user: Guest post by Andrew Grant 
  
 Combine the extraordinary precision of atomic clocks with the bizarre facets of quantum entanglement, and you get a global quantum network of clocks that does far more than just measure time. Quantum timekeeping could enable startlingly precise measurements of Earth’s terrain. 
  
 “You’d be able to see someone digging a tunnel under the U.S.-Mexico border from space,�? says Chris Monroe, a physicist at the Joint Quantum Institute at the University of Maryland in College Park. 
  
 The technology does not yet exist to build such a superclock, but “all the building blocks are there,�? says Harvard physicist Eric Kessler. He and colleagues lay out a blueprint June 15 in Nature Physics. 
  
 For more on the mind-blowing implications of linking a new generation of atomic clocks, read SN's Quantum timekeeping feature. ||||| Comstock/Getty 
  
 By exploiting the tricks of quantum physics, researchers say they could build a worldwide network of atomic clocks that are much more accurate than any single clock in existence today. 
  
 This global hook-up would enable countries to synchronize timekeeping standards and improve space navigation. It could also aid the exploration of fundamental physics concepts, such as the long-sought gravitational waves thought to be rippling through space and time. 
  
 “We’re trying to be a little bit visionary,” says co-author Eric Kessler, a physicist at Harvard University in Cambridge, Massachusetts. “All the building blocks have been demonstrated in principle, and we want to show what might lie ahead if all these fields merge together.” Kessler and his colleagues, led by Mikhail Lukin of Harvard, describe the quantum superclock today in Nature Physics1. 
  
 Tangled time 
  
 The idea combines two popular research trends. The first is atomic clocks, which are becoming more precise as scientists improve ways of measuring superfast fluctuations in the energy states of charged particles or atoms. The second is quantum entanglement, in which pairs of particles become linked in such a way that measuring a property of one of them instantaneously determines the same property for the other. Lukin’s group wants to take a number of atomic clocks and entangle atoms from one clock to the next. 
  
 Imagine a set of ten satellites orbiting Earth, each carrying its own atomic clock, says Kessler. One satellite, as the network's centre, would start by preparing its clock particles in an entangled state. It would then communicate with a neighbouring satellite to extend the entanglement there. The linking would eventually spread through the whole fleet, joining the satellites in one quantum network. 
  
 Entanglement improves the precision of an atomic clock by reducing measurement noise (see 'Atomic clocks use quantum timekeeping'). So all of the clocks in a joined network perform better than each would alone, Kessler says. “The clocks behave as if they have a single giant pendulum, which has a better accuracy,” he says. 
  
 Sharing information also makes the clocks more stable. Individual clocks could send occasional updates to the central satellite, reporting how well or poorly they are keeping time. The network could then fine-tune its overall performance. “If you have more clocks in the network, it just makes it a better machine,” says Christopher Monroe, a physicist at the University of Maryland in College Park. 
  
 Better timekeeping would enable global financial markets to operate in near-perfect synchronicity. The clock network could also be used to improve the performance of Global Positioning System (GPS) satellites, which rely on accurate time standards to pinpoint locations on the ground. Other, more fundamental applications might include exploring the effects of the general theory of relativity, because the linked clocks would also serve as a common frequency reference that could be used to detect subtle shifts in space and time. 
  
 If it all sounds too good to be true, that is because the superclock is a long way from reality. Although certain parts of the clock network have been made in laboratories, much work remains to be done. As many as 14 atoms have been entangled at a time2, for instance, but the fidelity of that link is much less than would be needed for each clock in a network. The type of entanglement states that would be most useful for linked clocks have been generated only for fewer than 10 atoms, says Eugene Polzik, a physicist at the Niels Bohr Institute in Copenhagen. And no one has come close to demonstrating entanglement in space. 
  
 James Thompson, a physicist at JILA in Boulder, Colorado — a joint institute of the University of Colorado and the National Institute of Standards and Technology — says that the paper is important because it lays out ways in which quantum entanglement can improve scientists' ability to make measurements, such as in atomic clocks. “That's a big deal,” he says. “Everyone kind of assumes we'll be able to generate a lot of entanglement and observe it and do things with it, but that just hasn't really been the case.” Thompson has been working to improve quantum entanglement in the lab to make it more practical to work with. 
  
 “There’s no doubt this is a very futuristic proposal,” says Kessler. “We’ve got a long way to go.” ||||| What does light from the most distant objects in the universe and quantum mechanics have in common? 
  
