We have developed a “digital twin” of the photon entanglement generation process to model the degradation of quantum entangled states produced via spontaneous parametric down-conversion (SPDC) when transmitted through lossy and noisy optical channels. These two-mode entangled states can exhibit entanglement in photon number, polarization, and spectral-temporal modes, making them valuable resources for quantum communication and information processing. Using a density matrix formalism, we perform numerical simulations to model the interaction of each mode with its environment, incorporating the effects of photon loss, incoherent noise processes, and intermodal correlations between polarization and frequency degrees of freedom. Purity and fidelity are key metrics used to quantify the mixedness and coherence of the transmitted state. The simulation results quantify the decoherence as noise and loss increase, highlighting the vulnerability of entanglement to environmental disturbances and the compounding impact of mode-dependent quantum correlations. This study provides insight into the performance limits of SPDC-based entangled photon sources and offers guidance for optimizing quantum systems operating under realistic conditions. The simulation platform serves as a predictive tool, enabling future integration with experimental control systems and benchmarking against measured entangled photon characteristics such as Hong-Ou-Mandel interference and polarization correlation visibility.
We have developed a “digital twin” of the photon entanglement generation process to model the degradation of quantum entangled states produced via spontaneous parametric down-conversion (SPDC) when transmitted through lossy and noisy optical channels. These two-mode entangled states can exhibit entanglement in photon number, polarization, and spectral-temporal modes, making them valuable resources for quantum communication and information processing. Using a density matrix formalism, we perform numerical simulations to model the interaction of each mode with its environment, incorporating the effects of photon loss, incoherent noise processes, and intermodal correlations between polarization and frequency degrees of freedom. Purity and fidelity are key metrics used to quantify the mixedness and coherence of the transmitted state. The simulation results quantify the decoherence as noise and loss increase, highlighting the vulnerability of entanglement to environmental disturbances and the compounding impact of mode-dependent quantum correlations. This study provides insight into the performance limits of SPDC-based entangled photon sources and offers guidance for optimizing quantum systems operating under realistic conditions. The simulation platform serves as a predictive tool, enabling future integration with experimental control systems and benchmarking against measured entangled photon characteristics such as Hong-Ou-Mandel interference and polarization correlation visibility.
Quantum entanglement represents a fundamental resource for quantum information processing, yet the dynamics of multi-photon entangled systems remain inadequately characterized for practical quantum computing applications. This research investigates the generation, manipulation, and characterization of entangled states involving two to eight photons using spontaneous parametric down-conversion techniques. Through theoretical analysis and numerical simulations, we developed a comprehensive framework that quantifies entanglement fidelity, decoherence rates, and Bell inequality violations across various multi-photon configurations. Our findings reveal that entanglement fidelity decreases exponentially with increasing photon number, exhibiting a decay constant of 0.087±0.012 per additional qubit in optical systems. The research demonstrates significant variations in decoherence times across different physical implementations, with ion trap systems maintaining coherence 1.7 times longer than photonic platforms for equivalent qubit numbers. We established mathematical relationships between system parameters and entanglement quality metrics, identifying critical thresholds for fault-tolerant quantum gate operations. The theoretical framework incorporates realistic noise models, environmental coupling mechanisms, and measurement-induced perturbations that affect practical implementations. Our analysis indicates that five-photon entangled states maintain sufficient fidelity (>83.5%) for error-corrected quantum algorithms when decoherence times exceed 75 microseconds. These results provide essential guidelines for designing scalable quantum processors and establishing benchmarks for experimental validation of multi-qubit entanglement protocols.
Quantum entanglement is a vital resource in quantum information processing. High-dimensional quantum entanglement offers advantages that classical systems cannot surpass, particularly in enhancing channel capacity, improving system noise resilience, and increasing sensitivity to external environments. The construction of multimode entanglement in the spectral domain is well-suited for fiber-optic systems. Here, we present a straightforward scheme for generating multimode frequency-bin entanglement using a semiconductor chip through a simple mode conversion. A general model for Hong–Ou–Mandel (HOM) interference with a multimode frequency-bin entangled state is presented and applied to the experiments. The multimode entangled photons we produced exhibit HOM interference with a high-visibility beating pattern, demonstrating a strong relationship with the mode number, mode spacing, and the profile of the single mode. Building on the Fisher information analysis, we explore the relationship between the features in multimode entangled state interference traces and the precision of interferometric measurements even in the presence of experimental nonidealities. This work may deepen the understanding of multimode frequency-bin entanglement and advance the application of multimode HOM interference in quantum sensing.
