Australian researchers believe they have solved a decades-old problem that could eventually lead to the development of larger and useful quantum computers.
Most important points:
- The solution could potentially manage millions of qubits at once
- This is a necessary step for the development of large quantum computers
- Billions are invested in the race to develop the first useful quantum computers
Quantum computing has the potential to solve certain problems millions of times faster than conventional computers.
But despite billions of dollars in investment, it remained tantalizingly out of reach.
In research published today in scientific progressUNSW’s team of researchers has proposed a new way to control millions of qubits, the building blocks of quantum computers.
Today’s largest quantum computers have only a few dozen qubits, but millions of qubits will be needed to develop new pharmaceutical drugs or build more accurate climate models.
Historically, the problem has been how to control millions of qubits at once without overcrowding or overheating the miniature-sized quantum chip.
Known as the problem of achieving “global control”, it was identified in the late 1990s and remained in the “unsolved” basket for two decades.
But one day in mid-2019, Jarryd Pla, a young researcher, walked into the office of Professor Andrew Dzurack, one of the leading quantum computing researchers in the country, with an idea about using magnetic fields to control qubits.
“I remember Jarryd coming into the office and talking to me about the idea,” Professor Dzurack said.
“I thought, ‘Oh my god, if that works, it will solve all our problems’.
“And it has.”
A very tight, very cold problem
Before we get to the idea of Dr. Pla, we need to understand why heat is the great enemy of quantum computers.
Qubits are very small and strange – traditional computers store information as zeros or ones, but in a quantum computer, qubits can be both numbers at the same time.
This ability, also known as superposition, means that quantum computers have the ability to perform multiple calculations at once.
But there is a catch. To maintain their quantum capabilities, qubits must be kept very cold.
“It’s incredibly cold,” said Andrew Doherty, a quantum physicist at the University of Sydney, who was not involved in the study.
Many of the quantum computing technologies operate at around 0.1 Kelvin, or just above absolute zero — that’s -273.15 degrees Celsius.
Quantum chips have to work in huge dilution refrigerators in which isotopes of liquid helium are pumped through a system of tubes that looks a bit like a chandelier or an inverted bird’s nest.
And that’s the easy thing.
To be useful for calculations, qubits must also be checked.
This is often done by passing a current through a wire close to the qubit to create a magnetic field that controls its spin.
The direction of rotation is equal to a zero or one in a binary code, Professor Dzurack explained.
“You can think of the spin-down as a zero and the spin-up as one,” he said.
These control wires transfer heat. As more qubits are installed, more wires are needed to control them and it becomes more difficult to keep the qubits cool.
Then there’s the real estate problem: Not only do qubits need to be kept cold, they also need to be packed close together to maintain their quantum properties.
In some models, they are 100 nanometers apart, which is one-tenth of a micron. A human hair is about 70 microns thick.
The control wires take up valuable space on a miniature-sized quantum chip that must also contain millions of qubits.
Manage four million qubits at once
To get around these problems, Dr. Plan on removing the wires altogether and replacing them with a magnetic field from above the chip that can manipulate all the qubits at once.
Enter something called a “dielectric resonator”.
While it sounds like the heart of a time machine, it’s actually a pretty standard device—as an antenna for microwave frequencies, it’s used in cell phones to transmit high-frequency signals.
The crystal prism developed by the researchers is made of potassium tantalate, which allows it to operate at very low temperatures.
“It’s actually a small transparent crystal,” said Dr. Pla.
The crystal is zapped with microwaves, which get trapped and bounce around.
In doing so, they generate a magnetic field that emanates from the bottom of the crystal in a flat and uniform plane.
The researchers found that the field generated by the resonator could control an area that could potentially contain four million qubits; enough for global control.
In addition, they found that relatively little current is required to create the magnetic field, which, crucially, means not too much heat.
Promising but only the ‘beginning of the story’
David Reilly, an experimental physicist and director of the University of Sydney’s quantum computer lab not involved in the study, said the results were “very interesting.”
“It’s an important development that solves a problem that many researchers have been concerned about,” he said.
Professor Doherty said the solution was a “promising approach to an important problem”.
“It’s great science with excellent results,” he said.
While promising, Professor Doherty said the results were preliminary and there was still a long way to go before definitive global control was proven.
“They have roughly one qubit sitting under a magnetic field,” he said.
“What they don’t show is they don’t have a million qubits.”
Another challenge is the production of qubits in large quantities.
“To get a million on a chip, qubits don’t have to be a handmade thing, but a large-scale fabrication,” said Professor Doherty.
“That will be quite a challenge.”
How far are we from seeing a quantum computer?
While the reality of quantum computing is still a while away, Professor Doherty, who recently returned from a two-year stint with Silicon Valley quantum computing company PsiQuantum, is confident we’ll get there.
He said he was encouraged by the amount of money being spent on quantum computer research.
In 2020, investors poured $557.5 million into 28 venture deals for quantum computing companies in the US and Canada; that’s more than three times the amount spent in 2019.
In July of this year, PsiQuantum, co-founded by two Australians, raised nearly $450 million to build what it believes will be the world’s first commercially viable quantum computer.
“The UNSW team’s solution to global control “is the beginning of a story rather than the end of one,” said Professor Doherty.
“I’m very curious where the research will go.”