 More than most would imagine --- potentially including answers to whether quantum encryption technology is hackable to whether physics on the smallest scales allows for complete philosophical “free will.” 
  
 A recent paper published in the journal Physical Review Letters details National Science Foundation-funded research that proposes a real-time experiment that would use incoming light from ancient quasars --- literally on opposite ends of the universe --- to close one of the last major theoretical loopholes in quantum mechanics. 
  
 “We wanted to come up with a potential test that could close one of the last major remaining quantum physics loopholes that could still allow entangled particle experiments to be interpreted according to classical physics,” said Andrew Friedman, an MIT postdoctoral fellow and the paper’s second author. 
  
 As Friedman explains, when pairs of particles are in an "entangled" quantum state, measuring one particle seems to affect the outcome of a measurement conducted on the other, even if the particles are so far apart that no known signals could have traveled between them during the measurements. Fifty years ago, physicist John Bell quantified the maximum amount that such measurements could be correlated, assuming both particles behaved independently. 
  
 No one would expect to find the roll of the dice to be the same across those two separate craps tables more often than chance, says David Kaiser, an MIT theoretical physicist and one of the paper’s co-authors. Bell’s Theorem, explains Kaiser, puts a mathematical limit on how often these measurements could line up and agree if the particles were independent. 
  
 However, Einstein, says Kaiser, called “quantum entanglement,” the concept that subatomic particles can be connected across space and time and, as a result, affect each other instantly ---“spooky action at a distance.” 
  
 Einstein thought that the theory must be wrong, says Kaiser, who says he would expect their own proposed experiment’s measurements to look like the predictions of quantum mechanics. 
  
 The concept, says Friedman, is to focus on whether the settings of the detectors that measure the entangled particles could have been correlated with any "hidden" information in their shared pasts. That is, correlations not accounted for by quantum mechanics. By selecting cosmic sources that have never been in causal contact with each other, or anything else in their shared past, he explains, the experiment would place the most stringent limit possible on this important, but often overlooked, loophole. 
  
 But as Friedman explains, “no matter how strange it sounds,” it is a logical possibility that some shared events could have somehow influenced both detector settings to make them correlated and thus not as completely free and independent as one would expect. If so, the physicists might not have complete free will to choose the settings of their detectors. 
  
 “We ultimately hope to collaborate with our experimentalist colleagues who have expressed sincere interest in performing the experiment,” said Friedman. “Our role would be mainly to help choose which cosmic sources to observe, help design an observational program, and potentially help interpret the results of the experiment from the theoretical side.” 
  
 To that end, the paper’s authors have been discussing their proposed experiment with an experimental physics group at the University of Vienna which has already performed similar experiments in the Canary Islands. 
  
 “They've been able to send entangled optical photons 144km through the open air between the islands of La Palma and Tenerife,” said Friedman. 
  
 The Vienna group, headed by longtime quantum physicist Anton Zeilinger, found that the outcomes of measurements using detector settings chosen with quantum random number generators were “correlated” and thus obeyed the laws of quantum mechanics. |||||
What is a one-paragraph summary of the above article?


assistant: – Sick of missing appointments by milliseconds because of inaccurate atomic clocks? Researchers say they could use quantum physics to create a timekeeper so accurate it could help explain some of the mysteries of time itself. The "quantum superclock" would involve multiple atomic clocks, each in its own satellite orbiting the Earth and each carrying pairs of linked particles entangled in such a way that "measuring a property of one of them instantaneously determines the same property for the other," Nature explains—a phenomenon known as "quantum entanglement" that Albert Einstein called "spooky action at a distance," notes Forbes. In the case of these satellites, a central satellite would fashion its clock particles in an entangled state, then extend the entanglement to another satellite, and so on, until the quantum network is created. The linked clock would be far more accurate than anything that exists today, allowing for precise linking of financial markets, better space navigation, and even the detection of subtle shifts in space and time. The researchers say that in addition to measuring time, the quantum timepiece could measure the Earth's terrain so accurately that somebody digging a tunnel under the US-Mexican border could be spotted from space, Science News reports. Much research remains to be done before such a superclock can become a reality, but "we're trying to be a little bit visionary," says Harvard physicist Eric Kessler. "All the building blocks have been demonstrated in principle, and we want to show what might lie ahead if all these fields merge together." In the meantime, the US has unveiled what it says is the most accurate clock in the world.