@article{65094,
  abstract     = {{<jats:p>
                    The development of practical sensors for optical coherence tomography (OCT) with undetected photons requires miniaturization via integration. To be practical, these sensors must exhibit a large spectral bandwidth and a high brightness, which are linked to a high axial resolution and a sufficient signal-to-noise ratio, respectively. Here, we combine these requirements in a scheme for OCT measurements with undetected photons based on nonlinear
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                    waveguides. We investigate the performance benchmarks of the commonly used SU(1,1) scheme in comparison to an induced-coherence scheme and find that the latter is actually better suited when implementing measurements with undetected photons in integrated systems. In both schemes, we perform pump-gain optimization and OCT measurements with undetected photons with an axial resolution as low as
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                      <d:mn>28</d:mn>
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                      <d:mrow>
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                    </d:math>
                    .
                  </jats:p>}},
  author       = {{Roeder, Franz and Pollmann, René and Quiring, Viktor and Eigner, Christof and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{2331-7019}},
  journal      = {{Physical Review Applied}},
  number       = {{3}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Toward integrated sensors for optimized optical coherence tomography with undetected photons}}},
  doi          = {{10.1103/cwsx-42c4}},
  volume       = {{25}},
  year         = {{2026}},
}

@article{65096,
  abstract     = {{<jats:p>
                    Precise measurements of both the arrival time and carrier frequency of light pulses are essential for time–frequency-encoded quantum technologies. Quantum mechanics, however, imposes fundamental limits on the simultaneous determination of these quantities. In this work, we derive and experimentally verify the quantum uncertainty bounds governing joint time–frequency measurements. We show that when detection is restricted to finite time windows, the problem is naturally described by a quantum rotor, rendering the commonly used Heisenberg uncertainty relation inapplicable. We further propose an optimal detection scheme that saturates these fundamental limits. By sampling the
                    <jats:italic toggle="yes">Q</jats:italic>
                    -function, we demonstrate the reconstruction of the Wigner function beyond the harmonic oscillator. Using an experimental implementation based on a quantum pulse gate, we confirm that the proposed scheme approaches the ultimate quantum limit for simultaneous time–frequency measurements. These results provide a framework for joint time–frequency detection with direct implications for precision measurements and quantum information processing.
                  </jats:p>}},
  author       = {{Folge, Patrick Fabian and Serino, Laura Maria and Mišta, Ladislav and Brecht, Benjamin and Silberhorn, Christine and Řeháček, Jaroslav and Hradil, Zdeněk}},
  issn         = {{2334-2536}},
  journal      = {{Optica}},
  number       = {{3}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Quantum-limited detection of the arrival time and the carrier frequency of time-dependent signals}}},
  doi          = {{10.1364/optica.579459}},
  volume       = {{13}},
  year         = {{2026}},
}

@article{63451,
  abstract     = {{<jats:p>Superconducting nanowire single-photon detectors (SNSPDs) can enable photon-number resolution (PNR) based on accurate measurements of the detector’s response time to few-photon optical pulses. In this work, we investigate the impact of the optical pulse shape and duration on the accuracy of this method. We find that Gaussian temporal pulse shapes yield cleaner arrival-time histograms and, thus, more accurate PNR, compared to bandpass-filtered pulses of equal bandwidth. For low system jitter and an optical pulse duration comparable to the other jitter contributions, photon numbers can be discriminated in our system with a commercial SNSPD. At 60 ps optical pulse duration, photon-number discrimination is significantly reduced. Furthermore, we highlight the importance of using the correct arrival-time histogram model when analyzing photon-number assignment. Using exponentially modified Gaussian distributions, instead of the commonly used Gaussian distributions, we can more accurately determine photon-number misidentification probabilities. Finally, we reconstruct the positive operator-valued measures of the detector, revealing sharp features that indicate the intrinsic PNR capabilities.</jats:p>}},
  author       = {{Schapeler, Timon and Mischke, Isabell and Schlue, Fabian and Stefszky, Michael and Brecht, Benjamin and Silberhorn, Christine and Bartley, Tim}},
  issn         = {{2835-0103}},
  journal      = {{APL Quantum}},
  number       = {{1}},
  publisher    = {{AIP Publishing}},
  title        = {{{Practical considerations for assignment of photon numbers with SNSPDs}}},
  doi          = {{10.1063/5.0304127}},
  volume       = {{3}},
  year         = {{2026}},
}

