Two milliseconds – or two thousandths of a second – is an extraordinarily long time in the world of quantum computing.
On these time scales, a blink of an eye – to a tenth of a second – is like an eternity.
Now a team of researchers from UNSW Sydney have broken new ground by proving that ‘spin qubits’ – properties of electrons representing the basic units of information in quantum computers – can hold information up to two milliseconds . Known as “coherence time,” the amount of time qubits can be manipulated into increasingly complicated calculations, the achievement is 100 times longer than previous benchmarks in the same quantum processor.
“Longer coherence time means you have more time for your quantum information to be stored – which is exactly what you need when performing quantum operations,” says PhD student Ms Amanda Seedhouse, whose work in theoretical quantum computing have contributed to the realization.
“Coherence time basically tells you how long you can do all the operations in whatever algorithm or sequence you want to do before you lose all the information from your qubits. »
In quantum computing, the longer you can keep spins moving, the better the chance that information can be retained during calculations. When the spin qubits stop spinning, the computation collapses and the values represented by each qubit are lost. The concept of coherence extension has already been confirmed experimentally by quantum engineers from UNSW in 2016.
Making the task even more difficult is the fact that working quantum computers of the future will need to track the values of millions of qubits if they are to solve some of humanity’s greatest challenges, such as finding effective vaccines, modeling systems weather and predict the impacts of climate change.
Late last year, the same team at UNSW Sydney solved a technical problem that had stuck engineers for decades on how to manipulate millions of qubits without generating more heat and interference. Rather than adding thousands of tiny antennas to control millions of electrons with magnetic waves, the research team found a way to use a single antenna to control all of the chip’s qubits by introducing a crystal called dielectric resonator. These results were published in Scientists progress.
This solved the problem of space, heat, and noise that would inevitably increase as more and more qubits come online to perform the mind-bending calculations that are possible when qubits represent not just 1 or 0 like conventional binary computers, but both at the same time. , using a phenomenon known as quantum superposition.
Global control vs individual control
However, this proof-of-concept achievement still left some challenges to be resolved. Lead researcher Ms Ingvild Hansen joined Ms Seedhouse in addressing these questions in a series of journal articles Physical examination B, Physical examination A and Applied physics exams — the latest article published just this week.
Being able to control millions of qubits with a single antenna was a big step forward. But while controlling millions of qubits at once is a great feat, working quantum computers will also need them to be manipulated individually. If all spin qubits spin at roughly the same frequency, they will have the same values. How can we control them individually so that they can represent different values in a calculation?
“We first showed theoretically that we can improve coherence time by continuously spinning qubits,” says Hansen.
“If you imagine a circus performer spinning plates, while they are still spinning, the performance can continue. Similarly, if we continuously drive qubits, they can retain information longer. We have shown that such “dressed” qubits have a time coherence of more than 230 microseconds [230 millionths of a second]. »
After the team showed that coherence times could be extended with so-called “dressed” qubits, the next challenge was to make the protocol more robust and show that globally controlled electrons can also be individually controlled so that they may contain different necessary values. for complex calculations.
This was achieved by creating what the team dubbed the “SMART” qubit protocol – sinusoidally modulated, always spinning and tailor-made.
Rather than spinning the qubits in circles, they manipulated them to rock back and forth like a metronome. Then, if an electric field is individually applied to any qubit – knocking it out of resonance – it can be put into a different tempo from its neighbors, but still moving at the same rate.
“Think of it like two kids on a seesaw moving forward and backward pretty much in sync,” Ms Seedhouse says. “If we nudge one of them, we can get them to reach the end of their arc at opposite ends, so one can be a 0 when the other is now a 1. . »
The result is that not only can a qubit be controlled individually (electronically) under the influence of a global control (magnetically), but the coherence time is, as mentioned before, significantly longer and suitable for quantum calculations.
“We have shown a simple and elegant way to control all qubits at once, which also comes with better performance,” says Dr. Henry Yang, one of the team’s principal investigators.
“The SMART protocol will be a potential route for large-scale quantum computers. »
The research team is led by Professor Andrew Dzurak, CEO and founder of Diraq, a UNSW spin-off company that develops quantum computing processors that can be fabricated using standard silicon chip fabrication.
Next steps
“Our next goal is to show it works with two-qubit computations after showing our proof of concept in our experimental paper with one qubit,” says Hansen.
“After that, we want to show that we can also do this for a handful of qubits, to show that the theory is proven in practice. »
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