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Conjuring a Neutron Star from a Nanowire
Using tiny mechanical devices to create accelerations equivalent to 100 million times the Earth’s gravitational field—mimicking the arena of quantum gravity in the lab.
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 Science 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 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:
”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 all 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.”
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:
“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.”
In remembrance of Jacob Bekenstein, a guest post by his friend and colleague Eduardo Guendelman, Physics Department, Ben Gurion University, Beer Sheva, Israel.
It is with great sorrow that we report on the passing of Professor Jacob D. Bekenstein from the Hebrew University in Jerusalem, Israel. Jacob was born in Mexico City in 1947 and obtained his undergraduate and M Sc degrees in 1969 from the Polytechnic Institute of Brooklyn. His Ph.D work obtained from Princeton University in 1972 under the guidance of John A. Wheeler contained his breakthrough discovery of black hole entropy which started the subject of black hole thermodynamics.
Bekenstein's findings were supported by the discovery of black hole radiation by Hawking, who had initially opposed his ideas. Bekenstein was a postdoctoral fellow at the University of Texas at Austin, and then faculty member (1974-1990) at the Ben Gurion University in Israel where he became full professor in 1978 and then Arnow Professor of Astrophysics in 1983. I had the fortune to be his post doctoral fellow at Ben Gurion University (1985-1988) at the time when he was developing his famous Entropy Bounds, We worked on this subject and this was a great opportunity to get to know not only a great scientist but also an outstanding human being.
Since 1990 he has been at the Hebrew University of Jerusalem (since 1993 as Polak Professor of Theoretical Physics). Other scientific interests Jacob had have been relativistic magnetohydrodynamics, galactic dynamics, physical aspects of information theory, development of consistent theories with a time dependent fine structure constant, the development of alternatives to dark matter by modifying gravity and more recently also designing realistic experiments to explore quantum gravity. .
Bekenstein was a member of the Israel Academy of Sciences and Humanities (since 1997) and of The World Jewish Academy of Sciences, and has been honored with the Landau Prize (1981), the Rothschild Prize (1988), the Israel National Prize (2005), the Weizmann Prize (2011), the Wolf Prize 2012 and most recently the Einstein Prize of the American Physical Society for 2015.
He will be greatly missed as a great scientist and as a great man. He is survived by his wife Bilha and their three children.
This month’s podcast is jam-packed, thanks to all the huge physics announcements made in July.
So, Brendan and I begin with a news round up, discussing the Pluto flyby (with some help from cosmologist Andrew Pontzen), the creation of the pentaquark at the LHC, and the discovery of the most Earth-like planet yet, Kepler 452b.
Then we’re back to our usual in-depth interviews. A couple of weeks ago, I chatted with Frank Drake, one of the pioneers of the Search for Extra Terrestrial Intelligence (SETI), in the run-up to the launch of a $100 million project to hunt for alien communications. I wrote an article for Nature about the project, which you can read here. But in our podcast interview, I had the chance to ask Drake more about his long-running history with SETI, why he sticks with it despite the lack of success, and his work on the Drake equation for estimating the number of technological civilisations on other worlds. He also talks about why he’s scared that the aliens might be sending us information encoded as holograms. And in the extended podcast interview, he tells us about new job opportunities in SETI.
Pluto, pentaquarks & Earth 2.0; Frank Drake talks about the new 100 million dollar hunt for alien life; conjuring a neutron star from a nanowire; & "Edge of the Sky" talks physics without the jargon.
The project is funded by Russian billionaire Yuri Milner. What do you think? If you had $100 million to spend on one question in science, what would choose?
Next up, FQXi member Keith Schwab talks about his quest to mimic the gravitational effects on the surface of a neutron star, by accelerating a nanowire. Reporter Carinne Piekema wrote about Schwab’s experiments for us here, and now you can listen to him discuss how they could help those who want to learn more about quantum gravity.
And finally, a “radical experiment in science communication” — which is what cosmologist Roberto Trotta of Imperial College London calls his new book, “The Edge of the Sky.” In it, he attempts to junk jargon by describing the workings of the universe using only the 1000 most common words in the English language, as he explains to Sophie Hebden.
This past winter, FQXi announced it's fifth Large Grant program, on the topic of The Physics of What Happens – a call for proposals for research and outreach projects on "Events". I am happy to announce that from an initial group of almost 250 proposals, we now have the list of 20 grantees. You may view the list here. These grants will give the research teams funding for the next two years, starting September 2015.
The total amount given out comes to $1.85M. This is a relatively tiny amount in the world of physics, especially considering that this is an international grant program. This fact means that, while our review panel ultimately preferred these 20, many of the other proposals were excellent, worthy projects, which we would gladly support if we had the funds.
For the sake of discussion, I’d like to mention a few research themes that showed up in multiple applications, possibly suggesting the directions that many researchers are looking these days. These hot topics include:
1. Nonlocality (i.e. Is an event defined by what happens in multiple locations?).
2. The Nature of Causality in a quatum world.
3. Noncontextuality, possibly as the prime indicator of quantum-ness (above the previous favorite, entanglement).
We again congratulate our new grantees. We also thank everyone who applied, especially those who were invited to submit full proposals, which took a great deal of time and resources to prepare. We wish to offer another round of grants in the near future, and we wish everyone will take a chance to apply.
The podcast features attendees at the New Directions in the Foundations of Physics meeting, held annually in Washington, DC. This meeting is one of the only recurring meetings that brings together physicists and philosophers in the same room to discuss the state of the art in their fields.
How do we communicate foundational physics to the public? Panel discussion with physicists and communicators Sabine Hossenfelder, Matt Leifer, Dagomir Kaszlikowski & Brendan Foster, from the New Directions meeting in Washington, DC.
Following Sabine, Matt, and Dag, the group at large turned to Star Trek, tiny books, physics slogans, and more. On the recording, you’ll hear from Michael Fisher, Alexei Grinbaum, Jos Uffink, Alex Wilce, David Wallace, Melissa Jacquert, and Mile Gu.
Visit the podcast page to listen and find links to much more, including Sabine and Matt’s blogs.
The Reality of the Wavefunction By ZEEYA MERALI
[picture]A couple of months ago we spoke with quantum physicists Martin Ringbauer and Alessandro Fedrizzi of the University of Queensland, in Australia, on the podcast, about their experiment looking into the nature of the wavefunction. Their results...
Collapsing Physics, Celebrating Ghirardi By ZEEYA MERALI
[picture]Yesterday afternoon at the quantum foundations meeting in Erice (supported by COST) we celebrated the 80th birthday (somewhat in advance) of GianCarlo Ghirardi who famously worked on collapse models, in an attempt to deal with the quantum...
Detecting Dark Matter Using Space-Based Quantum... By ZEEYA MERALI
[picture]I’m lucky enough to be attending a COST Action workshop on quantum foundations currently taking place in Erice, Italy, with lots of FQXi folk in attendance. (Thank you to the organisers, FQXi’s Angelo Bassi, Catalina Oana Curceanu and...
A mathematical philosophy - a digital view By JOSELLE KEHOE
I’ve become fascinated with Gregory Chaitin’s exploration of randomness in computing and his impulse to bring these observations to bear on physical, mathematical, and biological theories. His work inevitably addresses epistemological questions...
2014: Paradoxical Cats and the Physics Year in... By ZEEYA MERALI
[picture]It's become a bit of a tradition for quantum physicist and FQXi member Ian Durham to join us on the podcast each December to choose his favourite physics stories of the year. As always, Ian's gone for an unconventional top pick. I'd be...