# Quantum Foundations Domain: Fundamental & Quantum Physics · 244 papers indexed · newest 2026-10-29 Loophole-free Bell tests, the measurement problem, contextuality, Wigner's-friend scenarios and experimental tests of collapse models. Key concepts: Bell inequality, measurement problem, contextuality, Wigner's friend, collapse model, quantum foundations, nonlocality ## Most influential - **[Loophole-free Bell inequality violation using electron spins separated by 1.3 kilometres](https://space-kitty.com/papers/arxiv:1508.05949)**. Bas Hensen, Hannes Bernien, A. Dréau et al. · 2015 · Nature (arXiv:1508.05949 · doi:10.1038/nature15759 · 2,846 citations). Source: > For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. In his seminal work, John Bell proved that no theory of nature that obeys locality and realism can reproduce all the predictions of quantum theory. Bell showed that in any local realist theory the correlations between distant measurements satisfy an inequality and, moreover, that this inequality can be violated according to quantum theory. This provided a recipe for experimental tests of the fundamental principles underlying the laws of nature. In the past decades, num… - **[Significant-Loophole-Free Test of Bell’s Theorem with Entangled Photons](https://space-kitty.com/papers/arxiv:1511.03190)**. Marissa Giustina, Marijn A. M. Versteegh, Sören Wengerowsky et al. · 2015 · Physical Review Letters (arXiv:1511.03190 · doi:10.1103/physrevlett.115.250401 · 1,529 citations). Source: > Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell's theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell's inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-o… - **[Strong Loophole-Free Test of Local Realism](https://space-kitty.com/papers/arxiv:1511.03189)**. Lynden K. Shalm, Evan Meyer-Scott, Bradley Christensen et al. · 2015 · Physical Review Letters (arXiv:1511.03189 · doi:10.1103/physrevlett.115.250402 · 1,402 citations). Source: > We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p-values as small as $5.9\\times 10^{-9}$ for our Bell violation while maint… - **[Detection-Loophole-Free Test of Quantum Nonlocality, and Applications](https://space-kitty.com/papers/arxiv:1306.5772)**. Bradley Christensen, Kevin T. McCusker, Joseph B. Altepeter et al. · 2013 · Physical Review Letters (arXiv:1306.5772 · doi:10.1103/physrevlett.111.130406 · 477 citations). Source: > We present a source of entangled photons that violates a Bell inequality free of the "fair-sampling" assumption, by over 7 standard deviations. This violation is the first experiment with photons to close the detection loophole, and we demonstrate enough "efficiency" overhead to eventually perform a fully loophole-free test of local realism. The entanglement quality is verified by maximally violating additional Bell tests, testing the upper limit of quantum correlations. Finally, we use the source to generate secure private quantum random numbers at rates over 4 orders of magnitude beyond prev… - **[Loophole-free Bell inequality violation with superconducting circuits](https://space-kitty.com/papers/doi:10.1038/s41586-023-05885-0)**. Simon Storz, Josua Schär, Anatoly Kulikov et al. · 2023 · Nature (doi:10.1038/s41586-023-05885-0 · 219 citations). Source: > Abstract Superposition, entanglement and non-locality constitute fundamental features of quantum physics. The fact that quantum physics does not follow the principle of local causality1–3 can be experimentally demonstrated in Bell tests4 performed on pairs of spatially separated, entangled quantum systems. Although Bell tests, which are widely regarded as a litmus test of quantum physics, have been explored using a broad range of quantum systems over the past 50 years, only relatively recently have experiments free of so-called loopholes5 succeeded. Such experiments have been performed with sp… > _Abstract excerpt by Simon Storz et al., CC-BY, https://doi.org/10.1038/s41586-023-05885-0; truncated._ - **[Practical device-independent quantum cryptography via entropy accumulation](https://space-kitty.com/papers/doi:10.1038/s41467-017-02307-4)**. Rotem Arnon-Friedman, Frédéric Dupuis, Omar Fawzi et al. · 2018 · Nature Communications (doi:10.1038/s41467-017-02307-4 · 271 citations). Source: > Device-independent cryptography goes beyond conventional quantum cryptography by providing security that holds independently of the quality of the underlying physical devices. Device-independent protocols are based on the quantum phenomena of non-locality and the violation of Bell inequalities. This high level of security could so far only be established under conditions which are not achievable experimentally. Here we present a property of entropy, termed "entropy accumulation", which asserts that the total amount of entropy of a large system is the