This nanodevice can measure the absolute power of microwave radiation

It offers a significant step forward for quantum technologies.
Loukia Papadopoulos
Photo of a bolometer.jpg
Photo of a bolometer.


Scientists in Finland have engineered a nanodevice called a bolometer that can measure the absolute power of microwave radiation down to the femtowatt level at ultra-low temperatures.  

This is according to a press release by Aalto University published last month.

A team led by Mikko Möttönen, associate professor of quantum technology at Aalto and VTT, worked with researchers at the quantum-technology companies Bluefors, IQM, and VTT Technical Research Centre of Finland to create the new and improved bolometer. 

“We added a heater to the bolometer, so we can apply a known heater current and measure the voltage. Since we know the precise amount of power we’re putting into the heater, we can calibrate the power of the input radiation against the heater power. The result is a self-calibrating bolometer working at low temperatures, which allows us to accurately measure absolute powers at cryogenic temperatures,” Möttönen said. 

Microwave power

Furthermore, according to Russell Lake, director of quantum applications at Bluefors, the new bolometer is an important step forward in measuring microwave power.  

“Commercial power sensors typically measure power at the scale of one milliwatt. This bolometer does that accurately and reliably at one femtowatt or below. That’s a trillion times less power than used in typical power calibrations,” he said.

It could also serve to improve the performance of quantum computers

“For accurate results, the measurement lines used to control qubits should be at very low temperatures, void of any thermal photons and excess radiation. Now with this bolometer, we can actually measure that radiation temperature without interference from the qubit circuitry,” Möttönen said.

This is because it covers a very broad range of frequencies.  

“The sensor is broadband, which means that it can measure what is the power absorbed in various frequencies. This is not a given in quantum technology as usually the sensors are limited to a very narrow band,” said Jean-Philippe Girard, a scientist at Bluefors who also previously worked at Aalto on the device. 

The new invention will now provide a major boost to quantum technology fields.  

“Measuring microwaves happens in wireless communications, radar technology, and many other fields. They have their ways of performing accurate measurements, but there was no way to do the same when measuring very weak microwave signals for quantum technology. The bolometer is an advanced diagnostic instrument that has been missing from the quantum technology toolbox until now,” Lake said.

“That shows that this is not just a lucky break in a university lab, but something that both the industrial and the academic professionals working in quantum technology can benefit from,” Möttönen concluded in the statement.

The work was published in the Review of Scientific Instruments

Study abstract:

Recently, great progress has been made in the field of ultrasensitive microwave detectors, reaching even the threshold for utilization in circuit quantum electrodynamics. However, cryogenic sensors lack the compatibility with broad-band metrologically traceable power absorption measurements at ultralow powers, which restricts their range of applications. Here, we demonstrate such measurements using an ultralow-noise nanobolometer, which we extend by an additional direct-current (dc) heater input. The tracing of the absorbed power relies on comparing the response of the bolometer between radio frequency and dc-heating powers traced to the Josephson voltage and quantum Hall resistance. To illustrate this technique, we demonstrate two different methods of dc-substitution to calibrate the power that is delivered to the base temperature stage of a dilution refrigerator using our in situ power sensor. As an example, we demonstrate the ability to accurately measure the attenuation of a coaxial input line between the frequencies of 50 MHz and 7 GHz with an uncertainty down to 0.1 dB at a typical input power of −114 dBm.