quantum computing
Artistic image of a graphene bolometer controlled by electric field. Credit: Heikka Valja.

Physicists at VTT Technical Research Center of Finland and Aalto University have built up a new detector for measuring energy quanta at unprecedented resolution. This finding could help bring quantum processing out of the research lab and into real-world applications. The outcomes have been distributed today in Nature.

The kind of finder detector the group works is known as a bolometer, which measures the energy of approaching radiation by measuring the amount it heats up the detector. Professor Mikko Möttönen’s Quantum Computing and Devices group at Aalto has been building up their skill in bolometers for quantum computing over the past decade, and have now built up a gadget that can match current state-of-the-art detectors utilized in quantum PCs.

“It is amazing how we have been able to improve the specs of our bolometer year after year, and now we embark on an exciting journey into the world of quantum devices,” says Möttönen. 

Measuring the energy of qubits is at the core of how quantum computers work. Most quantum PCs presently measure a qubit’s energy state by measuring the voltage induced by the qubit. However, there are 3 issues with voltage measurements: right off the bat, firstly, measuring the voltage requires extensive amplification circuitry, which may restrict the scalability of the quantum computer; secondly, this circuitry consumes a lot of power; and thirdly, the voltage measurements carry quantum noise which presents mistakes in the qubit readout. Quantum computer scientists trust that by utilizing bolometers to measure qubit energy, they can conquer these complications, and now Professor Möttönen’s team has created one that is sufficiently fast and sensitive enough for the job.

“Bolometers are now entering the field of quantum technology and perhaps their first application could be in reading out the quantum information from qubits. The bolometer speed and accuracy seems now right for it,” says Professor Möttönen.

The team had previously produced a bolometer made of a gold-palladium alloy with unparalleled low noise levels in its measurements, but it was still too slow to measure qubits in quantum computers. The breakthrough in this new work was achieved by swapping from making the bolometer out of gold-palladium alloys to making them out of graphene. To do this, they collaborated with Professor Pertti Hakonen’s NANO group—also at Aalto University—who have expertise in fabricating graphene-based devices. Graphene has a very low heat capacity, which means that it is possible to detect very small changes in its energy quickly. It is this speed in detecting the energy differences that make it perfect for a bolometer with applications in measuring qubits and other experimental quantum systems. By swapping to graphene, the researchers have produced a bolometer that can make measurements in well below a microsecond, as fast as the technology currently used to measure qubits.

“Changing to graphene increased the detector speed by 100 times, while the noise level remained the same. After these initial results, there is still a lot of optimisation we can do to make the device even better,” says Professor Hakonen.

Now that the new bolometers can compete when it comes to speed, the hope is to utilize the other advantages bolometers have in quantum technology. While the bolometers reported in the current work performs on par with the current state-of-the-art voltage measurements, future bolometers have the potential to outperform them. Current technology is limited by Heisenberg’s uncertainty principle: voltage measurements will always have quantum noise, but bolometers do not. This higher theoretical accuracy, combined with the lower energy demands and smaller size—the graphene flake could fit comfortably inside a single bacterium—means that bolometers are an exciting new device concept for quantum computing.

The next steps for their research are to resolve the smallest packets ever observed using bolometers in real-time and to use the bolometer to measure the quantum properties of microwave photons, which not only have exciting applications in quantum technologies such as computing and communications, but also in the fundamental understanding of quantum physics.

Many of the scientists involved in the researchers also work at IQM, a spin-out of Aalto University developing technology for quantum computers. “IQM is constantly looking for new ways to enhance its quantum-computer technology and this new bolometer certainly fits the bill,” explains Dr. Kuan Yen Tan, Co-Founder of IQM who was also involved in the research.