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Cryogenic Sensor Enabling Broad-Band and Traceable Power Measurement

Recently, great progress has been made in the field of ultrasensitive microwave detectors, reaching even the threshold for utilization in circuit quantum electrodynamics (cQED). However, cryogenic sensors lack the compatibility with broad-band metrologically traceable power absorption measurements at ultralow powers, which restricts their range of applications. Our newest research publication presents a solution for this challenge.

Researchers from Bluefors, Aalto University’s Department of Applied PhysicsIQM, and VTT Technical Research Centre of Finland have developed a sensitive bolometer device for traceable microwave power absorption measurements relying on dc substitution. The tracing of the absorbed power relies on comparing the response of the bolometer between radio frequency (rf) and dc-heating powers traced to the Josephson voltage and quantum Hall resistance.  The device operates at low temperatures, exhibits a broad input bandwidth and is suitable for characterization of devices operating in the framework of cQED.

Image of the power sensor on a silicon chip. The measured radiation arrives from the horizontal transmission line on the left and the power level is read out using the line of the right. The red line is used to apply a known heater power to calibrate the power reading. the inset on the right shows a zoom-in of the absorber and thermometer parts of the device. Credit: Jean-Philippe Girard/Aalto University

Ultralow-Noise Nanobolometer

The team has developed a detector that is native to the cryogenic environment where quantum computers operate. Importantly, measurements from the detector can be traced to existing SI standards for greatly improved accuracy.

Much like a room-temperature microwave power meter, the device converts incoming microwave signals into heat that can be detected by an electrical circuit. Thanks to its engineered-nanoscale components, the sensor is extremely sensitive to small amounts of heating so that it can detect microwaves in the regime relevant for quantum technology.

At room temperature, microwave power sensors are ubiquitous and underpin technologies such as wireless communications, radar, Bluetooth, among others. However, accurate power meters for the cryogenic environment where quantum computers operate are still a great challenge because of the extremely small operation power levels. The new result improves the accuracy, dynamic range, and bandwidth of the measurements.

Demonstration of the equivalence of rf heating and dc heating.

Demonstration of the Sensor

The researchers have been testing the sensor and the results were published in the Review of Scientific Instruments. The article was chosen as the Editor’s Pick.

To illustrate the technique, the researchers demonstrated two different methods of dc-substitution to calibrate the power that is delivered to the base temperature stage of a dilution refrigerator using the in-situ power sensor.

As an illustration of the utility of the introduced microwave sensor, the demonstration included the calibration of a heavily attenuated rf line including several microwave components at low temperatures. It was achieved by comparing the response of the bolometer to a heater signal applied through the rf line and to heating applied through a dc line. From this comparison, the frequency-dependent attenuation of the rf line was accurately determined with an uncertainty of 0.33 dB for an absorbed power of −114 dBm.

Practical Benefits in Measuring the Power Radiated to a Qubit

Operating a power meter in situ in the cryostat has several benefits for characterizing cryogenic components. For example, the typical signals used in quantum technology are too weak to be brought back up to room temperature for detection. Therefore, the traceability of the new detector enables accurate measurements of real signals for debugging and characterizing quantum hardware. In future experiments, the bolometer is planned to be used for microwave signal calibration in qubit experiments, both for investigations of readout with thermal detectors, and pre- qualification of qubit wiring schemes through accurate measurements of line attenuation and noise power that reaches the sample space. The demonstration aims to facilitate the implementation of bolometers in experiments on cryogenic electronics as traceable power sensors and to enable highly accurate power measurements.

Bluefors Continues to Develop Solutions for Quantum Research and Applications

Bluefors is here to enable the quantum breakthrough and support scientists in their work. Through the work of our Quantum Applications team, we continue delivering new innovations to the community and create solutions for cryogenic measurement challenges. View our recent publications to learn more about our research.