@article{50840, abstract = {{Superconducting nanowire single-photon detectors (SNSPDs) have been widely used to study the discrete nature of quantum states of light in the form of photon-counting experiments. We show that SNSPDs can also be used to study continuous variables of optical quantum states by performing homodyne detection at a bandwidth of 400 kHz. By measuring the interference of a continuous-wave field of a local oscillator with the field of the vacuum state using two SNSPDs, we show that the variance of the difference in count rates is linearly proportional to the photon flux of the local oscillator over almost five orders of magnitude. The resulting shot-noise clearance of (46.0 ± 1.1) dB is the highest reported clearance for a balanced optical homodyne detector, demonstrating their potential for measuring highly squeezed states in the continuous-wave regime. In addition, we measured a CMRR = 22.4 dB. From the joint click counting statistics, we also measure the phase-dependent quadrature of a weak coherent state to demonstrate our device’s functionality as a homodyne detector.}}, author = {{Protte, Maximilian and Schapeler, Timon and Sperling, Jan and Bartley, Tim}}, issn = {{2837-6714}}, journal = {{Optica Quantum}}, number = {{1}}, publisher = {{Optica Publishing Group}}, title = {{{Low-noise balanced homodyne detection with superconducting nanowire single-photon detectors}}}, doi = {{10.1364/opticaq.502201}}, volume = {{2}}, year = {{2024}}, } @article{54815, abstract = {{Broadband quantum light is a vital resource for quantum metrology and spectroscopy applications such as quantum optical coherence tomography or entangled two photon absorption. For entangled two photon absorption in particular, very high photon flux combined with high time-frequency entanglement is crucial for observing a signal. So far these conditions could be met by using high power lasers driving degenerate, type 0 bulk-crystal spontaneous parametric down conversion (SPDC) sources. This naturally limits the available wavelength ranges and precludes deterministic splitting of the generated output photons. In this work we demonstrate an integrated two-colour SPDC source utilising a group-velocity matched lithium niobate waveguide, reaching both exceptional brightness 1.52⋅106pairssmWGHz and large bandwidth (7.8 THz FWHM) while pumped with a few mW of continuous wave (CW) laser light. By converting a narrow band pump to broadband pulses the created photon pairs show correlation times of Δτ ≈ 120 fs while maintaining the narrow bandwidth Δω p  ≪ 1 MHz of the CW pump light, yielding strong time-frequency entanglement. Furthermore our process can be adapted to a wide range of central wavelengths.}}, author = {{Pollmann, René and Roeder, Franz and Quiring, Victor and Ricken, Raimund and Eigner, Christof and Brecht, Benjamin and Silberhorn, Christine}}, issn = {{1094-4087}}, journal = {{Optics Express}}, number = {{14}}, publisher = {{Optica Publishing Group}}, title = {{{Integrated, bright broadband, two-colour parametric down-conversion source}}}, doi = {{10.1364/oe.522549}}, volume = {{32}}, year = {{2024}}, } @article{57862, abstract = {{The latest applications in ultrafast quantum metrology require bright, broadband bi-photon sources with one of the photons in the mid-infrared and the other in the visible to near infrared. However, existing sources based on bulk crystals are limited in brightness due to the short interaction length and only allow for limited dispersion engineering. Here, we present an integrated PDC source based on a Ti:LiNbO3 waveguide that generates broadband bi-photons with central wavelengths at 860 nm and 2800 nm. Their spectral bandwidth exceeds 25 THz and is achieved by simultaneous matching of the group velocities (GVs) and cancellation of GV dispersion for the signal and idler field. We provide an intuitive understanding of the process by studying our source’s behavior at different temperatures and pump wavelengths, which agrees well with simulations.