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Imaging the wavefunction of propagating microwave photons

Chen Qiming (Aalto University)

Abstract Propagating photons are essential carriers of quantum information in future quantum networks. While optical quantum networks thrive, detecting microwave photons at the quantum level remains a significant challenge. The superconductor-normal metal-superconductor (SNS) bolometer is a highly sensitive thermal detector that holds the current record of sensitivity for detecting the weakest signals down to the single-photon level. In this talk, I will first introduce the QCD efforts in pushing the power resolution of the SNS bolometer to 10^-18 W [1] and its energy resolution to 10^-21 J [2]. Then, I will show how this extreme sensitivity enables novel applications in quantum technology, including single-shot readout of superconducting qubits [3] and quantum correlation measurement of microwave photons [4]. Here, I will focus on our recent breakthrough in quantum state tomography [5]: By adapting the principles of computed tomography (CT) in medical imaging, we directly image the wavefunctions of propagating microwave photons using the SNS bolometer operating at millikelvin temperatures.

[1] R. Kokkoniemi, et al. Bolometer operating at the threshold for circuit quantum electrodynamics. Nature 586, 47–51 (2020)

[2] A. Gunyho, et al. Zeptojoule Calorimetry. arXiv:2412.14079 (2024)

[3] A. Gunyho, et al. Single-shot readout of a superconducting qubit using a thermal detector. Nat. Electron. 7, 288–298 (2024)

[4] A. Keranen and Q. Chen, et al. Measurement of microwave photon correlations at millikelvin with a thermal detector. Nat. Commun. 16, 3875 (2025)

[5] Q. Chen, et al. Computed tomography of propagating microwave photons, In preparation (2025)