Spookiness, it seems, is here to stay. Quantum theory has been put to its most stringent “loophole free” test yet, and it has come out victorious, ruling out more common sense views of reality (well, mostly). Many thanks to Matt Leifer for bringing this experiment -- by a collaboration of researchers in the Netherlands, Spain, and the UK -- to my attention (arXiv:1508.05949). A few years ago, I wrote a feature for [i]Science[/i] about the quest to close loopholes in quantum entanglement experiments, with a number of groups around the world vying to perform the perfect test. ("Quantum Mechanics Braces for the Ultimate Test.") In that article, I quote quantum physicist and FQXi member Nicolas Gisin saying: “This race is on because the group that performs the first loophole-free test will have an experiment that stands in history.” We may now have a winner. The test is a version of an experiment set out in the 1960s, by Irish physicist John Bell. He came up with a way of working out whether nature was really as spooky as it seems on the quantum level, or if a more common sense explanation was possible. The “sensible” view of the world, in this case, is taken to be “local” and “realistic.” “Local,” in this context, means that information cannot travel between objects faster than the speed of light, so instantaneous communication is impossible. “Realistic” means that the properties of particles are set before they are observed, and are not affected by measurements made on them. By contrast, quantum theory says that prior to measurement, particles can exist in a murky superposition state where their properties are not clearly defined; it’s only upon measurement that their properties click and become well-defined. And quantum theory allows two entangled particles to become linked in such a way that when a measurement is performed on one (breaking it out of superposition, and clicking it into a well-defined state), the properties of its entangled partner will likewise become defined, instantaneously — no matter how far apart they are separated. Bell suggested that experimenters should entangle a string of particles and measure how well their properties match up. He derived a theorem showing that the common sense view of the world (local realism) can only account for correlations between the particles up to a certain limit. If experiments measured a violation of that bound, then the common-sense view would have to be given up in favour of the spooky quantum one. Those experiments were first carried out in the 1970s and, more famously and strictly, in 1980s, and have been performed many times since, and always seem to come down on the side of quantum theory. This has convinced most physicists that the world truly is bizarre on tiny scales. But all experiments have loopholes, and to get a truly definitive result, these need to be closed. One such loophole is the “detection loophole”. In many Bell tests, experimenters entangle photons and then measure their properties. The trouble is photons zip about quickly, and often simply escape from the experiment before being detected and measured. Physicists can lose as many as 80 per cent of the photons in their test. That means that experimenters have to make a ‘fair sampling’ assumption that the ones that they *do* detect are representative of the ones that have gone missing. For the conclusions to be watertight, however, you really want to keep track of all the subjects in your test. It is easier to keep hold of entangled ions, which have been used in other experiments. The catch there, however, is that these are not often kept far enough apart to rule out the less spooky explanation that the two entangled partners simply influence each other, communicating at a speed that is less than the speed of light, during the experiment. This is known as the “communication loophole” or the “locality loophole.” In the new paper by Hensen et al, the authors describe measuring electrons with entangled spins. The entangled pairs have been separated by 1.3 km, to ensure that they do not have time to communicate (at a speed slower than the speed of light) over the course of the experiment. They cleverly use a technique known as "entanglement swapping" to tie up both loopholes, combining the benefits of photons (which can travel long distances) with electrons (which are easier to monitor). Their electrons are placed in two different labs, 13km apart. The spin of each electron is then entangled with a photon and those two photons are fired off to a third location, where they are entangled with each other. As soon as the photons are entangled, BINGO, so too are the two original electron spins, seated in vastly distant labs. The team carried out 245 trials of the experiment, comparing entangled electrons, and report that Bell’s bound is violated. From their paper: [i]”Our experiment realizes the first Bell test that simultaneously addresses both the detection loophole and the locality loophole. Being free of the experimental loopholes, the setup can test local realist theories of nature without introducing extra assumptions such as fair-sampling, a limit on (sub-)luminal communication or the absence of memory in the setup. Our observation of a loophole-free Bell inequality violation thus rules out [/i]all[i] local realist theories that accept that the number generators timely produce a free random bit and that the outputs are final once recorded in the electronics. This result places the strongest restrictions on local realistic theories of nature to date.”