New bolometer could lead to better cryogenic quantum technologies

A new type of bolometer that covers a broad range of microwave frequencies has been created by researchers in Finland. The work builds on previous research by the team and the new technique could potentially characterize background noise sources and thereby help to improve the cryogenic environments necessary for quantum technologies.

A bolometer is an instrument that measures radiant heat. Instruments have existed for 140 years and are conceptually simple devices.  They use an element that absorbs radiation in a specific region of the electromagnetic spectrum. This causes the device to heat up, resulting in a parameter change that can be measured.

Bolometers have found applications ranging from particle physics to astronomy and security screening. In 2019 Mikko Möttönen of Aalto University in Finland and colleagues developed a new ultra-small, ultralow-noise bolometer comprising a microwave resonator made of a series of superconducting sections joined by a normal gold-palladium nanowire. They found that the resonator frequency dropped when the bolometer was heated.

Measuring qubits

In 2020, the same group swapped the normal metal for graphene, which has a much lower thermal capacity and thus should measure temperature changes 100 times faster. The result could have advantages over current technologies used to measure the states of individual superconducting quantum bits (qubits).

Superconducting qubits, however, are notoriously prone to the classical noise of thermal photons, and in the new work Möttönen and colleagues, together with researchers at the quantum technology company Bluefors, set out to tackle this. The graphene bolometer focuses on sensing a single qubit, and on measuring the relative power level as quickly as possible to determine its state. In this latest work, however, the researchers were looking for noise from all sources, so they needed a broadband absorber. They also needed to measure the absolute power, which requires the calibration of the bolometer.

One of the applications that the team demonstrated in their experiments was the measurement of the amount of microwave loss and noise in the cables running from room temperature components to low-temperature components. Previously, researchers have done this by amplifying the low-temperature signal before comparing it to a reference signal at room temperature.

Very time consuming

“These lines have typically been calibrated by running a signal down, running it back up and then measuring what happens,” explains Möttönen, “but then I’m a little bit unsure whether my signal was lost on the way down or up so I have to calibrate many times…and warm up the fridge…and change the connections…and do it again – it’s very time consuming.”

Instead, therefore, the researchers integrated a tiny electrical direct-current heater into the thermal absorber of the bolometer, allowing them to calibrate the power absorbed from the surroundings against a power supply that they could control.

“You see what the qubit will see,” says Möttönen. The femtowatt-scale heating used for calibration – which is turned off during the operation of the quantum device – should have no meaningful effect on the system. The researchers eschewed graphene, reverting to a superconductor–normal metal–superconductor design for the junctions because of the greater ease of production and better durability of the finished product: “These gold-palladium devices will remain almost unchanged on the shelf for a decade, and you want your characterization tools to remain unchanged over time,” Möttönen says.

The researchers are now developing the technology for more detailed spectral filtering of noise. “The signal that comes into your quantum processing unit has to be heavily attenuated, and if the attenuator gets hot, that’s bad…We would like to see what is the temperature of that line at different frequencies to get the power spectrum,” Möttönen says. This could help to decide on what frequencies are best to choose or help to optimize equipment for quantum computing.

“It’s impressive work,” says quantum technologist Martin Weides of the University of Glasgow. “It adds to a number of existing measurements on the transfer of power in cryogenic environments required for quantum technologies. It allows you to measure from dc up to microwave frequencies, it allows you to compare both, and the measurement itself is straightforward…If you’re building a quantum computer, you’re building a cryostat, and you want to characterize all your components reliably, you probably would like to use something like this.”

The research is published in Review of Scientific Instruments.