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Monitoring qubits for the first time

A team of UNSW researchers has taken yet another important step towards realising a functional quantum computer by experimentally demonstrating a platform that will allow for quantum mistakes to be corrected.

Quantum computers have the potential to revolutionise the way scientists design new drugs, create complex simulations of weather patterns or take artificial intelligence to new heights.

They use a binary language of 1s and 0s as do traditional computers – in which strings of bits, which are related to transistors being on or off, are used to perform calculations that in turn perform the task that the computer was asked to do.

Instead of the transistor devices used to realise traditional bits, a quantum bit (qubit) in the UNSW design contains a single electron. Electrons have an intrinsic property called spin which can be either up or down, meaning the qubit binary information can be a 1 or a 0. What makes it exciting is a phenomenon called the superposition principle which, often explained using the famous analogy of ‘Schrodinger’s cat’, means that until it’s observed, the electron is spinning both up and down, so the qubit is both 1 and 0 simultaneously. This superposition principle, combined with another quantum phenomenon known as ‘entanglement’, means that a quantum computer can perform a range of useful calculations of unprecedented complexity at an astounding rate.

The UNSW team, supported by international collaborators in the US and Japan, is led by ANFF-NSW Director, Andrew Dzurak, who previously demonstrated the first quantum bit that could be made using a modified silicon transistor, also called a silicon CMOS quantum dot. This qubit state (or binary value) can be changed without influencing the surrounding qubits, and by combining this with a new readout process the new integrated device platform will allow larger numbers of qubits to be monitored for errors.

This means that when larger strings of qubits are made in the future, and set to a particular combination of 1s or 0s, and there will now be a way to check that they’ve actually done as they’re told.

“This is an important milestone for us on the path to performing quantum error correction with spin qubits, which is going to be essential for any universal quantum computer,” says Dzurak.

“Quantum error correction is a key requirement in creating large-scale useful quantum computing because all qubits are fragile, and you need to correct for errors as they crop up,” says lead author, Michael Fogarty, who performed the experiments as part of his PhD research with Professor Dzurak at UNSW.
The achievement is extraordinary, but what makes it still more exciting is that it’s been done using an established technology that’s been essential to the computing industry for years called silicon CMOS (complementary metal-oxide-semiconductor). Dzurak explained, “By using silicon CMOS technology we have the ideal platform to scale to the millions of qubits we will need, and our recent results provide us with the tools to achieve spin qubit error-correction in the near future.”

“It’s another confirmation that we’re on the right track. And it also shows that the architecture we’ve developed at UNSW using the ANFF-NSW fabrication facility has, so far, shown no roadblocks to the development of a working quantum computer chip.”
“And, what’s more, one that can be manufactured using well-established industry processes and components.”

How it’s done
The process for creating these qubit devices in silicon was developed by Dzurak’s team using the ANFF-NSW cleanroom laboratories at UNSW, incorporating both industry-style CMOS fabrication processes and cutting-edge nanofabrication techniques.
A series of electrical contacts to the device, which allow for qubit measurement, are built by repeatedly laying down and removing layers of metal before an insulating layer of silicon dioxide is added.

To control the qubits, layers of tiny gate electrodes, less than 30 nanometres in width, were fabricated using ANFF-NSW’s state-of-the-art electron-beam writer and an aluminium evaporator in a process known as ‘metal lift-off’. The electron-beam writer allows a nanoscale template to be defined against the wafer. Just like when spray painting over a stencil, evaporated aluminium can reach the silicon wafer, resulting in a conductive pattern of electrodes.
Subsequent layers of electrodes can be aligned to one another with better than 5-nanometre alignment accuracy, a capability that is essential to achieving working structures. It requires more than 40 precise fabrication steps, all conducted within the ANFF cleanroom at UNSW, to realise a final device that can be used for these qubit proving experiments.

More information
View the UNSW story here