@article{65095,
  abstract     = {{<jats:p>
                    We provide experimental validation of tight entropic uncertainty relations for the Shannon entropies of observables with mutually unbiased eigenstates in high dimensions. In particular, we address the cases of dimensions
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                    , 4, and 5 and consider from 2 to
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                    </b:math>
                    mutually unbiased bases. The experiment is based on pulsed frequency bins measured with a multioutput quantum pulse gate, which can perform projective measurements on a complete high-dimensional basis in the time-frequency domain. Our results fit the theoretical predictions: the bound on the sum of the entropies is never violated and is saturated by the states that minimize the uncertainty relations.
                  </jats:p>}},
  author       = {{Serino, Laura Maria and Chesi, Giovanni and Brecht, Benjamin and Maccone, Lorenzo and Macchiavello, Chiara and Silberhorn, Christine}},
  issn         = {{2469-9926}},
  journal      = {{Physical Review A}},
  number       = {{3}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Experimental entropic uncertainty relations in dimensions three to five}}},
  doi          = {{10.1103/f6c4-jtlc}},
  volume       = {{113}},
  year         = {{2026}},
}

@inproceedings{60022,
  author       = {{Brauckmann, Michael and Narvaez Castaneda, Emmanuel and Siebert, Dustin and Brecht, Benjamin and Förstner, Jens and Zentgraf, Thomas}},
  booktitle    = {{Proceedings of The 15th International Conference on Metamaterials, Photonic Crystals and Plasmonics}},
  location     = {{Malaga, Spain}},
  title        = {{{Enhancement Of Light-matter Interaction In Topological Waveguides And Resonators}}},
  year         = {{2025}},
}

@inproceedings{60587,
  author       = {{Schapeler, Timon and Schlue, Fabian and Stefszky, Michael and Brecht, Benjamin and Silberhorn, Christine and Bartley, Tim}},
  booktitle    = {{Advanced Photon Counting Techniques XIX}},
  editor       = {{Itzler, Mark A. and McIntosh, K. Alex and Bienfang, Joshua C.}},
  publisher    = {{SPIE}},
  title        = {{{Optimizing photon-number resolution with superconducting nanowire multi-photon detectors}}},
  doi          = {{10.1117/12.3054905}},
  year         = {{2025}},
}

@article{61110,
  abstract     = {{<jats:p>By analyzing the physics of multi-photon absorption in superconducting nanowire single-photon detectors (SNSPDs), we identify physical components of jitter. From this, we formulate a quantitative physical model of the multi-photon detector response that combines the local detection mechanism and local fluctuations (hotspot formation and intrinsic jitter) with the thermoelectric dynamics of resistive domains. Our model provides an excellent description of the arrival-time histogram of a commercial SNSPD across several orders of magnitude, both in arrival-time probability and across mean photon number. This is achieved with just three fitting parameters: the scaling of the mean arrival time of voltage response pulses, as well as the Gaussian and exponential jitter components. Our findings have important implications for photon-number-resolving detector design, as well as applications requiring low jitter, such as light detection and ranging (LIDAR).</jats:p>}},
  author       = {{Sidorova, Mariia and Schapeler, Timon and Semenov, Alexej D. and Schlue, Fabian and Stefszky, Michael and Brecht, Benjamin and Silberhorn, Christine and Bartley, Tim}},
  issn         = {{2378-0967}},
  journal      = {{APL Photonics}},
  keywords     = {{Jitter, PNR, SNSPD}},
  number       = {{8}},
  publisher    = {{AIP Publishing}},
  title        = {{{Jitter in photon-number-resolved detection by superconducting nanowires}}},
  doi          = {{10.1063/5.0273752}},
  volume       = {{10}},
  year         = {{2025}},
}

