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The search for ever higher computer performance is a strong motivation for scientists. The computers of the future will be quantum, allowing for very fast and complex computations, full simulation of molecules, or the development of innovative materials. However, before getting to it, it is first necessary to create the components for these supercomputers. Recently, engineers in Sydney demonstrated a quantum integrated circuit made of silicon, consisting of 10 phosphorous atoms. This represents an important step in the development of quantum computing that is useful in real conditions. By precisely controlling the quantum states of atoms – the different energy levels of the electrons belonging to an atom – the new silicon processor can simulate the structure and properties of an organic molecule with amazing precision.
The atomic-scale integrated circuit milestone is the culmination of 20 years of research led by Michael Simmons of Scientia, founder of the start-up Silicon Quantum Computing at the University of New South Wales (SQC). In 2012, his team had created the first “quantum transistor”.
Transistors are small electronic components that store bits of information. It is made of semiconductor materials, which allows the effect of switching and encoding of information. This is due to the presence of a large pool of electrons in a semiconductor. However, according to quantum mechanics, an electron can only occupy certain energy levels. This is how the levels of electrons forming a semiconductor correspond to the “bands” or differences in permissible energy values. When the transistor is turned on – the voltage is in the power range – current flows and the computer detects the value “1”. When the transistor is off – the voltage is outside the permissible power range – current no longer flows and the computer interprets it as a “0” value.
Remember that a quantum computer is equivalent to classical computers, but it performs its calculations using the laws of quantum physics directly. Whereas a classical computer deals with bits of information, which are either 0 or 1, a quantum computer uses qubits. These are generalizations of classical bits, which are a kind of simultaneous superposition of these two states.
Thus, recently, a team of quantum computing physicists from the University of New South Wales, in partnership with Silicon Quantum Computing, designed an atomic-scale quantum processor to simulate the behavior of a small organic molecule, simulating 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 achieved before. Their results are published in the journal temper nature.
Imitating nature, but in a very difficult way
This technological innovation addresses a challenge first posited by theoretical physicist Professor Richard Feynman in his famous 1959 lecture. Plenty of room downstairs. During this lecture, Feynman emphasized that in order to understand how nature works, it is necessary to be able to control matter at the same length scales that matter is made of – that is, to be able to control matter at the atomic level.
Scientific American, lead researcher on the study, Michelle Simmons, said in a statement: And that’s what we do, we literally build it from the bottom up, mimicking a polyacetylene molecule by placing atoms in silicon at the exact distances that represent the single and double carbon bonds This molecule is well known by researchers. So they can immediately determine the consistency of the result, and therefore the reliability of the chip.
To design the first integrated quantum circuit, the team had to perform three distinct technological feats of atomic engineering, in a near-absolute vacuum. Indeed, at this scale, a single hydrogen atom can dilute a complete manipulation.
The first achievement was the creation of tiny dots of atoms of uniform size, aligning their energy levels and enabling electrons to pass through them easily. These points, called quantum dots (QD), are points of phosphorous atoms. By configuring their layouts, they can act like real quantum transistors. In this study, a quantum integrated circuit includes a series of 10 quantum dots to simulate the exact location of atoms in the polyacetylene chain.
However, the permissible power range, as mentioned earlier for conventional transistors, is very small. This is where the second technological breakthrough comes in, the ability to adjust energy levels for each point individually, but also for all points collectively. Therefore, using the nanometric precision system, they added six control electrodes (G1 to G6 in the image below) to adjust the energy levels. This gives complete control over where the electrons are located in the polyacetylene chain. By adding a source (S) and drain (D) conductors, they can then measure the current flowing through the device as the electrons pass through a series of 10 quantum dots.
Finally, the third technical challenge was to achieve the ability to control the distances between points with sub-nanometer precision. If it is too close, the energy produced is too strong to be mastered. If they are too far apart, the interactions between them become risky. So the points must be close enough, but remain independent, to allow a coherent transfer of electrons across the chain.
To double check this consistency in the results produced by the circuit, the researchers simulated two different strands of polymer chains at 10 points of the molecule.
In the first machine they cut a piece of chain to leave double links at the end to give 10 peaks in the current. In the second device, they cut a different part of the chain to leave single links at the end, resulting in only two peaks in the current. So the current through each chain was drastically different due to the different bond lengths of the atoms at the end of the chain.
Professor Simmons explains: What this shows is that you can literally simulate what is actually happening in the molecule. Which is why it’s exciting because the two signatures of the series are completely different. Most other quantum computing architectures lack the ability to engineer atoms with sub-nanometer precision or allow atoms to be as close to this limit. This means that we can now begin to understand increasingly complex molecules by putting atoms in place as if they were simulating a real physical system. “.
And now? Quantum biology…
According to Professor Simmons, it is no coincidence that a 10-atom carbon chain was chosen, because it is on the order of magnitude that a conventional computer can calculate, with up to 1024 distinct interactions of electrons in this system. Increasing it to a series of 20 points would see the number of possible interactions increase exponentially, making it difficult for a typical computer to solve.
Says: ” We’re approaching the limit of what traditional computers can do, so this is a step into the unknown. […] We will be able to understand the world in a different way, by addressing fundamental questions that we could not answer before. “.
Moreover, we are talking about quantum biology. This modern disciplinary field deals with the study of processes in living organisms that involve the laws of quantum physics. Photosynthesis, the orientation of migratory birds or even bioluminescence, is governed by quantitative 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 path similar to the evolution of classical computers — from a transistor in 1947 to an integrated circuit in 1958, and then small computer chips that were incorporated into commercial products, such as 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 announced (in 2012) that they had built the world’s first single-atom transistor, completing two years ahead of schedule.
Finally, using fewer components in the circuit to control the qubits reduces the amount of any interference with quantum states, allowing the devices to scale to create more complex and powerful quantum systems.