Cold atoms realise quantum advantage predicted by CQT researchers
CQT’s Dimitris Angelakis and Supanut Thanaslip collaborated with researchers in China on the demonstration
CQT researchers and their collaborators performed a sampling task using a quantum processor of ultracold atoms, showing a computational advantage in calculating the probability distributions of the output bit strings. Image credit: Shutterstock.com/tomeqs.
There’s a new quantum speedup to note. Some years back, CQT researchers proved in theory that quantum advantage could be extracted from analogue quantum systems. Now, collaborators at the University of Science and Technology of China (USTC) have realised this advantage in an experiment with cold atoms. The team’s results are published on 18 June in Physical Review X.
CQT Principal Dimitris Angelakis and his former graduate student Supanut Thanaslip, co-authors on the original prediction, also contributed to the new work. Zhen-Sheng Yuan and Jian-Wei Pan led the experimental team at USTC.
Quantum devices are said to show advantage when they perform a task more efficiently than a classical device could, pointing to ways to build better technology. Past demonstrations of quantum advantage were done with random circuit sampling with superconducting circuits and boson sampling with photonic systems.
In this work, the researchers performed a sampling task using a quantum processor of ultracold atoms. They operated the processor as an analogue quantum system, meaning they made use of the atoms’ natural dynamics instead of implementing logic gates.
On the biggest task they tried, the quantum processor gave its output in 500 seconds, versus the eight days they estimated the Frontier supercomputer – currently the most powerful supercomputer in the world – would need.
Analogue quantum simulators
“With analogue quantum systems, you don’t need gates or digital-based fault tolerance. Our work shows that quantum advantage can be proven at the analogue Hamiltonian, in a way closer to the natural system state,” says Dimitris.
Dimitris holds joint appointments at the Institute for Quantum Computing and Quantum Technologies in Greece and the University of Southampton in the United Kingdom. He is also co-founder of quantum computing startup AngelQ, incorporated in Singapore. Supanut is currently a faculty member at Chulalongkorn University in Thailand.
Supanut had worked on the earlier theoretical prediction of quantum advantage early in his PhD. Researchers had long thought that analogue quantum simulators could outperform classical computers at simulating the dynamics of quantum many-body systems, which are large groups of quantum particles interacting with one another, but it had not been proven.
“What was lacking was some theoretical guarantee to say that the analogue quantum systems had an advantage over classical approaches,” said Supanut.
He, Dimitris and other groups members looked at analogue systems driven to thermalisation. That means energy is constantly given to the system, and the system is allowed to evolve into a thermal phase. Measuring the system gives a seemingly simple result – a string of 1s and 0s – but the researchers showed that classical computers struggle to predict the distributions of these bit strings.
The computational advantage comes from the probability distributions of the output bit strings. The researchers showed mathematically that it is classically intractable to calculate these distributions, thus predicting a quantum experiment could show advantage.
They first put their paper on the physics preprint server arXiv in 2020 before it was published in Quantum Science and Technology in 2023. The USTC team wrote to Dimitris after seeing the preprint to say they could realise the proof in their lab.
Ultracold demonstration
The USTC team implemented the experiment with up to 20 ultracold atoms spread across 64 sites. They start with rubidium atoms in a Bose-Einstein condensate, then drive a transition to the thermal phase. After the system evolves, they freeze the dynamics and expand the atoms. They measure the bit string by checking where the atoms end up, with 1 representing a site with an atom present and 0 an empty site.
By sampling from the system while varying how they drove it, the researchers could also distinguish between the system’s different phases and behaviour. Dimitris says, “It shows that probing a phase of quantum matter can itself constitute a quantum-advantage experiment – and that such an experiment can generate scientifically useful information about phases of matter.”