sum of its parts. We use this property to p… > _Abstract excerpt by Rotem Arnon-Friedman et al., CC-BY, https://doi.org/10.1038/s41467-017-02307-4; truncated._ - **[Device-independent quantum random-number generation](https://space-kitty.com/papers/arxiv:1807.09611)**. Yang Liu, Qi Zhao, Minghan Li et al. · 2018 · Nature (arXiv:1807.09611 · doi:10.1038/s41586-018-0559-3 · 258 citations). Source: > Randomness is critical for many information processing applications, including numerical modeling and cryptography. Device-independent quantum random number generation (DIQRNG) based on the loophole free violation of Bell inequality produces unpredictable genuine randomness without any device assumption and is therefore an ultimate goal in the field of quantum information science. However, due to formidable technical challenges, there were very few reported experimental studies of DIQRNG, which were vulnerable to the adversaries. Here we present a fully functional DIQRNG against the most gener… - **[Loopholes in Bell inequality tests of local realism](https://space-kitty.com/papers/arxiv:1407.0363)**. Jan-Åke Larsson · 2014 · Journal of Physics A Mathematical and Theoretical (arXiv:1407.0363 · doi:10.1088/1751-8113/47/42/424003 · 261 citations). Source: > Bell inequalities are intended to show that local realist theories cannot describe the world. A local realist theory is one where physical properties are defined prior to and independent of measurement, and no physical influence can propagate faster than the speed of light. Quantum-mechanical predictions for certain experiments violate the Bell inequality while a local realist theory cannot, and this shows that a local realist theory cannot give those quantum-mechanical predictions. However, because of unexpected circumstances or "loopholes" in available experiment tests, local realist theorie… - **[Conclusive quantum steering with superconducting transition-edge sensors](https://space-kitty.com/papers/arxiv:1111.0829)**. Devin H. Smith, Geoff Gillett, Marcelo Pereira de Almeida et al. · 2012 · Nature Communications (arXiv:1111.0829 · doi:10.1038/ncomms1628 · 237 citations). Source: > Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date all experimental tests with single photon states have relied on post-selection, allowing untrusted devices to cheat by hiding unfavourable events in losses. Here we close this "detection loophole" by combining a highly efficient source of entangled photon pairs with superconducting transition edge sens… - **[Toward a Photonic Demonstration of Device-Independent Quantum Key Distribution](https://space-kitty.com/papers/arxiv:2110.01480)**. Wen‐Zhao Liu, Yuzhe Zhang, Yi-Zheng Zhen et al. · 2022 · Physical Review Letters (arXiv:2110.01480 · doi:10.1103/physrevlett.129.050502 · 134 citations). Source: > In this paper, we employ theoretical and experimental efforts and realize a proof-of-principle verification of device-independent QKD based on the photonic setup. On the theoretical side, we enhance the loss tolerance for real device imperfections by combining different approaches, namely, random post-selection, noisy preprocessing, and developed numerical methods to estimate the key rate via the von Neumann entropy. On the experimental side, we develop a high-quality polarization-entangled photon source achieving a state-of-the-art (heralded) detection efficiency about 87.5%. This efficiency … - **[Tomography of the quantum state of photons entangled in high dimensions](https://space-kitty.com/papers/doi:10.1103/physreva.84.062101)**. Megan Agnew, Jonathan Leach, Melanie G. McLaren et al. · 2011 · Physical Review A (doi:10.1103/physreva.84.062101 · 162 citations). Source: - **[High-Speed Device-Independent Quantum Random Number Generation without a Detection Loophole](https://space-kitty.com/papers/arxiv:1709.06779)**. Yang Liu, Xiao Yuan, Minghan Li et al. · 2018 · Physical Review Letters (arXiv:1709.06779 · doi:10.1103/physrevlett.120.010503 · 140 citations). Source: > Quantum mechanics provides means of generating genuine randomness that is impossible with deterministic classical processes. Remarkably, the unpredictability of randomness can be certified in a self-testing manner that is independent of implementation devices. Here, we present an experimental demonstration of self-testing quantum random number generation based on an detection-loophole free Bell test with entangled photons. In the randomness analysis, without the assumption of independent identical distribution, we consider the worst case scenario that the adversary launches the most powerful a… - **[Violation of a Temporal Bell Inequality for Single Spins in a Diamond Defect Center](https://space-kitty.com/papers/arxiv:1103.4949)**. G. Waldherr, Philipp Neumann, Susana F. Huelga et al. · 2011 · Physical Review