From sand to superposition: A key step towards a powerful silicon quantum computer

Whether it’s our phones, cars, televisions, medical devices or even washing machines, we now have computers everywhere.

Using bigger computers, we solve bigger problems like managing the operation of a power grid, designing an aircraft, predicting the weather or providing  different types of Artificial Intelligence (AI). 

But all these machines work by manipulating data in the form of ones and zeros (bits) using classical techniques that have not changed since the abacus was invented in antiquity.

Realising the benefits of quantum computing

Humankind’s tireless technological advance now faces us with problems that cannot be solved even with the most powerful classical supercomputers.

For those challenges, we need a quantum computer. By employing the strange rules of quantum mechanics, we may be able to benefit many sectors including vaccine and drug design, financial risk management, industrial data processing, secure data and communications systems as well as machine learning and AI.

Quantum computers store information in quantum bits or ‘qubits’. A qubit can be any quantum object with two or more states – for example, a single electron that can be in a spin-up or spin-down state.

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Machine learning to scale up the quantum computer

Using microwaves or laser beams these qubits can be manipulated and even put into states that are a quantum mechanical mixture of one and zero – a condition known as ‘superposition’. This versatility builds the framework for the awesome potential of a quantum computer.

Quantum computers can deal with computational tasks so that otherwise impossible calculations – processes that would take centuries on classical supercomputers – can be performed within hours.

But tackling complex computational problems relevant to society will need a powerful quantum computer – with a chip architecture, size and complexity comparable to those of state-of-the-art classical processors.

In other words, a quantum processor with an immense number of physical qubits, arranged in ordered arrays, or ‘scalable arrays’. 

Building quantum devices

Silicon – made from beach sand – is the key material for today’s information technology industry because it is an abundant and versatile semiconductor.

We are already building quantum devices made from silicon engineered with dopant atoms – impurities intentionally added from other elements to change the properties of silicon.  

We have shown how these devices can be programmed with quantum states to form the qubits for a quantum computer.

However, the roadblock so far is that qubits are very susceptible even to tiny imperfections in their environment causing the qubit to lose its information (known as decoherence) requiring a reset of the computation.

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Lightning in a diamond to power the quantum revolution

Our previous work showed that qubits made from dopant atoms in silicon are very durable when exposed to changes in the environment. 

Now, our latest study demonstrates how to construct large arrays of single dopant atoms in a silicon chip, that could form the basis of a robust quantum computer.

The desirable attributes of silicon and its dopant atoms for making durable qubits allow the adaption of standard silicon fabrication processes for new purposes in the dawning quantum era. 

Our discovery is a way of building large-scale ordered arrays of atoms implanted into a silicon device. But not just any atoms. Our breakthrough is how to make these arrays out of elements that are promising new qubit candidates. 

Implanting dopant atoms into silicon is a standard technique for making silicon chips.

We discovered that if we equip our silicon chips with tiny surface electrodes, we can reliably register the implantation of a single atom from the electric signal it creates when stopping in the chip.  

These signals were found to be surprisingly strong, allowing us to construct atomic arrays in silicon devices with very high fidelity.

Now, new physical qubits, including antimony, bismuth and germanium offer powerful features that  provide new options for silicon quantum computers

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Making scalable donor arrays in silicon

As our new paper shows, the technique even works for diatomic antimony molecules so that each implantation event results in closely spaced pairs of antimony atoms.

These pairs are able to host many high-quality physical qubits that can be controlled with a single electronic gate, known as ‘multi-qubit-gate-operation’.

Now that we have shown that our novel technique works, the most important next step is to build a quantum processor from atomic arrays that are configured with the necessary circuitry to program and control the qubit interactions.

The ability to create scalable atomic arrays developed from well-established industry-manufacturing tools adapts silicon, the most important element for classical computing to build reliable quantum computers.

Banner image: Tony Melov. Artistic representation of an engineered atomic array in silicon. In this example, each site of the array hosts two closely spaced antimony atoms. This atomic pair could serve as a building block to realise and control up to six nuclear spin qubits with a single electronic gate.