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  • richardmitnick 2:41 pm on August 16, 2016 Permalink | Reply
    Tags: , , , Pan Jian-Wei, , Quantum entanglement   

    From Nature- “China’s quantum space pioneer: We need to explore the unknown” 

    Nature Mag
    Nature

    14 January 2016 [Just appeared in social media, probably because of new Chinese spacecraft that went up today.]
    Celeste Biever

    1
    Pan Jian-Wei is leading a satellite project that will probe quantum entanglement. Tengyun Chen

    Physicist Pan Jian-Wei is the architect of the world’s first attempt to set up a quantum communications link between Earth and space — an experiment that is set to begin with the launch of a satellite in June.

    The satellite will test whether the quantum property of entanglement extends over record-breaking distances of more than 1,000 kilometres, by beaming individual entangled photons between space and various ground stations on Earth. It will also test whether it is possible, using entangled photons, to teleport information securely between Earth and space.

    On 8 January, Pan, who works at the University of Science and Technology of China in Hefei, won a major national Chinese science prize (worth 200,000 yuan, or US$30,000) for his contributions to quantum science. He spoke to Nature about why his experiments are necessary and about the changing nature of Chinese space-science missions.

    How are preparations for the launch going?

    We always have two feelings. We feel, “Yes, everything is all right,” and then we are happy and excited. But we have, a couple of times, thought, “Probably our project will collapse and never work.” I think the satellite should be launched on time.
    What technical challenges do you face?

    The satellite will fly so fast (it takes just 90 minutes to orbit Earth) and there will be turbulence and other problems — so the single-photon beam can be seriously affected. Also we have to overcome background noise from sunlight, the Moon and light noise from cities, which are much stronger than our single photon.

    What is the aim of the satellite?

    Our first mission is to see if we can establish quantum key distribution [the encoding and sharing of a secret cryptographic key using the quantum properties of photons] between a ground station in Beijing and the satellite, and between the satellite and Vienna. Then we can see whether it is possible to establish a quantum key between Beijing and Vienna, using the satellite as a relay.

    The second step will be to perform long-distance entanglement distribution, over about 1,000 kilometres. We have technology on the satellite that can produce pairs of entangled photons. We beam one photon of an entangled pair to a station in Delingha, Tibet, and the other to a station in Lijiang or Nanshan. The distance between the two ground stations is about 1,200 kilometres. Previous tests were done on the order of 100 kilometres.

    Does anyone doubt that entanglement happens no matter how far apart two particles are?

    Not too many people doubt quantum mechanics, but if you want to explore new physics, you must push the limit. Sure, in principle, quantum entanglement can exist for any distance. But we want to see if there is some physical limit. People ask whether there is some sort of boundary between the classical world and the quantum world: we hope to build some sort of macroscopic system in which we can show that the quantum phenomena can still exist.

    In future, we also want to see if it is possible to distribute entanglement between Earth and the Moon. We hope to use the Chang’e programme (China’s Moon programme) to send a quantum satellite to one of the gravitationally-stable points [Lagrangian points] in the Earth-Moon system.

    How does entanglement relate to quantum teleportation?

    We will beam one photon from an entangled pair created at a ground station in Ali, Tibet, to the satellite. The quantum state of a third photon in Ali can then be teleported to the particle in space, using the entangled photon in Ali as a conduit.
    The quantum satellite is a basic-science space mission, as is the Dark Matter Particle Explorer (DAMPE), which China launched in December.

    Are basic-research satellites a new trend for China?

    Yes, and my colleagues at the Chinese Academy of Sciences (CAS) and I helped to force things in this direction. In the past, China had only two organizations that could launch satellites: the army and the Ministry of Industry and Information Technology. So scientists had no way to launch a satellite for scientific research. One exception is the Double Star probe, launched in collaboration with the European Space Agency in 2003 to study magnetic storms on Earth.

    What changed?

    We at CAS really worked hard to convince our government that it is important that we have a way to launch science satellites. In 2011, the central government established the Strategic Priority Program on Space Science, which DAMPE and our quantum satellite are part of. This is a very important step.

    I think China has an obligation not just to do something for ourselves — many other countries have been to the Moon, have done manned spaceflight — but to explore something unknown.

    Will scientists also be involved in China’s programme to build a space station, Tiangong?