}}, author = {{Roeder, Franz and Gnanavel, Abira and Pollmann, René and Brecht, Olga and Stefszky, Michael and Padberg, Laura and Eigner, Christof and Silberhorn, Christine and Brecht, Benjamin}}, issn = {{1367-2630}}, journal = {{New Journal of Physics}}, number = {{12}}, publisher = {{IOP Publishing}}, title = {{{Ultra-broadband non-degenerate guided-wave bi-photon source in the near and mid-infrared}}}, doi = {{10.1088/1367-2630/ad9f98}}, volume = {{26}}, year = {{2024}}, } @inproceedings{32407, abstract = {{Estimating the ground state energy of a local Hamiltonian is a central problem in quantum chemistry. In order to further investigate its complexity and the potential of quantum algorithms for quantum chemistry, Gharibian and Le Gall (STOC 2022) recently introduced the guided local Hamiltonian problem (GLH), which is a variant of the local Hamiltonian problem where an approximation of a ground state is given as an additional input. Gharibian and Le Gall showed quantum advantage (more precisely, BQP-completeness) for GLH with $6$-local Hamiltonians when the guiding vector has overlap (inverse-polynomially) close to 1/2 with a ground state. In this paper, we optimally improve both the locality and the overlap parameters: we show that this quantum advantage (BQP-completeness) persists even with 2-local Hamiltonians, and even when the guiding vector has overlap (inverse-polynomially) close to 1 with a ground state. Moreover, we show that the quantum advantage also holds for 2-local physically motivated Hamiltonians on a 2D square lattice. This makes a further step towards establishing practical quantum advantage in quantum chemistry.}}, author = {{Gharibian, Sevag and Hayakawa, Ryu and Gall, François Le and Morimae, Tomoyuki}}, booktitle = {{Proceedings of the 50th EATCS International Colloquium on Automata, Languages and Programming (ICALP)}}, number = {{32}}, pages = {{1--19}}, title = {{{Improved Hardness Results for the Guided Local Hamiltonian Problem}}}, doi = {{10.4230/LIPIcs.ICALP.2023.32}}, volume = {{261}}, year = {{2023}}, } @unpublished{48502, abstract = {{The prediction of photon echoes is an important technique for gaining an understanding of optical quantum systems. However, this requires a large number of simulations with varying parameters and/or input pulses, which renders numerical studies expensive. This article investigates how we can use data-driven surrogate models based on the Koopman operator to accelerate this process. In order to be successful, we require a model that is accurate over a large number of time steps. To this end, we employ a bilinear Koopman model using extended dynamic mode decomposition and simulate the optical Bloch equations for an ensemble of inhomogeneously broadened two-level systems. Such systems are well suited to describe the excitation of excitonic resonances in semiconductor nanostructures, for example, ensembles of semiconductor quantum dots. We perform a detailed study on the required number of system simulations such that the resulting data-driven Koopman model is sufficiently accurate for a wide range of parameter settings. We analyze the L2 error and the relative error of the photon echo peak and investigate how the control positions relate to the stabilization. After proper training, the dynamics of the quantum ensemble can be predicted accurately and numerically very efficiently by our methods.}}, author = {{Peitz, Sebastian and Hunstig, Anna and Rose, Hendrik and Meier, Torsten}}, title = {{{Accelerating the analysis of optical quantum systems using the Koopman operator}}}, year = {{2023}}, } @article{48349, abstract = {{We report a titanium indiffused waveguide resonator featuring an integrated electro-optic modulator for cavity length stabilisation that produces close to 5 dB of squeezed light at 1550 nm (2.4 dB directly measured). The resonator is locked on resonance for tens of minutes with 70 mW of SH light incident on the cavity, demonstrating that photorefraction can be mitigated. Squeezed light production concurrent with cavity length stabilisation utilising the integrated EOM is demonstrated. The device demonstrates the suitability of this platform for squeezed light generation in network applications, where stabilisation to the reference field is typically necessary.}}, author = {{Stefszky, M. and vom Bruch, F. and Santandrea, M. and Ricken, R. and Quiring, V. and Eigner, C. and Herrmann, H and Silberhorn, C}}, issn = {{1094-4087}}, journal = {{Optics Express}}, keywords = {{Atomic and Molecular Physics, and Optics}}, number = {{21}}, publisher = {{Optica Publishing Group}}, title = {{{Lithium niobate waveguide squeezer with integrated cavity length stabilisation for network applications}}}, doi = {{10.1364/oe.498423}}, volume = {{31}}, year = {{2023}}, } @article{48599, abstract = {{AbstractThe biexciton‐exciton emission cascade commonly used in quantum‐dot systems to generate polarization entanglement yields photons with intrinsically limited indistinguishability. In the present work, it focuses on the generation of pairs of photons with high degrees of polarization entanglement and simultaneously high indistinguishability. It achieves this goal by selectively