One step closer to error-free quantum computing

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If the race for quantum computing continues around the world, the practical applications of these computers with unparalleled computing power remain fairly theoretical. One of the major obstacles to their implementation is the complexity of correcting calculation errors. A team of Austrian researchers has just published research in which they claim to have found a way to overcome this key obstacle.

This is a subject on which physicists specializing in quantum physics seem to agree. The detection and correction of calculation errors are a necessity to make quantum computers usable in a concrete way. However, managing to correct these famous errors is not an easy task. Indeed, quantum computers have a very different operating principle from our classical computers.

A quantum computer makes it possible to make calculations based on the principles of quantum physics. That is to say, to sum up, the science which is interested in the behavior of matter and light at a microscopic or atomic level. By studying how matter behaves at this level, certain physical principles, very different from what we knew until then, became apparent to researchers.

In the field of quantum computers, what is the key to operation is what is called “quantum superposition”. Concretely, it is about the fact that something (here at the atomic level) can be “in two states at the same time”, as counter-intuitive as that may seem. In the case of a classic computer, remember that the basic unit of information is the “bit”. This can be either in the “0” state or in the “1” state. In a quantum computer, there is a sort of equivalent, called a “qubit”. Thanks to this law of superposition, qubits can be both 0 and 1, and even be in superimposed states, such as 01, 10, 11… This is the very thing that could allow them to deploy unparalleled computing power. .

However, this strength can also be a weakness when it comes to correcting calculation errors. However, errors, these famous quantum computers make a lot of them! “ Quantum computers are inherently much more sensitive to disturbances and therefore will likely always require error correction mechanisms, otherwise errors will propagate uncontrollably through the system and information will be lost. “, thus explains a statement from the University of Innsbruck, where the researchers who published their discovery work.

Redundancy, a countermeasure against calculation errors

As the technique has progressed a lot, and the manufacturing is of high quality, there are no longer many miscalculations in conventional computing. However, when an error occurs, there is always, for certain critical applications, a countermeasure. It is about the redundancy of the processed data, which makes it possible to obtain a certain security. Concretely, the idea is to have several copies of the data: it is then quite easy to detect if an error has been made, because the results of the calculations diverge.

However, this mechanism is theoretically not applicable in the case of a quantum computer. Indeed, according to the “no cloning” theorem, which is one of the laws of quantum physics, one cannot “copy” quantum information. However, and this is what the scientists explain, “redundancy can be achieved by distributing logical quantum information in an entangled state of several physical systems, for example several individual atoms”.

Indeed, it should be remembered that what is called a “qubit” is, very concretely, an atom. In quantum computing, the idea is to “organize” these qubits in various ways so that they interact with each other and place themselves in these quantum states required for the computer. Different types of atoms and different systems are used for this. For example, there are “magnetic traps” for positive ions, or even superconductors… As for quantum entanglement; it is an “entanglement”, between two qubits, for example, which makes it possible to make a single system. So that then, even if they are separated, it is possible to know the state of a qubit by measuring that of the other. This entanglement phenomenon is required to perform a quantum calculation.

We therefore measure the progress that a functional qubit calculation system can represent, which in addition would make it possible to correct errors. The team of experimental physicists claim that for the first time they have implemented a universal set of computational operations on fault-tolerant quantum bits. In other words, they have therefore demonstrated that it is possible to program an algorithm on a quantum computer and ensure that unexpected errors do not distort the final result.

They additionally implemented this patch on a “universal quantum gate”, which could prove to be all the more useful. To better understand it, let’s first recall what a “logic gate” is in classical computing. Logic gates allow the basic operations that can be performed on a bit (the famous basic unit mentioned above, symbolized by 0 or 1). This is what allows information to be entered to obtain a result. Different types of logic gates are ultimately used to program and give instructions to the computer.

Universal quantum gates

Different “logic gates” also exist in quantum computing, but “universal gates” are those that would allow any type of calculation to be performed: “ For a real-world quantum computer, we need a universal set of gates with which we can program all algorithms. explains Lukas Postler, an experimental physicist from Innsbruck. In concrete terms, this “gate” was made using 16 atoms confined in atom traps. Quantum information was stored in two logical quantum bits, each spread over seven atoms.

The researchers implemented the operations on the logical qubits in such a way that errors caused by the underlying physical operations could also be detected and corrected, thanks to this famous distribution of quantum information. So they realized the first fault-tolerant implementation of a universal set of gates on encoded logic quantum bits “.

The system obtained is thus more complex, but also more efficient, according to the scientists: “ The fault-tolerant implementation requires more operations than the non-fault-tolerant operations. This will introduce more errors at the scale of single atoms, but experimental operations on logical qubits are better than non-fault-tolerant logical operations says Thomas Monz, from the Department of Experimental Physics at the University of Innsbruck. “ Effort and complexity increase, but the resulting quality is better “. The continuation of this quantum adventure will be to implement these methods on “larger, and therefore more useful” quantum computers, say the researchers.

Source : Nature

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One step closer to error-free quantum computing

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