    The mechanism to make decisions for which projects can go to the space station has been significantly changed. Originally, the army wanted to take over the responsibility, but it was finally agreed that CAS is the right organization.

    We will have a quantum experiment on the space station and it will make our studies easier because we can from time to time upgrade our experiment (unlike on the quantum satellite). We are quite happy with this mechanism. We need only talk to the leaders of CAS — and they are scientists, so you can communicate with them much more easily.

    See the full article here .

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    Nature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public.

     
  • richardmitnick 10:52 am on July 13, 2016 Permalink | Reply
    Tags: , Quantum entanglement,   

    From UC Santa Barbara- “Entanglement : Chaos” 

    UC Santa Barbara Name bloc

    July 11, 2016
    Sonia Fernandez

    1
    A quantum qubit array. Photo Credit: Michael Fang/Martinis Lab

    2
    Experimental link between quantum entanglement (left) and classical chaos (right) found using a small quantum computer. Photo Credit: Courtesy Image

    3
    The Google and UCSB researchers, from left to right: Jimmy Chen, John Martinis, Pedram Roushan, Yu Chen, Anthony Megrant and Charles Neill. Photo Credit: Sonia Fernandez

    Using a small quantum system consisting of three superconducting qubits, researchers at UC Santa Barbara and Google have uncovered a link between aspects of classical and quantum physics thought to be unrelated: classical chaos and quantum entanglement. Their findings suggest that it would be possible to use controllable quantum systems to investigate certain fundamental aspects of nature.

    “It’s kind of surprising because chaos is this totally classical concept — there’s no idea of chaos in a quantum system,” Charles Neill, a researcher in the UCSB Department of Physics and lead author of a paper that appears in Nature Physics. “Similarly, there’s no concept of entanglement within classical systems. And yet it turns out that chaos and entanglement are really very strongly and clearly related.”

    Initiated in the 15th century, classical physics generally examines and describes systems larger than atoms and molecules. It consists of hundreds of years’ worth of study including Newton’s laws of motion, electrodynamics, relativity, thermodynamics as well as chaos theory — the field that studies the behavior of highly sensitive and unpredictable systems. One classic example of a chaotic system is the weather, in which a relatively small change in one part of the system is enough to foil predictions — and vacation plans — anywhere on the globe.

    At smaller size and length scales in nature, however, such as those involving atoms and photons and their behaviors, classical physics falls short. In the early 20th century quantum physics emerged, with its seemingly counterintuitive and sometimes controversial science, including the notions of superposition (the theory that a particle can be located in several places at once) and entanglement (particles that are deeply linked behave as such despite physical distance from one another).

    And so began the continuing search for connections between the two fields.

    All systems are fundamentally quantum systems, according Neill, but the means of describing in a quantum sense the chaotic behavior of, say, air molecules in an evacuated room, remains limited.

    Imagine taking a balloon full of air molecules, somehow tagging them so you could see them and then releasing them into a room with no air molecules, noted co-author and UCSB/Google researcher Pedram Roushan. One possible outcome is that the air molecules remain clumped together in a little cloud following the same trajectory around the room. And yet, he continued, as we can probably intuit, the molecules will more likely take off in a variety of velocities and directions, bouncing off walls and interacting with each other, resting after the room is sufficiently saturated with them.

    “The underlying physics is chaos, essentially,” he said. The molecules coming to rest — at least on the macroscopic level — is the result of thermalization, or of reaching equilibrium after they have achieved uniform saturation within the system. But in the infinitesimal world of quantum physics, there is still little to describe that behavior. The mathematics of quantum mechanics, Roushan said, do not allow for the chaos described by Newtonian laws of motion.

    To investigate, the researchers devised an experiment using three quantum bits, the basic computational units of the quantum computer. Unlike classical computer bits, which utilize a binary system of two possible states (e.g., zero/one), a qubit can also use a superposition of both states (zero and one) as a single state. Additionally, multiple qubits can entangle, or link so closely that their measurements will automatically correlate. By manipulating these qubits with electronic pulses, Neill caused them to interact, rotate and evolve in the quantum analog of a highly sensitive classical system.

    The result is a map of entanglement entropy of a qubit that, over time, comes to strongly resemble that of classical dynamics — the regions of entanglement in the quantum map resemble the regions of chaos on the classical map. The islands of low entanglement in the quantum map are located in the places of low chaos on the classical map.

