Atom by atom construction of a silicon quantum computer chip

A University of Melbourne led team have perfected a technique for embedding single atoms in a silicon wafer one-by-one. Credit: University of Melbourne
A University of Melbourne led team have perfected a technique for embedding single atoms in a silicon wafer one-by-one. Credit: University of Melbourne

A team led by the University of Melbourne has mastered a technique for embedding single atoms in a silicon wafer one at a time. Their approach has the ability to produce quantum computers using the same technologies that have produced low-cost, dependable conventional systems with billions of transistors.

"We could 'hear' the electrical click as each atom dropped into one of our prototype device's 10,000 locations. Our goal is to employ this technology to create a very large-scale quantum gadget "Professor David Jamieson of The University of Melbourne, who is the main author of the Advanced Materials publication outlining the procedure, agrees.

His co-authors include researchers from the University of New South Wales (UNSW), the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), the Leibniz Institute of Surface Engineering (IOM), and the RMIT Microscopy and Microanalysis Facility.

"We believe that utilizing our technology and making use of the manufacturing processes that the semiconductor industry has established, we may eventually construct large-scale devices based on single-atom quantum bits," he adds.

Until now, implanting atoms in silicon was a haphazard procedure in which a silicon chip was bombarded with phosphorus, which implanted in a random pattern, much like raindrops on a window.

"We placed phosphorus ions in a silicon substrate, accurately counting each one, to create a qubit chip," which may subsequently be used in lab trials to evaluate concepts for big-scale devices.

"We will be able to construct quantum logic operations amongst enormous arrays of individual atoms while maintaining very precise operations across the entire processor," explains UNSW Scientia Professor Andrea Morello, a co-author of the work. "Rather of implanting numerous atoms in random areas and picking the ones that perform best, they will now be positioned in an organized array, comparable to transistors in traditional semiconductor computer chips."

This novel technology can generate large-scale patterns of counted atoms that are managed in such a way that their quantum states may be changed, linked, and readout. Professor Jamieson and his colleagues' technique takes advantage of the atomic force microscope's precision, which has a sharp cantilever that gently 'touches' the surface of a chip with a positioning accuracy of just half a nanometre, about the same as the spacing between atoms in a silicon crystal. The researchers created a small hole in the cantilever so that when it was bombarded with phosphorus atoms, some would fall through the hole and lodge in the silicon substrate. The trick, however, was determining when exactly one atom, and no more than one, became buried in the substrate. The cantilever may then be moved to the next exact spot on the array.

The researchers discovered that the kinetic energy of the atom as it plows into the silicon crystal and releases its energy through friction may be used to generate a small electrical click. That is how scientists know an atom has become lodged in the silicon and how they may advance to the next precise spot.

According to Professor Jamieson, one atom colliding with a piece of silicon produces a very weak click, but we have devised incredibly sensitive electronics used to detect the click, which is greatly amplified and produces a loud and trustworthy signal. This gives us a lot of confidence in our technique. We may say something like, "Oh, there was a click." A new atom has just arrived. We may now shift the cantilever to the next position and await the next atom. We have previously generated ground-breaking results on single-atom qubits manufactured using this technology with our Centre partners, but the latest discovery will expedite our work on large-scale devices.

Quantum computers perform calculations by using the varied states of single atoms in the way that conventional computers use bits the most basic unit of digital information. But whereas a bit has only two possible values 1 or 0, true or false a quantum bit, or qubit, can be placed in a superposition of 0 and 1. Pairs of qubits can be placed in even more peculiar superposition states, such as "01 plus 10," called entangled states. Adding even more qubits creates an exponentially growing number of entangled states, which constitute a powerful computer code that does not exist in classical computers. This exponential density of information is what gives quantum processors their computational advantage.

This fundamental quantum mechanical oddity holds enormous promise for creating computers capable of addressing computational problems that conventional computers would find difficult to solve owing to their complexity. Practical applications include novel methods for optimizing schedules and budgets, unbreakable cryptography and computational medication design, and maybe the speedy production of new vaccinations.

According to Professor Jamieson, calculating the structure of the caffeine molecule, a very significant chemical for physics, is impossible with a traditional computer because there are too many electrons. All of these electrons are subject to quantum mechanics and the Schrödinger equation. However, calculating the structure of that molecule is impossible due to a large number of electron-electron interactions. Even the most powerful supercomputers in the world today are incapable of doing so. A quantum computer could accomplish that, but it would require a large number of qubits to rectify random mistakes and run a complex computer code.

Silicon chips containing arrays of single dopant atoms can be the material of choice for classical and quantum devices that exploit single donor spins. For example, group-V donors implanted in isotopically purified Si crystals are attractive for large-scale quantum computers. Useful attributes include 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 yields. Here, an on-chip detector electrode system with 70 eV root-mean-square noise (≈20 electrons) is employed to demonstrate near-room-temperature implantation of single 14 keV P+ ions.

The physics model for the ion–solid interaction shows an unprecedented upper-bound single-ion-detection confidence of 99.85 ± 0.02% for near-surface implants. As a result, the practically controlled silicon doping yield is limited by materials engineering factors including surface gate oxides in which detected ions may stop. For a device with 6 nm gate oxide and 14 keV P+ implants, a yield limit of 98.1% is demonstrated. Thinner gate oxides allow this limit to converge to the upper bound. Deterministic single-ion implantation can therefore be a viable materials engineering strategy for scalable dopant architectures in silicon devices.

Journal Information: Alexander M. Jakob et al, Deterministic Shallow Dopant Implantation in Silicon with Detection Confidence Upper‐Bound to 99.85% by Ion–Solid Interactions, Advanced Materials (2021). DOI: 10.1002/adma.202103235

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