainer Blatt are trying to ensure that there are only controlled interactions with the targeted laser pulses. You achieve this through low temperature, vacuum and precise manipulation tools: “We cool the atoms down to their absolute ground state, and pump at pressures of 10-11 until 10-12 Millibars remove everything else that is in the Paul trap and work with extremely narrow line widths, ”says Thomas Monz, a research associate in Blatt's work group. A narrow line width means that the frequency of the light varies only minimally. These measures enable the researchers to manipulate the ions in a targeted manner and to read out the stored information correctly.

The natural lifespan for an excited state of the ion qubits is one second. A superposition state, i.e. a quantum mechanical intermediate state, remains for about a tenth of a second. In this time window, the researchers can calculate with the quantum computer, that is, execute a quantum gate or a sequence of quantum gates. With an average gate time of one hundred microseconds, a thousand operations on the qubits are possible in the superposition state.

In order to read out the result after applying the algorithm, the researchers shoot another laser pulse with a different wavelength at the ions. They use fluorescence to indicate whether they are excited or not. A conventional computer can then use this information to determine the result.

What does quantum computer mean here anyway?

The question of whether or not quantum computers have already been implemented depends on how a quantum computer is defined. If you imagine it as a compact device with hard drive, processor, microchips and housing, the quantum computer does not yet exist. If you ask researchers like Rainer Blatt, the opposite is clear to him. “Here they are, the quantum computers,” he says, referring to two tables in the university laboratory on which there is a collection of devices and cables. In addition, there are laser structures that manipulate and cool the quantum system, and classic computers that the researchers use to operate. “These are our prototypes, and we're working on a portable model,” says Blatt.

Prototype of a quantum computer

This also corresponds to the view of Christian Ospelkaus, who researches quantum logic at the University of Hanover. He understands quantum computers as “very well controlled and interacting quantum systems such as ions in a trap”. According to this rather broad definition, the quantum computer already exists. However, the options for doing calculations and solving problems are still very limited and only interesting for research.

However, Blatt has the vision that in five to ten years the computing power will have increased so that quantum computers can be used remotely - just like mainframes up to now. "Then send your programs there and get the result back." However, the physicist admits: "A quantum computer for the home is really still a dream of the future."

Rainer Blatt in the laboratory

The research goals are currently aimed at expanding the systems and reducing the calculation errors per operation. So, on the one hand, researchers are working on controlling more qubits in parallel. To do this, they are developing various technical implementations of qubits around the world and testing how well they can be manipulated and at the same time isolated from the environment. On the other hand, the scientists are concerned with another major weak point: in quantum computers, the errors increase exponentially with the number of qubits. This is particularly problematic because the usual error corrections do not work. A classic method is to copy bits several times in order to calculate with the copies. If a result deviates from what most copies provide, this indicates a fault in the system. That doesn't work in the quantum world. Because here every disturbance, including copying, destroys the superposition of the qubits.

A wide variety of social groups are interested in the possibilities of quantum computers. For example, researchers want to use the new computers to simulate complex multi-particle systems such as electrons in a solid or the superposition of magnetic fields. Internet companies like Google hope for more efficient methods of searching databases. And secret services speculate on breaking high numbers into prime numbers and thus cracking encryption methods that were previously secure. "It is up to the user what he makes of it - with the classic computer as well as with the quantum mechanical", says Thomas Monz. But before quantum computers can effectively solve the problems with which conventional computers are overwhelmed today, a lot has to be done technically.