@misc{62639,
  author       = {{Kruse, Stephan and Silberhorn, Christine and Brecht, Benjamin and Schwabe, Tobias}},
  title        = {{{Optisch basierter Digital-Analog-Umsetzer}}},
  year         = {{2025}},
}

@article{63213,
  abstract     = {{<jats:p>Quantum uncertainty relations impose fundamental limits on the joint knowledge that can be acquired from complementary observables: Perfect knowledge of a quantum state in one basis implies maximal indetermination in all other mutually unbiased bases (MUBs). Uncertainty relations derived from joint properties of the MUBs are generally assumed to be uniform, irrespective of the specific observables chosen within a set. In this work, we demonstrate instead that the uncertainty relations can depend on the choice of observables. Through both experimental observation and numerical methods, we show that selecting different sets of three MUBs in a five-dimensional quantum system results in distinct uncertainty bounds, i.e., in varying degrees of complementarity, in terms of both entropy and variance.</jats:p>}},
  author       = {{Serino, Laura Maria and Chesi, Giovanni and Brecht, Benjamin and Maccone, Lorenzo and Macchiavello, Chiara and Silberhorn, Christine}},
  issn         = {{2643-1564}},
  journal      = {{Physical Review Research}},
  number       = {{3}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Complementarity-based complementarity: The choice of mutually unbiased observables shapes quantum uncertainty relations}}},
  doi          = {{10.1103/v24q-sl6n}},
  volume       = {{7}},
  year         = {{2025}},
}

@article{63214,
  abstract     = {{<jats:p>We study a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric down-conversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, which can be resolved by modern photodetectors. We show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of modes characterizing the entanglement between the two generated beams. We illustrate the proposed method by an example of photon pairs generated in a periodically poled potassium titanyl phosphate (KTP) crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes. Our theoretical model also shows that the magnified double-heralded autocorrelation function of one beam exhibits a local maximum around zero delay time, corresponding to photon bunching at a short time scale.</jats:p>}},
  author       = {{Horoshko, Dmitri B. and Srivastava, Shivang and Sośnicki, Filip and Mikołajczyk, Michał and Karpiński, Michał and Brecht, Benjamin and Kolobov, Mikhail I.}},
  issn         = {{2469-9926}},
  journal      = {{Physical Review A}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Time-resolved second-order autocorrelation function of parametric down-conversion}}},
  doi          = {{10.1103/7ckm-tm3r}},
  volume       = {{112}},
  year         = {{2025}},
}

@article{63215,
  abstract     = {{<jats:title>Abstract</jats:title>
               <jats:p>High-dimensional time-frequency encodings have the potential to significantly advance quantum information science; however, practical applications require precise knowledge of the encoded quantum states, which becomes increasingly challenging for larger Hilbert spaces. Self-guided tomography (SGT) has emerged as a practical and scalable technique for this purpose in the spatial domain. Here, we apply SGT to estimate time-frequency states using a multi-output quantum pulse gate. We achieve fidelities of more than 99% for 3- and 5-dimensional states without the need for calibration or post-processing. We demonstrate the robustness of SGT against statistical and environmental noise, highlighting its efficacy in the photon-starved regime typical of quantum information applications.</jats:p>}},
  author       = {{Serino, Laura Maria and Rambach, Markus and Brecht, Benjamin and Romero, Jacquiline and Silberhorn, Christine}},
  issn         = {{2058-9565}},
  journal      = {{Quantum Science and Technology}},
  number       = {{2}},
  publisher    = {{IOP Publishing}},
  title        = {{{Self-guided tomography of time-frequency qudits}}},
  doi          = {{10.1088/2058-9565/adb0ea}},
  volume       = {{10}},
  year         = {{2025}},
}