[/i] As a test of the foundations of reality, for most physicists, these experiments dot the i’s and cross the t’s. It seemed unlikely that given the other Bell tests performed so far — even with their loopholes — that quantum theory would be found wanting, in a loophole-free test. That’s because each of the earlier experiments were so different from each other, and had different weaknesses, that nature would have to have been cunning, in quite different and particular kinds of ways in each previous experiment, to keep fooling us into thinking quantum theory was correct, if it is not. But it is important, nonetheless, to test quantum theory to its limits. After all, you never know. There are also huge practical applications, though. A major motivation, as I explain in the Science feature, is that loophole free Bell tests are an essential step towards ‘device-independent quantum cryptography’ — creating a security system so tight that you could trust it even if you bought it from your worst enemy. Such a device would go beyond those quantum cryptographic systems that are already in place, which use entanglement to add create “unhackable” keys. In those systems, you share a string of entangled particle pairs between two parties (the sender and receiver) and they each independently perform measurements of their set of particles to generate a matching string of 0s and 1s to make up a key that only they should know. If a hacker tries to eavesdrop on the system, their presence will disrupt the quantum key, alerting the legitimate users and raising an alarm. Those systems are fine, assuming you really have been sold a quantum cryptographic system. But an unsuspecting buyer could be tricked by a hacker purporting to sell a genuine quantum cryptographic device, who actually just gives them a black box, preprogrammed with a string of 0s and 1s that she’s set up beforehand. The user would be none the wiser. To get around this, in 1991, Artur Ekert came up with the idea for a device that had to verify its quantum credentials using a Bell test at the same time as generating the key, so the user would know that it was working correctly, and was genuinely using a quantum process to produce the key. But such “device independent quantum cryptography” can only be trusted if the Bell tests are watertight. As Gisin told me for the Science piece, “It’s unlikely that nature is so malicious that it conspires with the apparatus to hold back particular photons just to fool us into thinking that quantum mechanics works,” but, a “hacker—by definition—is malicious enough to exploit the detection loophole to fool us into thinking that a quantum process has taken place.” There is still another way that nature could be tricking us in quantum tests. It seems a bit outlandish, but it’s possible that experimenters are somehow being manipulated into measuring certain properties in tests and not others, distorting the results. This is sometimes called the “freedom-of-choice” loophole. Last year, I wrote about a fun experiment that used light from distant quasars to help experimenters choose what measurements to make in the lab — in an attempt to rule out the possibility that the experimenters choices were being mysteriously biased by stuff in the experiment itself. That article appeared in Nature, “Cosmic Light Could Close Quantum Weirdness Loophole”. The authors touch on remaining loopholes at the end of their paper: [i]“Strictly speaking, no Bell experiment can exclude the infinite number of conceivable local realist theories, because it is fundamentally impossible to prove when and where free random input bits and output values came into existence. Even so, our loophole-free Bell test opens the possibility to progressively bound such less conventional theories: by increasing the distance between A and B (testing e.g. theories with increased speed of physical influence), using different random input bit generators (testing theories with specific free-will agents, e.g. humans), or repositioning the random input bit generators (testing theories where the inputs are already determined earlier, sometimes referred to as “freedom-of-choice” ). In fact, our experiment already excludes all models that predict that the random inputs are determined a maximum of 690 ns before we record them, because the inequality is still violated for a much shorter spin readout.”[/i] (Updated to include my write up about this experiment for [i]Nature[/i], 27 August 2015: Quantum 'spookiness' passes toughest test yet. With comments from FQXi members Nicolas Gisin, Anton Zeilinger, and Matt Leifer.)

It has been brought to my attention that the discussion threads for Parts 1, 2 and 3 of "To Be or Not to Be (a Local Realist)" and "On the Origins of Quantum Correlations" are getting increasingly unwieldy and slow, due to the huge numbers of comments. Since there have also been recent further contributions to this discussion on the arXiv, from Richard Gill and from Joy Christian, I am opening up this thread for new discussions about these recent papers. Please be polite and reasonable in your responses. No personal comments are necessary.