reducing the biexciton lifetime with an optical resonator. It demonstrates that a suitably tailored circular Bragg reflector fulfills the requirements of sufficient selective Purcell enhancement of biexciton emission paired with spectrally broad photon extraction and twofold degenerate optical modes. The in‐depth theoretical study combines (i) the optimization of realistic photonic structures solving Maxwell's equations from which model parameters are extracted as input for (ii) microscopic simulations of quantum‐dot cavity excitation dynamics with full access to photon properties. It reports non‐trivial dependencies on system parameters and use the predictive power of the combined theoretical approach to determine the optimal range of Purcell enhancement that maximizes indistinguishability and entanglement to near unity values, here specifically for the telecom C‐band at 1550 nm.}}, author = {{Bauch, David and Siebert, Dustin and Jöns, Klaus and Förstner, Jens and Schumacher, Stefan}}, issn = {{2511-9044}}, journal = {{Advanced Quantum Technologies}}, keywords = {{tet_topic_qd}}, publisher = {{Wiley}}, title = {{{On‐Demand Indistinguishable and Entangled Photons Using Tailored Cavity Designs}}}, doi = {{10.1002/qute.202300142}}, year = {{2023}}, } @unpublished{43246, abstract = {{The biexciton-exciton emission cascade commonly used in quantum-dot systems to generate polarization entanglement yields photons with intrinsically limited indistinguishability. In the present work we focus on the generation of pairs of photons with high degrees of polarization entanglement and simultaneously high indistinguishibility. We achieve this goal by selectively reducing the biexciton lifetime with an optical resonator. We demonstrate that a suitably tailored circular Bragg reflector fulfills the requirements of sufficient selective Purcell enhancement of biexciton emission paired with spectrally broad photon extraction and two-fold degenerate optical modes. Our in-depth theoretical study combines (i) the optimization of realistic photonic structures solving Maxwell's equations from which model parameters are extracted as input for (ii) microscopic simulations of quantum-dot cavity excitation dynamics with full access to photon properties. We report non-trivial dependencies on system parameters and use the predictive power of our combined theoretical approach to determine the optimal range of Purcell enhancement that maximizes indistinguishability and entanglement to near unity values in the telecom C-band at $1550\,\mathrm{nm}$.}}, author = {{Bauch, David and Siebert, Dustin and Jöns, Klaus and Förstner, Jens and Schumacher, Stefan}}, keywords = {{tet_topic_phc, tet_topic_qd}}, title = {{{On-demand indistinguishable and entangled photons at telecom frequencies using tailored cavity designs}}}, year = {{2023}}, } @article{31189, abstract = {{Given a geometrically finite hyperbolic surface of infinite volume it is a classical result of Patterson that the positive Laplace-Beltrami operator has no $L^2$-eigenvalues $\geq 1/4$. In this article we prove a generalization of this result for the joint $L^2$-eigenvalues of the algebra of commuting differential operators on Riemannian locally symmetric spaces $\Gamma\backslash G/K$ of higher rank. We derive dynamical assumptions on the $\Gamma$-action on the geodesic and the Satake compactifications which imply the absence of the corresponding principal eigenvalues. A large class of examples fulfilling these assumptions are the non-compact quotients by Anosov subgroups.}}, author = {{Weich, Tobias and Wolf, Lasse Lennart}}, journal = {{Communications in Mathematical Physics}}, title = {{{Absence of principal eigenvalues for higher rank locally symmetric spaces}}}, doi = {{https://doi.org/10.1007/s00220-023-04819-1}}, volume = {{403}}, year = {{2023}}, } @unpublished{51206, abstract = {{We present a numerical algorithm for the computation of invariant Ruelle distributions on convex co-compact hyperbolic surfaces. This is achieved by exploiting the connection between invariant Ruelle distributions and residues of meromorphically continued weighted zeta functions established by the authors together with Barkhofen (2021). To make this applicable for numerics we express the weighted zeta as the logarithmic derivative of a suitable parameter dependent Fredholm determinant similar to Borthwick (2014). As an additional difficulty our transfer operator has to include a contracting direction which we account for with techniques developed by Rugh (1992). We achieve a further improvement in convergence speed for our algorithm in the case of surfaces with additional symmetries by proving and applying a symmetry reduction of weighted zeta functions.