@article{63091,
  abstract     = {{We present the design and characterization of a guided-wave, bright, and highly frequency non-degenerate parametric down-conversion (PDC) source in thin-film lithium niobate. The source generates photon pairs with wavelengths of 815 nm and 1550 nm, linking the visible wavelength regime with telecommunication wavelengths. We confirm the high quality of the generated single photons by determining a value for the heralded second-order correlation function as low as g_h^(2)=(6.7+/-1.1)*10^8-3). Furthermore, we achieve a high spectral brightness of 0.44·10pairs/(smWGHz) which is two orders of magnitude higher than sources based on weakly guiding waveguides. The shape of the PDC spectrum and the strong agreement between the effective and nominal bandwidth highlight our high fabrication quality of periodically poled waveguides. The good agreement between the measured and simulated spectral characteristics of our source demonstrates our excellent understanding of the PDC process. Our result is a valuable step towards practical and scalable quantum communication networks as well as photonic quantum computing.}},
  author       = {{Babel, Silia and Bollmers, Laura and Roeder, Franz and Ridder, Werner and Golla, Christian and Köthemann, Ronja and Reineke, Bernhard and Herrmann, Harald and Brecht, Benjamin and Eigner, Christof and Padberg, Laura and Silberhorn, Christine}},
  issn         = {{1094-4087}},
  journal      = {{Optics Express}},
  number       = {{25}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Ultrabright, two-color photon pair source based on thin-film lithium niobate for bridging visible and telecom wavelengths}}},
  doi          = {{10.1364/oe.571605}},
  volume       = {{33}},
  year         = {{2025}},
}

@article{63733,
  abstract     = {{<jats:p>We study a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric down-conversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, which can be resolved by modern photodetectors. We show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of modes characterizing the entanglement between the two generated beams. We illustrate the proposed method by an example of photon pairs generated in a periodically poled potassium titanyl phosphate (KTP) crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes. Our theoretical model also shows that the magnified double-heralded autocorrelation function of one beam exhibits a local maximum around zero delay time, corresponding to photon bunching at a short time scale.</jats:p>}},
  author       = {{Horoshko, Dmitri B. and Srivastava, Shivang and Sośnicki, Filip Maksymilian and Mikołajczyk, Michał and Karpiński, Michał and Brecht, Benjamin and Kolobov, Mikhail I.}},
  issn         = {{2469-9926}},
  journal      = {{Physical Review A}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Time-resolved second-order autocorrelation function of parametric down-conversion}}},
  doi          = {{10.1103/7ckm-tm3r}},
  volume       = {{112}},
  year         = {{2025}},
}

@article{54544,
  abstract     = {{The biphoton correlation time, a measure for the conditional uncertainty in the temporal arrival of two photons from a photon pair source, is a key performance identifier for many quantum spectroscopy applications, with shorter correlation times typically yielding better performance. Furthermore, it provides fundamental insight into the effects of dispersion on the biphoton state. Here, we show that a characteristic dependence of the width of the temporal interferogram can be exploited to obtain insights into the amount of second-order dispersion inside the interferometer and to retrieve actual and Fourier-limited ultrashort biphoton correlation times of around 100 fs. In the presented scheme, we simultaneously measure spectral and temporal interferograms at the output of an SU(1,1) interferometer based on an integrated broadband parametric down conversion source in a Ti:LiNbO3 waveguide.}},
  author       = {{Roeder, Franz and Pollmann, René and Stefszky, Michael and Santandrea, Matteo and Luo, Kai Hong and Quiring, V. and Ricken, Raimund and Eigner, Christof and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{2691-3399}},
  journal      = {{PRX Quantum}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Measurement of Ultrashort Biphoton Correlation Times with an Integrated Two-Color Broadband SU(1,1)-Interferometer}}},
  doi          = {{10.1103/prxquantum.5.020350}},
  volume       = {{5}},
  year         = {{2024}},
}

