New Super-Sensitive Bolometer Could Find its Way into Quantum Computers

Scientists from Aalto University and VTT Technical Research Centre of Finland have developed a super-sensitive bolometer, a kind of thermal radiation detector. The new radiation detector was developed using a gold-palladium mixture and makes it simpler to determine the strength of electromagnetic radiation in real time.

Colored electron microscope image of the nanobolometer. The dark oval at the bottom left represents a 1.3-micrometer-long Ralstonia mannitolilytica bacterium. (Image credit: Roope Kokkoniemi/Aalto University)

Bolometers are used extensively in thermal cameras in the construction sector, and to measure cosmic radiation in satellites.

The latest progress may help bolometers make their way into quantum computers. In case the new radiation detector succeeds in working as well in space as it does in the lab, it can even be used for more accurate measurement of cosmic microwave background radiation in space.

The new detector is extremely sensitive, and its noise level—how much the signal bounces around the correct value, is only one-tenth of the noise of any other bolometer. It is also a hundred times faster than previous low-noise radiation detectors.

Mikko Möttönen, Joint Professor of Quantum Technology, Aalto University and VTT

Initially, the research team developed a radiation detector using gold. However, it broke within a few weeks since gold does not match with aluminum, which is utilized as a superconductor in the detector. The researchers overcame this by using a combination of gold and palladium, which is very sturdy but an uncommon material in bolometers.

In addition to the material, the secret of the new radiation detector lies in its really small scale. The nanowire running through the middle of the radiation detector is only about a micrometre long, two hundred nanometres wide and a few tens of nanometres thick,” stated Roope Kokkoniemi, who examined the bolometer at Aalto University.

A bolometer functions by measuring the heating effect of radiation. Upon getting heated, the electrical properties of a bolometer vary, and this can be measured very accurately. If the bolometer is smaller, the radiation needed to heat it can be minimized.

A small radiation detector has a low heat capacity, so weak radiation provides a stronger signal,” Kokkoniemi explains.

Better Protection

Quantum computers operate in cryostats, extremely cold super-freezers, in which even the smallest amount of excess radiation causes a lot of disturbance. As nanobolometers are very sensitive, they could conveniently measure the level of excess radiation in the cryostat in order to reduce the radiation through better protection.

Mikko Möttönen, Joint Professor of Quantum Technology, Aalto University and VTT

In addition, the bolometer could be employed to read the value of quantum bits (qubits). But to achieve that, the bolometer has to be even faster.

In order to read quantum information in superconducting quantum computers several times in a row without it degrading in between, the bolometer would have to be about a hundred times faster.

Mikko Möttönen, Joint Professor of Quantum Technology, Aalto University and VTT

During this research, microwave amplifiers were also developed. Their function is to reinforce the signal, but they also generate noise. The superconducting microwave amplifier built by VTT successfully halved the bolometer noise when compared to the leading commercial amplifier used.

The bolometer was built by the Quantum Computing and Devices research group headed by Mikko Möttönen. His team collaborates with Quantum Technology Finland, and uses the facilities at Micronova, which is part of the Otanano research center. The research paper has been published in the October 11th issue of Communications Physics journal.


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