    “There’s a very clear connection between entanglement and chaos in these two pictures,” said Neill. “And, it turns out that thermalization is the thing that connects chaos and entanglement. It turns out that they are actually the driving forces behind thermalization.

    “What we realize is that in almost any quantum system, including on quantum computers, if you just let it evolve and you start to study what happens as a function of time, it’s going to thermalize,” added Neill, referring to the quantum-level equilibration. “And this really ties together the intuition between classical thermalization and chaos and how it occurs in quantum systems that entangle.”

    The study’s findings have fundamental implications for quantum computing. At the level of three qubits, the computation is relatively simple, said Roushan, but as researchers push to build increasingly sophisticated and powerful quantum computers that incorporate more qubits to study highly complex problems that are beyond the ability of classical computing — such as those in the realms of machine learning, artificial intelligence, fluid dynamics or chemistry — a quantum processor optimized for such calculations will be a very powerful tool.

    “It means we can study things that are completely impossible to study right now, once we get to bigger systems,” said Neill.

    See the full article here .

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    UC Santa Barbara Seal
    The University of California, Santa Barbara (commonly referred to as UC Santa Barbara or UCSB) is a public research university and one of the 10 general campuses of the University of California system. Founded in 1891 as an independent teachers’ college, UCSB joined the University of California system in 1944 and is the third-oldest general-education campus in the system. The university is a comprehensive doctoral university and is organized into five colleges offering 87 undergraduate degrees and 55 graduate degrees. In 2012, UCSB was ranked 41st among “National Universities” and 10th among public universities by U.S. News & World Report. UCSB houses twelve national research centers, including the renowned Kavli Institute for Theoretical Physics.

     
  • richardmitnick 12:13 pm on June 9, 2016 Permalink | Reply
    Tags: , , Quantum entanglement   

    From MIT Tech Review: “Proof That Quantum Computers Will Change Everything” 

    MIT Technology Review
    MIT Technology Review

    First Demonstration of 10-Photon Quantum Entanglement

    June 9, 2016
    Emerging Technology from the arXiv

    The ability to entangle 10 photons should allow physicists to prove, once and for all, that quantum computers really can do things classical computers cannot.

    Entanglement is the strange phenomenon in which quantum particles become so deeply linked that they share the same existence. Once rare, entangling particles has become routine in labs all over the world.

    Quantum approach to big data. MIT
    Quantum approach to big data. MIT

    Physicists have learned how to create entanglement, transfer it from one particle to another, and even distil it. Indeed, entanglement has become a resource in itself and a crucial one for everything from cryptography and teleportation to computing and simulation.

    But a significant problem remains. To carry out ever more complex and powerful experiments, physicists need to produce entanglement on ever-larger scales by entangling more particles at the same time.

    The current numbers are paltry, though. Photons are the quantum workhorses in most labs and the record for the number of entangled photons is a mere eight, produced at a rate of about nine events per hour.

    Using the same techniques to create a 10-photon count rate would result in only 170 per year, too few even to measure easily. So the prospects of improvement have seemed remote.

    Which is why the work of Xi-Lin Wang and pals at the University of Science and Technology of China in Heifu is impressive. Today, they announce that they’ve produced 10-photon entanglement for the first time, and they’ve done it at a count rate that is three orders of magnitude higher than anything possible until now.

    The biggest bottleneck in entangling photons is the way they are produced. This involves a process called spontaneous parametric down conversion, in which one energetic photon is converted into two photons of lower energy inside a crystal of beta-barium borate. These daughter photons are naturally entangled.

    2
    Experiment setup for generating ten-photon polarization-entangled GHZ, from the science paper

    By zapping the crystal continuously with a laser beam, it is possible to create a stream of entangled photon pairs. However, the rate of down conversion is tiny, just one photon per trillion. So collecting the entangled pairs efficiently is hugely important.

    That’s no easy tasks, not least because the photons come out of the crystal in slightly different directions, neither of which can be easily predicted. Physicists collect the photons from the two points where they are most likely to appear but most of the entangled photons are lost.

    Xi-Lin and co have tackled this problem by reducing the uncertainty in the photon directions. Indeed, they have been able to shape the beams of entangled photons so that they form two separate circular beams, which can be more easily collected.