Letters (arXiv:1103.4949 · doi:10.1103/physrevlett.107.090401 · 145 citations). Source: > Quantum non-locality has been experimentally investigated by testing different forms of Bell's inequality, yet a loophole-free realization has not been achieved up to now. Much less explored are temporal Bell inequalities, which are not subject to the locality assumption, but impose a constrain on the system's time-correlations. In this paper, we report on the experimental violation of a temporal Bell's inequality using a nitrogen vacancy defect (NV) in diamond and provide a novel quantitative test of quantum coherence. We therefore present a new technique to initialize the electronic state of… - **[Generation of Fresh and Pure Random Numbers for Loophole-Free Bell Tests](https://space-kitty.com/papers/arxiv:1506.02712)**. Carlos Abellán, Waldimar Amaya, Daniel Mitrani et al. · 2015 · Physical Review Letters (arXiv:1506.02712 · doi:10.1103/physrevlett.115.250403 · 116 citations). Source: > We demonstrate extraction of randomness from spontaneous-emission events less than 36 ns in the past, giving output bits with excess predictability below $10^{-5}$ and strong metrological randomness assurances. This randomness generation strategy satisfies the stringent requirements for unpredictable basis choices in current "loophole-free Bell tests" of local realism \[Hensen et al., Nature (London) 526, 682 (2015); Giustina et al., Phys. Rev. Lett. 115, 250401 (2015); Shalm et al., Phys. Rev. Lett. 115, 250402 (2015)\]. - **[Loophole-free Bell test using electron spins in diamond: second experiment and additional analysis](https://space-kitty.com/papers/arxiv:1603.05705)**. Bas Hensen, Norbert Kalb, Machiel Blok et al. · 2016 · Scientific Reports (arXiv:1603.05705 · doi:10.1038/srep30289 · 95 citations). Source: > The recently reported violation of a Bell inequality using entangled electronic spins in diamonds (Hensen et al., Nature 526, 682-686) provided the first loophole-free evidence against local-realist theories of nature. Here we report on data from a second Bell experiment using the same experimental setup with minor modifications. We find a violation of the CHSH-Bell inequality of $2.35 \\pm 0.18$, in agreement with the first run, yielding an overall value of $S = 2.38 \\pm 0.14$. We calculate the resulting $P$-values of the second experiment and of the combined Bell tests. We provide an addition… ## Newest - **[Quantum field theory with scale-dependent weighted measures: nonlinear momentum representations, vacuum fluctuations, and causal structure](https://space-kitty.com/papers/arxiv:2610.39285)**. S. González-Salud, R. Cartas-Fuentevilla · 2026 (arXiv:2610.39285). Source: - **[Digital-Analog Quantum Neural Network with Long-Range Interactions](https://space-kitty.com/papers/arxiv:2610.39847)**. Zongyuan Ge, Daili Li, Zewen Zhang · 2026 (arXiv:2610.39847). Source: - **[Process matrices on generalised theories](https://space-kitty.com/papers/arxiv:2610.40448)**. Matt Wilson, James Hefford, Timothée Hoffreumon · 2026 (arXiv:2610.40448). Source: - **[On the gauge-invariant orbital angular momentum of vector optical vortices in the paraxial approximation and beyond](https://space-kitty.com/papers/arxiv:2610.40523)**. E. V. Maliutina, E. A. Vashukevich, V. A. Leonov et al. · 2026 (arXiv:2610.40523). Source: - **[Experimental Observation of Error-Resilience Phase Transition on Superconducting Quantum Processor](https://space-kitty.com/papers/arxiv:2610.38148)**. Emanuel Dallas, Mario Morford-Oberst, Eric Weine · 2026 (arXiv:2610.38148). Source: - **[Momentum-space entanglement and mutual information in Kaluza-Klein reduction](https://space-kitty.com/papers/arxiv:2610.37009)**. Udaykrishna Thattarampilly, Yunlong Zheng · 2026 (arXiv:2610.37009). Source: - **[Quantum Foundations Beyond Bell's Theorem: A Topical Review](https://space-kitty.com/papers/arxiv:2610.37854)**. Flavio Del Santo, Nicolas Gisin · 2026 (arXiv:2610.37854). Source: - **[Electron-photon entanglement in real space from quantum electrodynamics](https://space-kitty.com/papers/arxiv:2610.35467)**. Ivan P. Christov · 2026 (arXiv:2610.35467). Source: - **[Network nonlocality with tripartite entangled sources](https://space-kitty.com/papers/arxiv:2610.35838)**. Sudip Chakrabarty, Amit Kundu · 2026 (arXiv:2610.35838). Source: - **[Dynamical Tensor Train Approximation for Wigner--Fokker--Planck Equation with Applications to Open Quantum Systems](https://space-kitty.com/papers/arxiv:2610.36257)**. Geshuo Wang, Jingwei Hu · 2026 (arXiv:2610.36257). Source: --- Space Kitty Archive · https://space-kitty.com · API: https://space-kitty.com/api/v1 · MCP: https://space-kitty.com/mcp · Premium: $0.20/request via x402 (Base Sepolia (testnet)). Metadata: arXiv (CC0), OpenAlex (CC0), Europe PMC (CC BY/CC0 works), ClinicalTrials.gov.