@article{54812,
  author       = {{Weinbrenner, Lisa T. and Prasannan, Nidhin and Hansenne, Kiara and Denker, Sophia and Sperling, Jan and Brecht, Benjamin and Silberhorn, Christine and Gühne, Otfried}},
  issn         = {{0031-9007}},
  journal      = {{Physical Review Letters}},
  number       = {{24}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Certifying the Topology of Quantum Networks: Theory and Experiment}}},
  doi          = {{10.1103/physrevlett.132.240802}},
  volume       = {{132}},
  year         = {{2024}},
}

@article{55174,
  abstract     = {{<jats:p>We apply principal component analysis (PCA) to a set of electrical output signals from a commercially available superconducting nanowire single-photon detector (SNSPD) to investigate their photon-number-resolving capability. We find that the rising edge as well as the amplitude of the electrical signal have the most dependence on photon number. Accurately measuring the rising edge while simultaneously measuring the voltage of the pulse amplitude maximizes the photon-number resolution of SNSPDs. Using an optimal basis of principal components, we show unambiguous discrimination between one- and two-photon events, as well as partial resolution up to five photons. This expands the use case of SNSPDs to photon-counting experiments, without the need of detector multiplexing architectures.</jats:p>
          <jats:sec>
            <jats:title/>
            <jats:supplementary-material>
              <jats:permissions>
                <jats:copyright-statement>Published by the American Physical Society</jats:copyright-statement>
                <jats:copyright-year>2024</jats:copyright-year>
              </jats:permissions>
            </jats:supplementary-material>
          </jats:sec>}},
  author       = {{Schapeler, Timon and Lamberty, Niklas and Hummel, Thomas and Schlue, Fabian and Stefszky, Michael and Brecht, Benjamin and Silberhorn, Christine and Bartley, Tim}},
  issn         = {{2331-7019}},
  journal      = {{Physical Review Applied}},
  number       = {{1}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Electrical trace analysis of superconducting nanowire photon-number-resolving detectors}}},
  doi          = {{10.1103/physrevapplied.22.014024}},
  volume       = {{22}},
  year         = {{2024}},
}

@misc{57089,
  author       = {{Kruse, Stephan and Brecht, Benjamin and Silberhorn, Christine and Serino, Laura Maria}},
  title        = {{{Quantenoptisch-unterstütztes Sende-/Empfangssystem}}},
  year         = {{2024}},
}

@article{57743,
  author       = {{Krishnaswamy, Suchitra and Schlue, Fabian and Ares, L. and Dyachuk, V. and Stefszky, Michael and Brecht, Benjamin and Silberhorn, Christine and Sperling, Jan}},
  issn         = {{2469-9926}},
  journal      = {{Physical Review A}},
  number       = {{2}},
  publisher    = {{American Physical Society (APS)}},
  title        = {{{Experimental retrieval of photon statistics from click detection}}},
  doi          = {{10.1103/physreva.110.023717}},
  volume       = {{110}},
  year         = {{2024}},
}

@inproceedings{57108,
  author       = {{Kruse, Stephan and Brecht, Benjamin and Silberhorn, Christine}},
  title        = {{{A Quantum Pulse Gate Enhanced Photonic Radar Architecture }}},
  doi          = {{10.5281/zenodo.14934743}},
  year         = {{2024}},
}

@article{56267,
  author       = {{Serino, Laura and Ridder, Werner and Bhattacharjee, Abhinandan and Gil López, Jano and Brecht, Benjamin and Silberhorn, Christine}},
  issn         = {{2837-6714}},
  journal      = {{Optica Quantum}},
  publisher    = {{Optica Publishing Group}},
  title        = {{{Orchestrating time and color: a programmable source of high-dimensional entanglement}}},
  doi          = {{10.1364/opticaq.532334}},
  year         = {{2024}},
}