    In this way, the team has generated entangled photon pairs at the rate of about 10 million per watt of laser power. This is brighter than previous entanglement generators by a factor of about four. It is this improvement that makes 10-photon entanglement possible.

    Their method is to collect five successively generated pairs of entangled photons and pass them into an optical network of four beam splitters. The team then introduces time delays that ensure the photons arrive at the beam splitters simultaneously and so become entangled.

    This creates the 10-photon entangled state, albeit at a rate of about four per hour, which is low but finally measureable for the first time. “We demonstrate, for the first time, genuine and distillable entanglement of 10 single photons,” say Xi-Lin and co.

    That’s impressive work that immediately opens the prospect of a new generation of experiments. The most exciting of these is a technique called boson sampling that physicists hope will prove that quantum computers really are capable of things classical computers are not.

    That’s important because nobody has built a quantum computer more powerful than a pocket calculator (the controversial D-Wave results aside). Neither are they likely to in the near future. So boson sampling is quantum physicists’ greatest hope that will allow them to show off the mind-boggling power of quantum computation for the first time.

    Other things also become possible, such as the quantum teleportation of three degrees of freedom in a single photon and multi-photon experiments over very long distances.

    But it is the possibility of boson sampling that will send a frisson through the quantum physics community.

    Ref: arxiv.org/abs/1605.08547: Experimental Ten-Photon Entanglement

    See the full article here .

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  • richardmitnick 7:54 am on June 3, 2016 Permalink | Reply
    Tags: , , Quantum entanglement   

    From Daily Galaxy: “‘Quantum Entanglement in Space’ – A New Global Satellite-Based Quantum Network” 

    Daily Galaxy
    The Daily Galaxy

    June 02, 2016
    No writer credit found

    1
    Image credit: With thanks to shockingscience.com

    “We are reaching the limits of how precisely we can test quantum theory on Earth,” says Daniel Oi at the University of Strathclyde. Researchers from the National University of Singapore (NUS) and the University of Strathclyde, UK, have become the first to test in orbit technology for satellite-based quantum network nodes. With a network that carries information in the quantum properties of single particles, you can create secure keys for secret messaging and potentially connect powerful quantum computers in the future. But scientists think you will need equipment in space to get global reach.

    They have put a compact device carrying components used in quantum communication and computing into orbit. And it works: the team report* first data in a paper published 31 May 2016 in the journal Physical Review Applied. The team’s device, dubbed SPEQS, creates and measures pairs of light particles, called photons. Results from space show that SPEQS is making pairs of photons with correlated properties — an indicator of performance.

    “This is the first time anyone has tested this kind of quantum technology in space,” said team-leader Alexander Ling, an Assistant Professor at the Centre for Quantum Technologies (CQT) at NUS.

    The team had to be inventive to redesign a delicate, table-top quantum setup to be small and robust enough to fly inside a nanosatellite only the size of a shoebox. The whole satellite weighs just 1.65-kilogramme.

    Making correlated photons is a precursor to creating entangled photons. Described by Einstein as “spooky action at a distance,” entanglement is a connection between quantum particles that lends security to communication and power to computing.

    “Alex and his team are taking entanglement, literally, to a new level. Their experiments will pave the road to secure quantum communication and distributed quantum computation on a global scale,” said Artur Ekert, Director of CQT, invented the idea of using entangled particles for cryptography. He said, I am happy to see that Singapore is one of the world leaders in this area.”

    Local quantum networks already exist. The problem Ling’s team aims to solve is a distance limit. Losses limit quantum signals sent through air at ground level or optical fibre to a few hundred kilometers — but we might ultimately use entangled photons beamed from satellites to connect points on opposite sides of the planet. Although photons from satellites still have to travel through the atmosphere, going top-to-bottom is roughly equivalent to going only 10 kilometres at ground level.

    The group’s first device is a technology pathfinder. It takes photons from a BluRay laser and splits them into two, then measures the pair’s properties, all on board the satellite. To do this it contains a laser diode, crystals, mirrors and photon detectors carefully aligned inside an aluminum block. This sits on top of a 10 centimetres by 10 centimetres printed circuit board packed with control electronics.

