CM – Build a silicon quantum computer chip atom by atom


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January 12, 2022

from Science in Public

A team led by the University of Melbourne has perfected a technique for embedding individual atoms in a silicon wafer one after the other. Their technology has the potential to make quantum computers using the same methods that made cheap and reliable conventional devices with billions of transistors possible.

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“We could ‘hear’ the electronic click as each atom in our prototype fell into one of 10,000 places. Our vision is to use this technique to build a very, very large quantum device, ”says Professor David Jamieson of the University of Melbourne, lead author of the Advanced Materials paper, which describes the process.

His co-authors are from UNSW Sydney, the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Leibniz Institute for Surface Technology (IOM) and the RMIT Microscopy and Microanalysis Facility.

« We believe that with our method and the advantages of manufacturing techniques, which the semiconductor industry has perfected, can ultimately produce large machines based on quantum bits from individual atoms, ”he says implanted in a random pattern, like raindrops on a window.

« We embedded phosphorus ions in a silicon substrate and counted each one precisely, w o is created by a qubit chip, which can then be used in laboratory experiments to test designs for large devices. « 

 » This will enable us to develop the quantum logic operations between large arrays of individual atoms while performing highly precise operations over the to retain the entire processor, ”says Scientia professor Andrea Morello of UNSW, co-author of the paper. « Instead of implanting many atoms in random places and choosing the ones that work best, they are now arranged in an orderly arrangement, similar to the transistors in traditional semiconductor computer chips. »

« We have advanced technology for sensitive X-ray detectors and a special atomic force microscope, which was originally developed for the Rosetta space mission, as well as a comprehensive computer model for the trajectory of ions implanted in silicon, which was developed in cooperation with our colleagues in Germany. ”says Dr. Alexander (Melvin) Jakob, first author of the paper, also from the University of Melbourne.

This new technique can generate large-scale patterns of counted atoms, which can be controlled in such a way that their quantum states can be manipulated, coupled and read out.

The technology developed by Professor Jamieson and his colleagues makes use of the precision of the atomic force microscope, whose sharp cantilever gently “touches” the surface of a chip with a positioning accuracy of only half a nanometer. Distance between atoms in a silicon crystal.

The team drilled a tiny hole in this cantilever so that when it was showered with phosphorus atoms, you would occasionally fall through the hole and become embedded in the silicon substrate.

The key however, was knowing exactly when an atom – and no more than one – was embedded in the substrate. Then the cantilever could move to the next exact position on the array.

The team discovered that the kinetic energy of the atom as it plows into the silicon crystal and releases its energy through friction can be harnessed to create a tiny electronic one To generate a “click”.

This way they know that an atom is embedded in the silicon and move to the next exact position.

“An atom that collides with a piece of silicon makes a very weak click , but we have invented very sensitive electronics to detect the click, it is strongly amplified and gives a loud signal, a loud and reliable signal, « says Professor Jamieson.

 » This allows us to be very sure of our method being. We can say, “Oh, it clicked. An atom has just arrived. « Now we can move the cantilever to the next position and wait for the next atom. »

« We have already achieved groundbreaking results with our center partners on single-atom qubits using this technology but the new discovery will speed up our work on large-scale devices, « he says.

Quantum computers perform calculations using the different states of individual atoms in the same way that conventional computers use bits – the most basic unit of digital information.

But while a bit has only two possible values ​​- 1 or 0, true or false – a quantum bit or qubit can be placed in an overlay of 0 and 1 as « 01 plus 10 », so-called entangled states. The addition of even more qubits creates an exponentially growing number of entangled states that form powerful computer code that does not exist in classic computers. This exponential information density gives quantum processors their computational advantage.

This fundamental quantum mechanical weirdness has great potential to develop computers that are able to solve certain computational problems that conventional computers would consider impossible due to their complexity.

Practical applications include new ways of optimizing schedules and finances, unbreakable cryptography and computational drug design, maybe even the rapid development of new vaccines.

« If you wanted to calculate the structure of the caffeine molecule, a very important molecule in physics, go not with a classic computer because there are too many electrons, « says Professor Jamieson.

 » All of these electrons obey quantum physics and the Schrödinger equation. But if you want to calculate the structure of this molecule, there are so many electron-electron interactions that even the most powerful supercomputers in the world can’t do it today.

« A quantum computer could do that, but you need a lot of qubits because They have to correct random errors and execute very complicated computer code. « 

Silicon chips, which contain arrays of individual dopant atoms, can be the material of choice for classical and quantum components that use individual donor spins. For example, group V donors implanted in isotopically purified Si crystals are attractive to large quantum computers. Useful attributes are long nuclear and electron spin lifetimes of P, hyperfine clock transitions in Bi or electrically controllable Sb nuclear spins.

Promising architectures require the ability to fabricate arrays of individual near-surface dopant atoms with high yield. Here an on-chip detector electrode system with 70 eV square noise (≈20 electrons) is used to demonstrate the implantation of individual 14 keV P ions at room temperature.

The physics model for the ion-solid interaction shows an unprecedented one Upper limit of the single ion detection reliability of 99.85 ± 0.02% for implants close to the surface. As a result, the practically controlled silicon doping yield is limited by material-related factors including surface gate oxides in which detected ions can stop.

For a device with 6 nm gate oxide and 14 keV P implantations, a yield limit of 98.1% is demonstrated. Thinner gate oxides allow this limit to converge to the upper limit. The deterministic single ion implantation can therefore be a viable material development strategy for scalable dopant architectures in silicon devices.

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Related title :
Atom by atom: New silicon computer chip technology opens up construction possibilities for quantum computers
Construction of a silicon quantum computer chip atom by atom


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