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an atomic-scale integrated quantum circuit propels us towards quantum computers

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The search for ever higher computer performance is a strong motivation for scientists. The computers of tomorrow will be quantum, allowing rapid and extremely complex calculations, the complete simulation of molecules, or the development of innovative materials. However, before accessing it, it is first necessary to create the components of these supercomputers. Recently, engineers in Sydney demonstrated a quantum integrated circuit made of silicon, made up of 10 phosphorus atoms. This represents an important step in the development of useful quantum computing in real conditions. By precisely controlling the quantum states of atoms — the different energy levels of the electrons belonging to the atom — the new silicon processor can simulate the structure and properties of an organic molecule with astonishing precision.

The atomic-scale integrated circuit milestone is the culmination of 20 years of research led by Scientia’s Michelle Simmons, founder of UNSW start-up Silicon Quantum Computing (SQC). In 2012, his team had created the very first “quantum transistor”.

Transistors are small electronic components that store bits of information. They are made with semiconductor materials, allowing a switching effect and the encoding of information. This is because in semiconductors there is a large group of electrons. However, according to quantum mechanics, an electron can only occupy certain energy levels. This is how the levels of the electrons making up the semiconductor correspond to “bands” or variations in permitted energy values. When a transistor is turned on — the electrical voltage is in the energy band — current flows and the computer detects the value “1”. When a transistor is in off mode—the electrical voltage is outside the permitted energy band—current no longer flows and the computer interprets this as a “0” value.

Remember that a quantum computer is the equivalent of classical computers, but performing its calculations using the laws of quantum physics directly. While a classical computer manipulates bits of information, which are either 0s or 1s, a quantum computer uses qubits. These are generalizations of the classical bits, which are sort of a simultaneous superposition of these two states.

Thus, recently, a team of quantum computing physicists from UNSW Sydney, in partnership with the start-up Silicon Quantum Computing, designed an atomic-scale quantum processor to simulate the behavior of a small organic molecule, mimicking its structure and energy states. This represents a major milestone in the race to build the world’s first quantum computer, and demonstrates the team’s ability to control the quantum states of electrons and atoms in silicon to a level never before achieved. Their results are published in the journal Nature.

Imitate nature, but in a very demanding way

This technological innovation addresses a challenge first postulated by pioneering theoretical physicist Professor Richard Feynman in his famous 1959 lecture. Plenty of Room at the Bottom. During this lecture, Feynman asserted that in order to understand how nature works, it is essential to be able to control matter at the same length scales from which matter is constructed — that is, to be able to controlling matter on the atomic scale.

Scientia Professor Michelle Simmons, lead researcher on the study, said in a statement: And so that’s what we do, we literally build it from the bottom up, where we mimic the polyacetylene molecule by putting atoms in the silicon with the exact distances that represent the single and double carbon-carbon bonds “. This molecule has the advantage of being well known by researchers. They can therefore immediately determine the consistency of the result, and by extension the reliability of the chip.

To design the first quantum integrated circuit, the team had to perform three distinct technological feats of atomic engineering, in near-absolute vacuum. Indeed, at this scale, a single hydrogen atom can compromise the whole manipulation.

The first feat was to create small dots of uniformly sized atoms, so their energy levels would line up and electrons could easily pass through them. These dots, called Quantum Dots (QD), are dots of phosphorus atoms. By configuring their layouts, they can behave like real quantum transistors. In the present study, the quantum integrated circuit includes a chain of 10 quantum dots to simulate the precise location of atoms in the polyacetylene chain.

Nevertheless, the tolerable energy band, as mentioned earlier for conventional transistors, is extremely small. This is where the second technological feat comes in, the ability to adjust the energy levels of each point individually, but also of all the points collectively. So, using a nanometric precision system, they added six control electrodes (G1 to G6 in the image below) to adjust the energy levels. This gives complete control of where electrons exist in the polyacetylene chain. By adding source (S) and drain (D) conductors, they could then measure the current flowing through the device as electrons passed through the string of 10 quantum dots.

A scanning tunneling microscope image of a 10 quantum dot quantum analog simulator, mimicking a polyacetylene molecule. © SM Simmons, et al., 2022

Finally, the third technical challenge was to achieve the ability to control distances between points with sub-nanometer precision. If they are too close, the energy produced is too powerful to be mastered. If they are too far apart, interactions between them become risky. The points must therefore be close enough, but remain independent, to allow the coherent transport of electrons through the chain.

To be doubly sure of this consistency of the results produced by the circuit, the researchers simulated two different strands of the polymer chains at 10 points of the molecule.

In the first device they cut a piece of chain to leave double bonds at the end giving 10 peaks in the current. In the second device, they cut a different fragment of the chain to leave single bonds at the end, resulting in only two peaks in the current. The current through each chain was therefore radically different due to the different bond lengths of the atoms at the end of the chain.

Theoretical comparison and under scanning tunneling microscopy of the two different strands of the polymer chains at 10 points of the molecule. At the top (Device I), the chain ends with a double bond; at the bottom (Device II), it ends with a simple link. © SM Simmons et al., 2022 (modified by Laurie Henry for Trust My Science)

Professor Simmons explains: “ What this shows is that you can literally mimic what is actually going on in the molecule. And that’s why it’s exciting because the signatures of the two chains are very different. Most other quantum computing architectures lack the ability to engineer atoms with sub-nanometer precision or allow atoms to be that close. This means that we can now begin to understand increasingly complicated molecules by putting the atoms in place as if they were mimicking the real physical system. “.

And now ? Quantum biology…

According to Professor Simmons, it is not by chance that a carbon chain of 10 atoms was chosen, because it is within the size limit of what a conventional computer is able to calculate, with up to 1024 distinct interactions of electrons in this system. Increasing it to a chain of 20 points would see the number of possible interactions increase exponentially, making it difficult for a typical computer to solve.

She says: ” We are approaching the limit of what conventional computers can do, so this is like a step into the unknown. […] We are going to be able to understand the world in a different way, by addressing fundamental questions that we have never been able to answer before “.

Moreover, we are talking about quantum biology. This recent disciplinary field deals with the study of processes at work in living organisms involving the laws of quantum physics. Photosynthesis, the orientation of migratory birds or even bioluminescence are governed by quantum processes. Understanding these phenomena paves the way for many innovations in the field of biomimicry.

The team believes that the development of quantum computers is on a trajectory comparable to the evolution of classical computers — from a transistor in 1947 to an integrated circuit in 1958, then small computer chips that have been integrated into commercial products, like calculators or so, five years later. Incidentally, the production of this atomic-scale integrated circuit, which functions as an analog quantum processor, came less than a decade after the team declared (in 2012) that they had made the first transistor. single atom in the world, completed two years ahead of schedule.

Finally, using fewer components in the circuit to control the qubits minimizes the amount of any interference with quantum states, allowing devices to be scaled up to create more complex and powerful quantum systems.

Source: Nature

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