Department of Applied Physics

Ultrasensitive bolometers

Bolometers used for qubit readout

Microwave photon detector

Detecting extremely low levels of microwave radiation has wide variety of applications. For example, astronomer could measure the cosmic microwave background radiation with much greater accuracy with the help of more sensitive bolometers. Furthermore, ultrasensitive microwave detectors are needed in the search of the hypothetical dark matter particle, axion, since it is predicted to decay into microwave frequency photons. Outside of astronomy, microwave detectors have applications in the field of quantum technology. When combined with a single photon source, a broad-band single-photon detector can be used as a quantum level network analyzer. However, perhaps the most important application for single-photon microwave detector is as a building block of a superconducting quantum computer.

We have developed an ultrasensitive microwave detector operating on the bolometric principle [Phys. Rev. B 90, 064505 (2014)]. The detector exploits the strong temperature dependence of superconductor–normal-metal–superconductor Josephson junctions. We demonstrated detection of packets containing 200 photons at 8.4-GHz frequency, which corresponds to 1.1 zJ of energy [Phys. Rev. Lett. 117, 030802 (2016)]. Lately, we have also measured that the noise-equivalent power (NEP) of a similar detector can reach values down to 20 zW/rtHz with the use of a JPA (Josephson parametric amplifier) [Communications Physics, vol.2, 124 (2019)], which is an order of magnitude smaller than in previously demonstrated detectors.
Recently, we developed a new detector for measuring energy quanta at unprecedented resolution [Nature586, 47–51 (2020)]. We experimentally demonstrated a bolometer that meet the threshold for circuit quantum electrodynamics, with a noise-equivalent power of 30 zeptowatts per square-root hertz, together with a thermal time constant of 500 ns. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution.
In the future, we will continue to increase the sensitivity of the detector in the pursuit of the elusive single-photon regime for thermal microwave detectors.

Artistic image of a bolometer based on SNS junctions

Figure. Artistic image of a hybrid superconductor-metal microwave detector based on our work in [Phys. Rev. Lett. 117, 030802 (2016)]. Figure credit: Ella Maru Studio.

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