Quantum scientists accurately measure power levels a trillion times lower than normal

(Nanowerk News) Scientists in Finland have developed a nanodevice that can measure the absolute power of microwave radiation down to the femtowatt level at very low temperatures – a scale trillions of times lower than that used routinely for verifiable power measurements. This device has the potential to significantly advance microwave measurements in quantum technology.

Measures very low power

Quantum science mostly happens at very low temperatures using devices called diluent refrigerators. Experiments must also be carried out at low energy levels – down to the energy level of a single photon or even less. Researchers must measure these very low energy levels as accurately as possible, which also means calculating heat – a persistent problem for quantum devices.

To measure heat in quantum experiments, scientists use a special type of thermometer called a bolometer. A very accurate bolometer was recently developed at Aalto by a team led by Mikko Möttönen, professor of quantum technology at Aalto and VTT, but the device has more uncertainties than they expected. Even though it allowed them to observe relative strength levels, they couldn’t determine the absolute amount of energy with great accuracy.

In the new study, Möttönen’s team worked with researchers at quantum technology companies Bluefors and IQM, and Finland’s VTT Technical Research Center to improve the bolometer. The work was published as an Editor’s Choice at Review of Scientific Instruments (“Cryogenic sensors enable wide-band, traceable power measurements”). Image of a power sensor on a silicon chip. The measured radiation comes from the horizontal transmission line on the left and the power level is read using the channel on the right. The red line is used to apply known heating power to calibrate the power reading. The inset on the right shows a zoom-in of the device’s absorber and thermometer sections. (Image: Jean-Philippe Girard, Aalto University)

‘We added a heater to the bolometer, so we could apply a known heater current and measure the voltage. Since we know the exact amount of power we put into the heater, we can calibrate the input radiated power against the heater power. The result is a self-calibrating bolometer that operates at low temperatures, which allows us to accurately measure absolute strength at cryogenic temperatures,’ says Möttönen.

According to Russell Lake, director of quantum applications at Bluefors, the new bolometer is a significant step forward in measuring microwave power.

‘Commercial power sensors typically measure power on the one milliwatt scale. This bolometer performs accurately and reliably at one femtowatt or lower. That’s one trillion times less power than used in a typical power calibration.’

Covers deep and wide scales

Möttönen explains that the new bolometer can improve the performance of quantum computers. ‘For accurate results, the measurement lines used to control the qubits must be at very low temperatures, free of thermal photons and excess radiation. Now with this bolometer, we can actually measure the temperature of the radiation without interference from the qubit circuitry,” he said.

The bolometer also covers a very wide frequency range.

‘The sensor is broadband, which means it can measure how much power is absorbed over a wide range of frequencies. This is not a given in quantum technology because normally sensors are limited to a very narrow band,’ says Jean-Philippe Girard, a scientist at Bluefors who also previously worked at Aalto on the device.

The team says the bolometer provides a major boost to the field of quantum technology.

‘Measuring microwaves occurs in wireless communications, radar technology and many other fields. They have their own way of making accurate measurements, but there is no way to do the same when measuring very weak microwave signals for quantum technology. The bolometer is a sophisticated diagnostic instrument that has been missing from the quantum technology toolbox until now,’ said Lake.