    Through a series of pre-launch tests — and one unfortunate incident — the team became more confident that their design could survive a rocket launch and space conditions. The team had a device in the October 2014 Orbital-3 rocket which exploded on the launch pad. The satellite containing that first device was later found on a beach intact and still in working order.

    Even with the success of the more recent mission, a global network is still a few milestones away. The team’s roadmap calls for a series of launches, with the next space-bound SPEQS slated to produce entangled photons. SPEQS stands for Small Photon-Entangling Quantum System.

    With later satellites, the researchers will try sending entangled photons to Earth and to other satellites. The team are working with standard “CubeSat” nanosatellites, which can get relatively cheap rides into space as rocket ballast. Ultimately, completing a global network would mean having a fleet of satellites in orbit and an array of ground stations.

    In the meantime, quantum satellites could also carry out fundamental experiments — for example, testing entanglement over distances bigger than Earth-bound scientists can manage.

    *Science paper:
    Generation and Analysis of Correlated Pairs of Photons aboard a Nanosatellite

    See the full article here .

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  • richardmitnick 7:48 am on May 8, 2016 Permalink | Reply
    Tags: , Quantum entanglement, ,   

    From Ethan Siegel: “Ask Ethan: Can we use quantum entanglement to communicate faster-than-light?” 

    Starts with a Bang

    5.7.16
    Ethan Siegel

    NanoSail-D poses after a successful laboratory deployment test  NASA
    NanoSail-D poses after a successful laboratory deployment test NASA

    Einstein called it spooky, but if we figure it out right, can we learn about distant star systems instantaneously?

    “Trying to understand the way nature works involves a most terrible test of human reasoning ability. It involves subtle trickery, beautiful tightropes of logic on which one has to walk in order not to make a mistake in predicting what will happen.”

    1
    -Richard Feynman

    Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

    It’s a long shot, but could quantum entanglement be used for communication?

    Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

    2
    The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

    In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

    A quantum optics setup. Image credit Matthew Broome
    A quantum optics setup. Image credit Matthew Broome

    So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

    3
    Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

    Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

    4
    The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

    It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

    5
    A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

    This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

    6
    Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

    Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

    If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

    The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

    See the full article here .

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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

     
  • richardmitnick 2:36 pm on April 30, 2016 Permalink | Reply
    Tags: , Quantum entanglement, ,   

    From Ethan Siegel: “Can We Use Quantum Entanglement To Communicate Faster-Than-Light?” 

    Starts with a Bang

    4.30.16
    Ethan Siegel

    1
    The concept art of a solar sail (Japan’s IKAROS project) at a distant planet or star system. Image credit: Andrzej Mirecki of Wikimedia Commons, under a c.c.a.-s.a.-3.0 license.

    Earlier this month, billionaire Yuri Milner and astrophysicist Stephen Hawking teamed up to announce the Breakthrough Starshot, an incredibly ambitious plan to send the first human-created spacecraft to other star systems within our galaxy. While a giant laser array could, feasibly, launch a low mass, microchip-sized spaceship towards another star at some ~20% the speed of light, it’s unclear how such an underpowered, small device like that would ever communicate across the vastness of interstellar space. But Olivier Manuel had an idea that he submitted for Ask Ethan:

    It’s a long shot, but could quantum entanglement be used for communication?

    It’s certainly worth considering. Let’s take a look at the idea.

    1
    Two coins: one showing heads and the other showing tails. Image credit: United States Mint, public domain.

    Imagine you have two coins, where each one can turn up either heads or tails. You have one and I have one, and we’re located extremely far away from each other. We each toss them up in the air, catch them, and slap them down on the table. When we reveal the flip, we fully expect that there’s a 50/50 chance that each one of us will uncover a “heads” result and a 50/50 shot we’ll each get a “tails.” In the normal, unentangled Universe, your results and my results are completely independent of one another: if you get a “heads” result, there’s still a 50/50 shot for my coin to either display “heads” or “tails.” But under some circumstances, these results could be entangled, meaning that if we do this experiment and you get a “heads” result, you’ll know with 100% certainty that my coin is displaying “tails,” even before I told you. You’d know it instantaneously, even if we were separated by light years and not even a single second had passed.

    2
    The quantum mechanical Bell test for half-integer spin particles. Image credit: Wikimedia Commons user Maksim, under a c.c.a.-s.a.-3.0 license.

