Major breakthrough! Ultra-pure silicon is expected to trigger a quantum computin

Recently, the University of Manchester in the UK, in collaboration with the University of Melbourne in Australia, has developed a type of ultra-pure silicon that can be used to construct high-performance quantum bit devices.

Silicon is one of the most abundant elements on Earth. By heating high-purity silicon sand (silicon dioxide) in an electric furnace, one can obtain silicon with a purity of 99%. Subsequently, various methods are employed to transform the 99% "pure silicon" into "polycrystalline silicon" with a purity of eleven 9s after the decimal point. Next, by using the Czochralski process or the floating zone method, the pure silicon is melted and grown into single-crystal silicon ingots, which serve as raw materials for manufacturing chips or solar cells. However, the purity of this single-crystal silicon is still not sufficient for quantum computers.

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Isotopes of Silicon

Unlike classical computers, quantum computation fundamentally involves manipulating quantum bits at the physical level. We use coherence time to describe the duration for which a quantum system can maintain its quantum properties, such as superposition or entanglement states. This is a key parameter for measuring the performance of quantum computers.

A long coherence time implies that more complex quantum computations can be executed, enabling more sophisticated quantum algorithms. In experiments with silicon-based quantum computers, scientists have found that no matter how many 9s after the decimal point the purity of silicon reaches, its performance in the quantum realm is still unsatisfactory.

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This is because silicon has three stable isotopes: silicon-28, silicon-29, and silicon-30. Regardless of how high the purity of silicon extracted from nature is, the ratio of these three isotopes remains constant, with silicon-28 accounting for 92.23%, silicon-29 for 4.67%, and silicon-30 for 3.10%.There are slight differences in the isotopic ratios of silicon in rocks from different locations on Earth. Geologists, biologists, and environmental scientists are using these subtle differences to use the silicon ratio as a sensitive tracer in geology.

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Quantum Mechanical Effects of Isotopes

In the quantum world, nuclear spin is used to describe the angular momentum of atomic nuclei, which is one of the fundamental quantum mechanical properties of atomic nuclei. Nuclear spin is quantized, meaning it can only take on specific values. For a given atomic nucleus, the value of nuclear spin I can be 0, 1/2, 1, 3/2, etc.

In the nucleus of silicon-28, both the number of protons and neutrons are even (there are 14 protons and 14 neutrons in the nucleus). According to the rules of nuclear spin, even-even nuclei typically have zero spin. However, in silicon-29 and silicon-30, at least one nucleon (proton or neutron) is unpaired, resulting in a non-zero spin. Silicon-29 (with 15 neutrons) has a nuclear spin of 1/2, and silicon-30 (with 16 neutrons) has a nuclear spin of 2.

Scientists have achieved the highest purity of silicon-28 to date.

The spin state of electrons can be used as quantum bits in quantum computers, but silicon-29 and silicon-30 with spin will couple with electron spin, leading to a shorter coherence time for electrons. Silicon-28 with zero nuclear spin can significantly increase the coherence time of electrons. Therefore, as long as the purity of silicon-28 can be increased, the robustness of silicon-based quantum computers can be significantly improved.

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How to Obtain Purer Silicon-28Common isotope enrichment methods include centrifugal separation, electromagnetic separation, chemical vapor deposition (CVD), laser separation, gas diffusion, and ion implantation, among others.

Heavier isotopes are pushed towards the outer wall of the centrifuge; for example, uranium-235 is 3% lighter than uranium-238, and its purity can be gradually increased through continuous centrifugation.

Electromagnetic separation takes advantage of the deflection of charged particles in a magnetic field; isotopes of different masses will be deflected to varying degrees, allowing the use of a baffle to remove isotopes other than the target.

Chemical vapor deposition exploits the slight differences in reaction rates when different isotope gases undergo chemical reactions; for instance, lighter silicon-28 is more likely to participate in the reaction and form a silicon film on the substrate surface.

A research team from the University of Manchester in the UK and the University of Melbourne in Australia has found the current method to obtain the purest silicon-28. They used a Wien filter to obtain a high-purity silicon-28 ion beam, then focused it to achieve a high-density ion beam. By irradiating natural silicon wafers with this high-density ion beam, they were able to displace silicon-29 and silicon-30 in the target area's lattice.

According to analysis by ion mass spectrometry and transmission electron microscopy, they observed a 250nm thick layer of high-purity silicon-28 on the silicon wafer surface, with residual silicon-29 at 2.3 ± 0.7 ppm and silicon-30 at 0.6 ± 0.4 ppm; no residual carbon or oxygen elements were detected. This experimental result is the best among current global experimental projects.

In the future, with further optimization and process improvements, this technology is expected to produce the purest silicon in the world, offering a possibility for creating quantum computers with 1 million quantum bits.