Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km

there is no way for nature to "trick" you in this way, "enough" is 83% for the standard CHSH inequality or 66% if you use a slightly modified. Recent experiments (in the last decade or so) are substantially over the threshold for the detection loophole to be a problem. This is one of the earliest papers where this loophole was closed with space-like seperated detectors from 2015. In this paper they used entangled NV centers in diamond as their qubits of choice, and so essentialy had zero events lost. https://arxiv.org/abs/1508.05949 This is a second one from the same time. This one uses a more standard setup with photons and worked with about 75% detector efficiency for each party (well above the 66% required) https://arxiv.org/abs/1511.03189 And this is a third with an efficiency of 78% for Alice and 76% for Bob https://arxiv.org/abs/1511.03190 I therefore have a new challenge - break the CHSH inequality, while rejecting fewer than 17% of the events, or break the (easier) modified Bell inequality used in papers 2 & 3 while rejecting fewer than a third. Edit: This is another, more recent paper where they use superconducting qubits and again lose no events https://www.nature.com/articles/s41586-023-05885-0

equipment. Since you don't specify what this hidden variable is, it encapsulates anything that is allowed by classical physics (remember we started out with a classical state). Now lets allow entangled states between the kids (states that can only be specified if you have both subsystems, locally if you measured such a state your result will look at least a bit random or even completely random). Then you can prove that the two kids can do the task (that has classical success probability 75%) with success probability ~85% all of a sudden. You can write down general inequalities that describe such behaviour and that's what Bell inequalities are. The fact that entangled states in quantum mechanics violate the classical bound (which you cannot break if you demand your two kids can only share states allowed by classical physics, even if you allow unspecified hidden variables), proves that nature is not classical. It doesn't prove that nature is quantum, it just proves that nature is not classical. Bell inequalities have also been unambiguously experimentally verified [1]. In conclusion, nature allows for two-party (or multi-party) states that cannot be described by just describing the local parts of it. There is no information in the local parts, but if you look at both parts together then there is information. Think "the whole is more than the sum of its parts". Classically, the whole is the sum of its parts. Classically, you read a book page by page. If your pages are entangled with each other, you could only read your book by looking at all pages together, because if you looked a single pages separately anything written there would look like random garbage to you. The question that remains is: Where is the information then, physically? In some sense you could say it does not exist before measurement, because only when you look at the correlations between the local measurement results of the two kids will you find out that the underlying state must have been entangled (which you get to know if the state violates a Bell inequality). But that is just one possible interpretation, there still is a lot of scientific debate about what quantum mechanics really means. These are exactly the scenarios that make quantum so weird but fascinating at the same time. [1] https://www.nature.com/articles/nature15759

realism, 2007 https://arxiv.org/abs/0704.2529 Wherein the authors show, using previously untested correlations between two entangled photons, that maintaining realist interpretations of quantum mechanics (such as pilot-wave theory) as a fundamental concept require the introduction of locality-defying 'action at a distance' actions. Quantum erasure with causally disconnected choice, 2012 https://arxiv.org/abs/1206.6578 Wherein the authors, using an interferometric quantum eraser experiment, eliminate past and future communication channels by enforcing Einstein locality via a mechanism that I don't understand yet. They conclude that, "No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether." Experimental non-classicality of an indivisible quantum system, 2011 https://arxiv.org/abs/1106.4481 I understand even less of this one, looks like it has to do with performing the kinds of simultaneous measurements of various properties that one would expect to be able to, if realist interpretations were true. They conclude, "Not only is a single qutrit the simplest system in which such a contradiction is possible, but, even more importantly, the contradiction cannot result from entanglement, because such a system is indivisible, and it does not even allow the concept of entanglement between subsystems." UPDATE: Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km 2015 https://arxiv.org/abs/1508.05949v1 "For more than 80 years, the counterintuitive predictions of quantum theory have stimulated debate about the nature of reality. ... In the past decades, numerous ingenious Bell inequality tests have been reported. However, because of experimental limitations, all experiments to date required additional assumptions to obtain a contradiction with local realism, resulting in loopholes. Here we report on a Bell experiment that is free of any such additional assumption and thus directly tests the principles underlying Bell's inequality. ... This result rules out large classes of local realist theories, and paves the way for implementing device-independent quantum-secure communication and randomness certification." I'm not aware of any significant challenges to these studies.