Objective 1A: Scalable Detector Tomography
In our study “Electrical trace analysis of superconducting nanowire photon-number-resolving detectors” (Schapeler et al., Physical Review Applied 22, 014024 (2024)), we utilized Principal Component Analysis (PCA) to evaluate the photon number resolving capability of SNSPDs. Our work revealed that almost all information about photon number is contained in the rising edge of the SNSPD trace. This definite answer will enable future work that can focus on the optimization of photon-number resolution with SNSPDs.
We have also successfully demonstrated scalable quantum detector tomography (Schapeler et al., “Scalable quantum detector tomography by high-performance computing”. Quantum Science and Technology 10, 015018 (2025) - in press) in collaboration with the Paderborn Center for Parallel Computing (PC2). In this work, we have reconstructed the POVMs of a temporally multiplexed detector capable of detecting millions of photons.
Objective 1B: Quantum enhanced coherent detection
Currently, a theoretical study on homodyne detection with squeezed local oscillator is in progress. Alongside, we are developing a spontaneous parametric down-conversion (SPDC) source to serve as a bright squeezed light source to investigate this approach experimentally.
We have also investigated balanced homodyne detection with SNSPDs (Protte et al., “Low-noise balanced homodyne detection with superconducting nanowire single-photon detectors.” Optica Quantum 2.1: 1-6 (2024)) and showed that SNSPDs can be used to measure continuous variables of optical quantum states. By interfering a continuous-wave local oscillator with the vacuum state, we could show that the variance of the difference in count rates of two SNSPDs is linearly proportional to the incident photon flux and that the resulting shot-noise clearance is suited to measure large amounts of squeezing.
Objective 2A: Single-photon microspectrometry with superconducting detector arrays
Work on this objective began in earnest with the acquisition of the superconducting thin-film sputtering system.
Objective 2B: Spectral decomposition & optimisation of detector response
We made first steps toward spectrally resolved POVMs by investigating the manual polarization control and optical attenuation in optical fibers for the telecom range. In a Bachelor project, we used wavelength ranges from 910 nm to 980 nm and from 1440 nm to 1640 nm together with manual fiber polarization controllers optimized for a wavelength of 1550 nm and investigated whether it is possible to access the full Poincaré-Sphere.
Objectives 3A & 3B: Temporal mode selectivity with ultrafast cryogenic bias electronics; High dynamic range measurement of continuous illumination
Our work on cryogenic electronic gating of SNSPDs began prior to the start of the project, with the first publication (Thomas Hummel, et al "Nanosecond gating of superconducting nanowire single-photon detectors using cryogenic bias circuitry," Opt. Express 31, 610-625 (2023)) submitted outside the reporting period. Nevertheless, we have continued this research line with a number of other cryogenic electronic components in other contexts, as well as optical approaches to SNSPD bias control (Thiele et al., APL Photonics 9, 076118 (2024)).
