New bolometer could enhance quantum circuit analysis by detecting Josephson radiation up to 100 GHz

Josephson junctions are fundamental components in many superconducting quantum devices, including the qubits that form the heart of quantum computers. As the field of quantum computing rapidly advances, understanding and improving the performance of these crucial components becomes a priority. A central challenge lies in detecting and characterizing Josephson radiation, a high-frequency alternating current generated by these junctions that carries information about their behavior and energy dissipation. Now, researchers have developed a groundbreaking technique, described in Nature Nanotechnology, that promises to overcome this challenge.

At the heart of this new approach lies an on-chip hot-electron bolometer (HEB), a highly sensitive thermometer comprised of a normal metal nano-absorber whose temperature is measured by a normal-metal–insulator–superconductor (NIS) thermometer. This device precisely measures the temperature change caused by the heat from Josephson radiation.

Understanding Josephson junctions’ dissipative dynamics—how they lose energy to their surroundings—could potentially lead to gains in performance and coherence of quantum devices. In turn, that would directly impacts the ability to perform complex quantum calculations. By understanding how energy is dissipated, for example, researchers could design junctions and circuits that minimize energy loss, leading to longer qubit coherence times and more robust quantum computations.

In 1983, scientists predicted that the Josephson junction would become insulating as it nears absolute zero when connected in series with a resistance greater than about 6.5  k⁢Ω. Microwave measurements from recent years have opened up new lines of inquiry on the matter, as a 2023 Nature Communications noted.

Breaking down the bolometer research

Senior author Jukka P. Pekola, Ph.D., from Aalto University highlights the innovative nature of the bolometer research. In the abstract they note: “Here we design and build an engineered on-chip reservoir connected to a Josephson junction that acts as an efficient bolometer.” This on-chip bolometer, essentially a specialized reservoir designed to absorb energy, provides a new way to detect Josephson radiation. The bolometer “converts the a.c. Josephson current at microwave frequencies up to about 100 GHz into a temperature rise measured by d.c. thermometry,” the paper noted. This conversion allows researchers to study the typically elusive Josephson radiation using simple and readily available d.c. measurement techniques.

The team also developed a detailed circuit model. It accurately captures both the electrical behavior of the Josephson junction and the measured power. “The present experiment demonstrates an efficient, wide-band, thermal detection scheme of microwave photons and provides a sensitive detector of Josephson dynamics beyond the standard conductance measurements,” Pekola concluded.
This new thermal detection approach offers several notable features:

1. Efficiency: It provides a way to study JJ behavior by converting the a.c. signal into a d.c. one.
2. Wide-band detection: The technique can capture radiation up to about 100 GHz, covering the typical frequencies of Josephson radiation in these devices.
3. Complementary measurements: It allows for detailed investigations of JJ dynamics that complement standard conductance measurements. For instance, it enables the direct measurement of the power emitted by the Josephson junction, providing insights into energy dissipation mechanisms that are not readily accessible through conventional electrical measurements.

Looking ahead

This latest breakthrough builds on a history of advances in quantum computing. In 2016, for instance, researchers at Aalto University broke the world record for energy resolution in thermal photodetection, developing a superconducting microwave detector with potential applications in quantum computing. Two years later, in 2018, scientists at Caltech demonstrated how superconducting metamaterials could trap quantum light, offering a new platform for building complex quantum circuits.

The recent research opens up new possibilities for studying Josephson junction behavior and could lead to improvements in quantum device performance. The technique’s ability to provide complementary information to standard electrical measurements may prove valuable in future quantum technology developments. It’s part of a broader trend of innovation in quantum computing, including alternative approaches using microwave signals developed at Aalto University, demonstrating the field’s ongoing evolution.