The race for quantum computers and simulators is in full swing even if the outcome is still uncertain. Several approaches are possible and a team of researchers from MIT has just announced that it has found a new way to store bits of quantum information, qubits. The breakthrough is promising because these qubits are particularly resistant to disturbances, making it difficult to pursue quantum calculations and which therefore further limit the possible applications of the machines under construction.
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[EN VIDÉO] Interview: How is a quantum computer different? The quantum world is fascinating: at this scale, for example, objects can be in several states simultaneously. Exploiting this principle, a quantum computer would have much wider possibilities than a classical model. As part of its series of Expert Questions on physics and astrophysics, the publisher De Boeck interviewed Claude Aslangul, professor at UPMC, to explain to us how this strange machine works.
Our world, that of the Noosphere, is more and more intensive in information processing by means ofcomputers. This means being able to produce machines that are more and more powerful in terms of volume of calculation, the speed of their execution and, of course, increasingly miniaturized.
In the early 1980s, the brilliant Nobel laureate of physical Richard Feynman realized that the equations of the quantum physics and the phenomena ofentanglement and quantum superposition they contained should allow calculations to be made directly with atoms or particles. In particular, we could use quantum systems to simulate the behavior of others much more easily than with classical computers inherited from the work of John von Neumann and Alan Turing.
Discover in animation-video the history of quantum physics: from the ultraviolet catastrophe to the promises of the quantum computer, passing through the first and the second quantum revolution with the ideas of Feynman and Peter Shor. A video-animation co-produced with The Sorcerer’s Spirit. © CEA Research
The rise of quantum computing
The concept ofquantum computer universal in theory programmable for any task, or more simply as a quantum simulator to solve a very specific problem, was to be studied more and more seriously over the decades that were to follow, as explained in the video above which is also an introduction to quantum world whose keys had been found in 1925 by Werner Heisenberg as explained in one of his latest works by Carlo Rovelli.
The pioneers of what can be called quantum information have thus transposed the concepts and theories describing the algorithms and bits of information for these new machines.
Thus, during a seminar given in 1992, Benjamin Schumacher introduced for the first time the concept of qubit, the quantum transposition of bits of information from Claude Shannon.
They also discovered that, in certain cases, so-called quantum algorithms could in theory make it possible to solve very quickly problems that could take centuries of calculation to supercomputers classics. Even bearing in mind that it is possible, and has even happened, that we eventually find algorithms new classics that do as well as algorithms giving quantum supremacy over current computers, the stakes are such that many companies have embarked on the race for quantum computers, such as google and IBM.
A further introduction to the concepts of quantum computers and algorithms and, of course, qubits. © CEA Research
The obstacle of decoherence
However, there is a catch in all this because quantum computers or simulators need to solve what is called the problem of decoherence. Let’s take the image already exposed by Futura on several occasions to understand this problem.
To achieve a quantum computer surpassing a classical computer, it is indeed necessary to have a large number of qubits remaining as long as possible in a state of quantum entanglement and superposition to have time to perform the requested calculations. We can imagine them as the elements of a house of cards. The higher it gets, the more unstable it is.
When it reaches a few floors, a tiny stream ofair or a small vibration of the table is enough for the whole castle to crumble. In general, therefore, the larger the castle, the more likely it is to collapse quickly, unless it is placed in a vacuum chamber or on a table isolating it from ground vibrations, for example.
The problem is similar with qubits. It usually needs to cool almost to absolute zero quantum systems made up of just a few atoms that carry these qubits to insulate them long enough from the ambient, often thermal, background noise generated by the rest of theuniverse. Even so, the time available so far is too short to be able to perform anything more than a few timid quantum calculations, although recently some quantum simulators do indeed seem able to achieve quantum supremacy over classical computers in concerns certain very specific calculations.
It is therefore always necessary to find ways to protect quantum calculations as much as possible from these disturbances or to use quantum corrector codescousins of those of conventional computers to fight against calculation errors produced by decoherence.
From physicists from MIT have just found not only a new way to produce qubits but also in a promising form to combat the problem of decoherence. As explained in the article published in Nature but freely available on arXivthese are qubits obtained by cooling by laser atoms of potassium 40 and trapping them, still by laser, in the sites of a sort of artificial crystal formed by a network of rays of light.
Quantum analogues of classical coupled pendulum modes
Martin Zwierlein and his colleagues Thomas Hartke, Botond Oreg, and Ningyuan Jia discovered these new qubits quite by chance. Initially, they were just exploring the research area of ultracold atoms which makes it possible to look at states exotic of the matter like the famous Bose-Einstein condensates.
But they realized that the atoms they were trapping and plunging into a magnetic field could begin to oscillate according to quantum modes which are in correspondence with the classical modes of the oscillating systems made up of masses connected by springs and also forming pendulums. These classic systems have apps numerous and all the beginners in physics study them as one can be convinced by reading the famous Feynman’s physics course.
The pairs of atoms considered in the experiments of the MIT researchers can therefore behave like the quantum analogues of the classical systems with pendulums that we see oscillating according to the two modes of the image above. There are therefore two quantum states that can be superimposed to have a qubit. The decoherence effect would cause thecollapse from this superposition to a single mode.
But it turns out that the state superposition is particularly robust and can be maintained for about 10 seconds, which, according to physicists, would in principle allow 10,000 operations to be done before the effects of decoherence take over. . It’s really remarkable but remember that in this case, we must maintain the atoms at only 100 nanokelvins and that for the moment it is only a proof of principle of the existence of a quantum memory robust with the new qubits.
We still have to find a way to really be able to entangle these qubits and manipulate them to perform operations with them embodying quantum calculations. Physicists have all the same already been able to manipulate up to a certain point the oscillation states of around 400 pairs of atoms in their experiments, as explained by a press release from MIT about this job.
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