}}, author = {{Schütte, Philipp and Weich, Tobias}}, booktitle = {{arXiv:2308.13463}}, title = {{{Invariant Ruelle Distributions on Convex-Cocompact Hyperbolic Surfaces -- A Numerical Algorithm via Weighted Zeta Functions}}}, year = {{2023}}, } @article{49607, abstract = {{In this work, we utilize thin dielectric meta-atoms placed on a silver substrate to efficiently enhance and manipulate the third-harmonic generation. We theoretically and experimentally reveal that when the structural symmetry of the meta-atom is incompatible with the lattice symmetry of an array, some generalized nonlinear geometric phases appear, which offers new possibilities for harmonic generation control beyond the accessible symmetries governed by the selection rule. The underlying mechanism is attributed to the modified rotation of the effective principal axis of a dense meta-atom array, where the strong coupling among the units gives rise to a generalized linear geometric phase modulation of the pump light. Therefore, nonlinear geometric phases carried by third-harmonic emissions are the natural result of the wave-mixing process among the modes excited at the fundamental frequency. This mechanism further points out a new strategy to predict the nonlinear geometric phases delivered by the nanostructures according to their linear responses. Our design is simple and efficient and offers alternatives for the nonlinear meta-devices that are capable of flexible photon generation and manipulation.}}, author = {{Liu, Bingyi and Geromel, René and Su, Zhaoxian and Guo, Kai and Wang, Yongtian and Guo, Zhongyi and Huang, Lingling and Zentgraf, Thomas}}, issn = {{2330-4022}}, journal = {{ACS Photonics}}, keywords = {{Electrical and Electronic Engineering, Atomic and Molecular Physics, and Optics, Biotechnology, Electronic, Optical and Magnetic Materials}}, number = {{12}}, pages = {{4357--4366}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Nonlinear Dielectric Geometric-Phase Metasurface with Simultaneous Structure and Lattice Symmetry Design}}}, doi = {{10.1021/acsphotonics.3c01163}}, volume = {{10}}, year = {{2023}}, } @article{55901, author = {{Grisard, Stefan and Trifonov, Artur V. and Rose, Hendrik and Reichhardt, Rilana and Reichelt, Matthias and Schneider, Christian and Kamp, Martin and Höfling, Sven and Bayer, Manfred and Meier, Torsten and Akimov, Ilya A.}}, issn = {{2330-4022}}, journal = {{ACS Photonics}}, number = {{9}}, pages = {{3161--3170}}, publisher = {{American Chemical Society (ACS)}}, title = {{{Temporal Sorting of Optical Multiwave-Mixing Processes in Semiconductor Quantum Dots}}}, doi = {{10.1021/acsphotonics.3c00530}}, volume = {{10}}, year = {{2023}}, } @article{42158, author = {{Lüders, Carolin and Gil-Lopez, Jano and Allgaier, Markus and Brecht, Benjamin and Aßmann, Marc and Silberhorn, Christine and Bayer, Manfred}}, issn = {{2331-7019}}, journal = {{Physical Review Applied}}, keywords = {{General Physics and Astronomy}}, number = {{1}}, publisher = {{American Physical Society (APS)}}, title = {{{Tailored Frequency Conversion Makes Infrared Light Visible for Streak Cameras}}}, doi = {{10.1103/physrevapplied.19.014072}}, volume = {{19}}, year = {{2023}}, } @inproceedings{31872, abstract = {{Savitch's theorem states that NPSPACE computations can be simulated in PSPACE. We initiate the study of a quantum analogue of NPSPACE, denoted Streaming-QCMASPACE (SQCMASPACE), where an exponentially long classical proof is streamed to a poly-space quantum verifier. Besides two main results, we also show that a quantum analogue of Savitch's theorem is unlikely to hold, as SQCMASPACE=NEXP. For completeness, we introduce Streaming-QMASPACE (SQMASPACE) with an exponentially long streamed quantum proof, and show SQMASPACE=QMA_EXP (quantum analogue of NEXP). Our first main result shows, in contrast to the classical setting, the solution space of a quantum constraint satisfaction problem (i.e. a local Hamiltonian) is always connected when exponentially long proofs are permitted. For this, we show how to simulate any Lipschitz continuous path on the unit hypersphere via a sequence of local unitary gates, at the expense of blowing up the circuit size. This shows quantum error-correcting codes can be unable to detect one codeword erroneously evolving to another if the evolution happens sufficiently slowly, and answers an open question of [Gharibian, Sikora, ICALP 2015] regarding the Ground State Connectivity problem. Our second main result is that any SQCMASPACE computation can be embedded into "unentanglement", i.e. into a quantum constraint satisfaction problem with unentangled provers. Formally, we show how to embed SQCMASPACE into the Sparse Separable Hamiltonian problem of [Chailloux, Sattath, CCC 2012] (QMA(2)-complete for 1/poly promise gap), at the expense of scaling the promise gap with the streamed proof size. As a corollary, we obtain the first systematic construction for obtaining QMA(2)-type upper bounds on arbitrary multi-prover interactive proof systems, where the QMA(2) promise gap scales exponentially with the number of bits of communication in the interactive proof.