    In quantum physics, we normally entangle not coins but individual particles like electrons or photons, where, for example, each photon can have a spin of either +1 or -1. If you measure the spin of one of them, you instantaneously know the spin of the other, even if it’s halfway across the Universe. Until you measure the spin of either one, they both exist in an indeterminate state; but once you measure even one, you immediately know both. We’ve done an experiment on Earth where we’ve separated two entangled photons by many miles, measuring their spins within nanoseconds of one another. What we find is that if we measure one of them to be +1, we know the other to be -1 at least 10,000 times faster than the speed of light would enable us to communicate.

    3
    By creating two entangled photons from a pre-existing system and separating them by great distances, we can know information about the state of one by measuring the state of the other. Image credit: Melissa Meister, of laser photons through a beam splitter, under c.c.-by-2.0 generic, from https://www.flickr.com/photos/mmeister/3794835939.

    So now to Olivier’s question: could we use this property — quantum entanglement — to communicate from a distant star system to our own? The answer to that is yes, if you consider making a measurement at a distant location a form of communication. But when you say communicate, typically you want to know something about your destination. You could, for example, keep an entangled particle in an indeterminate state, send it aboard a spacecraft bound for the nearest star, and tell it to look for signs of a rocky planet in that star’s habitable zone. If you see one, make a measurement that forces the particle you have to be in the +1 state, and if you don’t see one, make a measurement that forces the particle you have to be in the -1 state.

    4
    Artist’s impression of a sunset from the world Gliese 667 Cc, in a trinary star system. Image credit: ESO/L. Calçada.

    Therefore, you reason, the particle you have back on Earth will then either be in the -1 state when you measure it, telling you that your spacecraft found a rocky planet in the habitable zone, or it will be in the +1 state, telling you that it didn’t find one. If you know the measurement has been made, you should then be able to make your own measurement, and instantly know the state of the other particle, even if it’s many light years away.

    5
    The wave pattern for electrons passing through a double slit. If you measure “which slit” the electron goes through, you destroy the quantum interference pattern shown here. Image credit: Dr. Tonomura and Belsazar of Wikimedia Commons, under c.c.a.-s.a.-3.0.

    It’s a brilliant plan, but there’s a problem: entanglement only works if you ask a particle, “what state are you in?” If you force an entangled particle into a particular state, you break the entanglement, and the measurement you make on Earth is completely independent of the measurement at the distant star. If you had simply measured the distant particle to be +1 or -1, then your measurement, here on Earth, of either -1 or +1 (respectively) would give you information about the particle located light years away. But by forcing that distant particle to be +1 or -1, that means, no matter the outcome, your particle here on Earth has a 50/50 shot of being +1 or -1, with no bearing on the particle so many light years distant.

    6
    A quantum eraser experiment setup, where two entangled particles are separated and measured. No alterations of one particle at its destination affect the outcome of the other. Image credit: Wikimedia Commons user Patrick Edwin Moran, under c.c.a.-s.a.-3.0.

    This is one of the most confusing things about quantum physics: entanglement can be used to gain information about a component of a system when you know the full state and make a measurement of the other component(s), but not to create-and-send information from one part of an entangled system to the other. As clever of an idea as this is, Olivier, there’s still no faster-than-light communication.

    7
    Quantum teleportation, an effect (erroneously) touted as faster-than-light travel. In reality, no information is being exchanged faster than light. Image credit: American Physical Society, via http://www.csm.ornl.gov/SC99/Qwall.html.

    Quantum entanglement is a wonderful property that we can exploit for any number of purposes, such as for the ultimate lock-and-key security system. But faster-than-light communication? Understanding why that’s not possible requires us to understand this key property of quantum physics: that forcing even part of an entangled system into one state or another doesn’t allow you to gain information about that forcing from measuring the remainder of the system. As Niels Bohr once famously put it:

    If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.

    The Universe plays dice with us all the time, much to Einstein’s chagrin. But even our best attempts to cheat at the game are thwarted by nature itself. If only all referees and umpires were as consistent as the laws of quantum physics!

    See the full article here .