As the LHC supposedly gears up, Harvard physicist Kevin Black, based at CERN, investigates rumors that the particle accelerator may, in fact, soon be shut down—by ripples from the future. From Kevin Black: I came across a bizarre paper recently suggesting that the LHC might be shut down. Not because of the funding cuts that have been threatening particle physics projects around the world, nor because of law suits accusing the LHC of threatening life on Earth. (Not even because we at the LHC have recently been accused of having far too much fun rapping.) No, the paper suggested that future effects caused by the production of particles, such as the Higgs, could ripple backwards in time and prevent the LHC from ever operating. If it hadn't been written by two very well respected and accomplished theoretical physicists, I would have stopped reading at the title alone:"Test of Effect from Future in Large Hadron Collider; A Proposal". To be completely honest, the title reads like titles that occasionally appear in my inbox—“Relativity Principle Untenable," "Quantum Mechanics a Hoax," and other nerdy versions of the emails from the supposed attorney of my long-lost Nigerian uncle who apparently has died and left me millions of dollars, if I can only send him $50,000 so that he can get it to me. But I didn't stop. I read the article. I read it for another reason other than the somewhat awkwardly sounding title and not just because the authors, Holger Nielsen, of the University of Copenhagen, and Masao Ninomiya, of Kyoto University, are somewhat famous. I read it because when I come across such things it tends to remind me of the first time I learned about quantum mechanics. To be honest, if it hadn't come from a professor at a university and a published text book I would have thought that the whole thing was some sort of a scam as well. I mean, really? Sometimes it acts as a wave and sometimes it acts as a particle? The first time I heard about wave/particle duality I was expecting to be asked to send the authors money (perhaps to Nigeria?). So what did the article say? Well, it started out with a reasonable enough point. One of the basic assumptions of classical physics is that time flows in one direction and that when describing a physical system one needs to know the equations of motion and the initial conditions in order to predict the future behavior of a classical system. However, quantum mechanics changes this a bit. Classical mechanics can be formulated in such a way that one sets up an “action” integral. The solution to the physical system can be expressed as the path that minimizes the action integral. It turns out that in quantum mechanics one needs to not simply take one path—but take the sum over all possible paths. For example, if you want to work out how a photon gets from a lightbulb to your eye, you need to take into account not just its straight-line trajectory, but contributions of all possible paths it could have taken, including paths where the photon bounces round the room. It's a bit strange, but it seems to work and 60 years+ of detailed experiments have confirmed this description over and over again to remarkable quantitative precision. The authors of this paper claim to show that other terms can be added to the quantum mechanical action that are consistent with current theory and experiment. However, some of these possible terms include conditions in the future that need to be taken into account and summed over. That is to say, what happens in the future could (according to this paper) affect what happens in the present. Why the LHC? The authors argue that these sorts of time-violating interactions could be associated with whatever new particles we create at the LHC. For example, the production of a large number of Higgs particles in the future could have a backwards-in-time causal effect on the machine that produced them, stopping the machine from ever running. As possible “evidence” for such a backwards-in-time effect, the authors cite the now-canceled Superconducting Super Collider (SSC)—a particle accelerator that was meant to hunt the Higgs and was partially constructed in Texas before Congress pulled the plug on the project. As the authors write in their paper: “Such a cancellation after a huge investment is already in itself an unusual event that should not happen too often. We might take this event as experimental evidence for our model in which an accelerator with the luminosity and beam energy of the SSC will not be built.” It’s as though the Higgs plays the role of the time traveler who goes back to the past and murders his grandfather, thus preventing his own birth. Now, I must admit that this is where I started to get a bit skeptical. The authors go on to suggest that the LHC is also under threat from a possible “miraculous” shutdown caused by the backward influence of particles like the Higgs, which it may create in the future: “Since the LHC has a performance approaching the SSC, it suggests that also the LHC may be in danger of being closed under mysterious circumstances.” Visions of the X-Files’ Fox Mulder launching into some hour-long diatribe about future conditions (and of course government conspiracies) started to appear. But I read on nonetheless, becoming a little bit more amused and a lot more confused as I tried to finish the paper. In order to make it topical they proposed an experiment. Play a game of cards—a kind of particle physics tarot—to determine if future LHC conditions could affect the draw of the cards. The idea being that if the cards were arranged to represent different possible LHC outcomes—discovery of the Higgs particle, discovery of SUSY, failure or cancellation of the LHC, destruction of the world by the creation of mini-black holes, etc—then what actually happens in the future could affect the outcome of the card drawn now. As crazy as it sounds, it’s at least a novel idea. Certainly, time travel and causality violation are topics that physicists, such as FQXi’s Ken Olum, are seriously investigating (see “Charting the River of Time”); so why not look for backwards causation at the LHC? As an experimentalist working at CERN, I’m ready to pull cards out of a hat in the name of science, even at the risk of prognosticating the demise of the LHC. However, I can't get my head around one basic thing. How can this possibly prove or disprove the theory? They went through some argument (which I can't say I completely understood) claiming that a concrete test of this idea could be put in place, but I am not convinced. Even if I pull the “shutdown card” and the LHC is indeed shut down, how will I know that this is proof and not just a strange coincidence? Conventionally, scientific hypotheses are considered “scientific” if and only if they can be falsified by some experiment (at least in principle). That is to say, it may not be technically possible to conduct such an experiment right now but at least in principle such an experiment could be made. That's the part I couldn't get my head around. How could you ever design an experiment that would disprove future causal influence on a current condition? I just can't imagine how one could do that, but I am open to suggestions. Any ideas? So, for now, I will just put down the paper and get back to the grind of my usual days—debugging software and trying to commission the ATLAS muon spectrometer. I will pass this paper off as quirky, but probably not likely to lead to any major discoveries. But then again, one day I might be teaching this experiment out of a text book to incredulous students who think I am trying to sell them the Brooklyn bridge... -- Kevin Black is a postdoc at Harvard University. He works with the ATLAS experiment at CERN, and sincerely hopes that LHC isn’t shut down any time soon.