}}, author = {{Gharibian, Sevag and Rudolph, Dorian}}, booktitle = {{14th Innovations in Theoretical Computer Science (ITCS)}}, pages = {{53:1--53:23}}, title = {{{Quantum space, ground space traversal, and how to embed multi-prover interactive proofs into unentanglement}}}, doi = {{10.4230/LIPIcs.ITCS.2023.53}}, volume = {{251}}, year = {{2023}}, } @article{43421, abstract = {{The achievement of a flat metasurface has realized extraordinary control over light–matter interaction at the nanoscale, enabling widespread use in imaging, holography, and biophotonics. However, three-dimensional metasurfaces with the potential to provide additional light–matter manipulation flexibility attract only little interest. Here, we demonstrate a three-dimensional metasurface scheme capable of providing dual phase control through out-of-plane plasmonic resonance of L-shape antennas. Under circularly polarized excitation at a specific wavelength, the L-shape antennas with rotating orientation angle act as spatially variant three-dimensional tilted dipoles and are able to generate desire phase delay for different polarization components. Generalized Snell's law is achieved for both in-plane and out-of-plane dipole components through arranging such L-shape antennas into arrays. These three-dimensional metasurfaces suggest a route for wavefront modulation and a variety of nanophotonic applications.}}, author = {{Li, Tianyou and Chen, Yanjie and Wang, Yongtian and Zentgraf, Thomas and Huang, Lingling}}, issn = {{0003-6951}}, journal = {{Applied Physics Letters}}, keywords = {{Physics and Astronomy (miscellaneous)}}, number = {{14}}, publisher = {{AIP Publishing}}, title = {{{Three-dimensional dipole momentum analog based on L-shape metasurface}}}, doi = {{10.1063/5.0142389}}, volume = {{122}}, year = {{2023}}, } @article{37280, author = {{Rose, Hendrik and Vasil'ev, A. N. and Tikhonova, O. V. and Meier, Torsten and Sharapova, Polina}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{1}}, publisher = {{American Physical Society (APS)}}, title = {{{Quantum-optical excitations of semiconductor nanostructures in a microcavity using a two-band model and a single-mode quantum field}}}, doi = {{10.1103/physreva.107.013703}}, volume = {{107}}, year = {{2023}}, } @article{44050, author = {{Sperling, Jan and Agudelo, Elizabeth}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{4}}, publisher = {{American Physical Society (APS)}}, title = {{{Entanglement of particles versus entanglement of fields: Independent quantum resources}}}, doi = {{10.1103/physreva.107.042420}}, volume = {{107}}, year = {{2023}}, } @article{40477, author = {{Sperling, Jan and Gianani, Ilaria and Barbieri, Marco and Agudelo, Elizabeth}}, issn = {{2469-9926}}, journal = {{Physical Review A}}, number = {{1}}, publisher = {{American Physical Society (APS)}}, title = {{{Detector entanglement: Quasidistributions for Bell-state measurements}}}, doi = {{10.1103/physreva.107.012426}}, volume = {{107}}, year = {{2023}}, } @article{42973, author = {{Lüders, Carolin and Pukrop, Matthias and Barkhausen, Franziska and Rozas, Elena and Schneider, Christian and Höfling, Sven and Sperling, Jan and Schumacher, Stefan and Aßmann, Marc}}, issn = {{0031-9007}}, journal = {{Physical Review Letters}}, keywords = {{General Physics and Astronomy}}, number = {{11}}, publisher = {{American Physical Society (APS)}}, title = {{{Tracking Quantum Coherence in Polariton Condensates with Time-Resolved Tomography}}}, doi = {{10.1103/physrevlett.130.113601}}, volume = {{130}}, year = {{2023}}, } @inproceedings{20841, author = {{Gharibian, Sevag and Watson, James and Bausch, Johannes}}, booktitle = {{Proceedings of the 40th International Symposium on Theoretical Aspects of Computer Science (STACS)}}, pages = {{54:1--54:21}}, title = {{{The Complexity of Translationally Invariant Problems beyond Ground State Energies}}}, doi = {{https://doi.org/10.4230/LIPIcs.STACS.2023.54}}, volume = {{254}}, year = {{2023}}, }