Twin beams from spontaneous parametric down-conversion (SPDC) enable sub-shot-noise transmission estimation, yet practical advantages are constrained by loss. We map joint photon-number-resolved (PNR) statistics to channel parameters using an exact, analytically solvable binomial-damping model for the lossy two-mode squeezed vacuum, removing the need for numerical master-equation integration. Benchmarking against an energy-matched coherent probe, we show the optimal linear estimator saturates the Cramer-Rao bound in closed form. For a fiber-coupled SPDC source interrogated with PNR superconducting nanowire detectors, we demonstrate that no quantum advantage survives at 14.7 to 23.5 dB signal loss and 14.8 dB idler loss. We conclude by providing a parameter-space design chart specifying the precise loss budget required for quantum-enhanced transmission estimation.
Twin beams from spontaneous parametric down-conversion (SPDC) enable sub-shot-noise transmission estimation, yet practical advantages are constrained by loss. We map joint photon-number-resolved (PNR) statistics to channel parameters using an exact, analytically solvable binomial-damping model for the lossy two-mode squeezed vacuum, removing the need for numerical master-equation integration. Benchmarking against an energy-matched coherent probe, we show the optimal linear estimator saturates the Cramer-Rao bound in closed form. For a fiber-coupled SPDC source interrogated with PNR superconducting nanowire detectors, we demonstrate that no quantum advantage survives at 14.7 to 23.5 dB signal loss and 14.8 dB idler loss. We conclude by providing a parameter-space design chart specifying the precise loss budget required for quantum-enhanced transmission estimation.
Quantum interference plays an essential role in understanding the concepts of quantum physics. Moreover, the interference of photons is indispensable for large-scale quantum information processing. With the development of quantum networks, interference of photons transmitted through long-distance fiber channels has been widely implemented. However, quantum interference of photons using free-space channels is still scarce, mainly due to atmospheric turbulence. Here, we report an experimental demonstration of Hong-Ou-Mandel interference with photons transmitted by free-space channels. Two typical photon sources, i.e., correlated photon pairs generated in spontaneous parametric down conversion (SPDC) process and weak coherent states, are employed. A visibility of 0.744 ± 0.013 is observed by interfering with two photons generated in the SPDC process, exceeding the classical limit of 0.5. Our results demonstrate that the quantum property of photons remains even after transmission through unstable free-space channels, indicating the feasibility and potential application of free-space-based quantum interference in quantum information processing.
Entangled photon pairs play a crucial role in emerging quantum technologies, acting as tamper-proof padlocks in secure quantum communication, ultra-precise probes in quantum metrology and high-fidelity information carriers in photonic quantum computing. Amongst the many technological possibilities for generating photon pairs, sources based on spontaneous parametric down-conversion (SPDC) in second-order nonlinear crystals are still the workhorse tool in quantum optics laboratories, and more recently even in long-distance quantum communication with satellites. SPDC is a widely used method to create entangled photon pairs. The number of photon pairs that can be detected in a particular choice of collection modes, typically being optical single-mode fibers, depends critically on the spatial characteristics of the multi-mode SPDC emission as well as the particular beam waist of the collection modes. While several studies have already addressed the issue of optimal fiber coupling of SPDC photons in theory and experiment, the results of these studies arrive at different conclusions. Here, we present the results of a comprehensive experimental study on the optimal collection of photon pairs into single-mode optical fibers. Our approach is based on quasi-phase-matched type II SPDC from a periodically poled KTiOPO4 (ppKTP) crystal. We discuss the influence of pump and collection focal parameters on the spectral brightness and heralding efficiency, as well as practical issues of alignment tolerances into an optical single mode fiber. Further, Using two-photon interference (Hong-Ou-Mandel interference), we assess the spectral bandwidth of the photon pairs for variable crystal lengths. The results are in good agreement with our theoretical model, thus providing the recipe of building ultra bright and stable entangled photon sources.