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  • richardmitnick 9:11 am on April 17, 2016 Permalink | Reply
    Tags: , Quantum entanglement, ,   

    From Science Alert: “Physicists find a way to probe the quantum realm without wrecking everything” 

    ScienceAlert

    Science Alert

    15 APR 2016
    BRENDAN COLE

    1
    solarseven/Shutterstock.com

    In 1930, German theoretical physicist Werner Heisenberg came up with a thought experiment, now known as Heisenberg’s microscope, to try to show why it’s impossible to measure an atom’s location with unlimited precision. He imagined trying to measure the position of something like an atom by shooting light at it.

    Light travels as a wave, and Heisenberg knew that different wavelengths could give you different degrees of confidence when used to measure where something is in space. Short wavelengths can give a more precise measurement than long ones, so you’d want to use light with a tiny wavelength to measure where an atom is, since atoms are really small. But there’s a problem: light also carries momentum, and short wavelengths carry more momentum than long ones.

    That means if you use light with a short wavelength to find the atom, you’ll hit the atom with all of that momentum, and that kicks it around and risks completely changing its location (and other properties) in the process. Use longer wavelengths, and you’ll move the atom less, but you’ll also be more uncertain about your measurement.

    You’re in a bind: any measurement changes what you’re measuring, and better measurements lead to bigger changes.

    It’s also possible to prepare atoms in what’s called an entangled state, which means they act cooperatively like a single atom, no matter how far away they are from each other. If you push one, the rest move like you pushed them all individually. And if you mess up one atom by shooting some light at it, you generally mess up the whole collection.

    In the past, these two effects made it impossible to measure how entangled atoms are arranged without destroying the arrangement and the entanglement – which were presumably prepared for some specific purpose, like to make a quantum computer.

    But now, physicists led by T. J. Elliott from the University of Oxford in the UK have proposed a way to measure large-scale properties of a group of entangled atoms without messing up the entanglement. It isn’t measuring individual atoms – that’s permanently off-limits – but it’s more than physicists have managed to do before.

    Usually, when physicists entangle atoms, they have to be careful that the atoms are all more or less the same when they start out. If there are lots of different kinds of atoms in there, they become a lot harder to match up, so the entanglement becomes more fragile.

    But it’s still possible to make stable groups of entangled atoms that have some outliers among them that are unlike the main group, and the paper’s authors have shown that these outliers can be used to measure things about the main group without messing up their entanglement.

    This includes really basic information like the density of atoms – how closely they are to one another – while they’re entangled, which historically has been out of physicists’ reach in individual experiments.

    Before, physicists had to measure a whole bunch of the entangled atoms really quickly, and they’d have to accept that they were changing things around as soon as they measured that first atom. More measurements might check more atoms, but they’d be increasingly uncertain as time went on.

    Now, all they have to do is measure what the outliers are doing and they can figure out how the atoms are distributed without the chaos. Within some limits, knowledge about the atoms’ density gets better – not worse – as more measurements are made.

    Admittedly, measurements still change things a little bit (light is still being used and Heisenberg’s microscope still applies) but the measurements won’t wreck the whole system like they would have before.

    This method of measuring the outliers is a window into a new realm for physicists, who could previously only see what entangled atoms did, not what they’re doing.

    The researchers simulated a simple system as a proof-of-concept, but they showed mathematically that this should work with a wide range of quantum systems where entanglement plays a key role. And small changes to the method could make it possible to measure properties like the magnetisation of entangled atoms, instead of just their density.

    All with atoms that shouldn’t be in the group in the first place. Not bad, physicists. Not bad.

    The research has been published in the journal Physical Review A.

    Science paper:
    Nondestructive probing of means, variances, and correlations of ultracold-atomic-system densities via qubit impurities

    See the full article here .

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  • richardmitnick 1:27 pm on March 22, 2016 Permalink | Reply
    Tags: , Quantum entanglement,   

    From TUM: “Sensitive quantum particles” 

    Techniche Universitat Munchen

    Techniche Universitat Munchen

    Important quantum physics phenomenon now more readily measurable

    21.03.2016
    Dr. Markus Hey
    Technical University of Munich
    Physics Department, Chair for Condensed Matter and Many-Body Theory (T34)
    James-Franck-Str., 85747 Garching, Germany
    markus.heyl@tum.de

    1
    Measuring multipartite entanglement (Illustration: Harald Ritsch / IQOQI)

    The quantum mechanical entanglement of particles plays an important role in many technical applications. To date, however, the effect has been difficult to measure experimentally. Physicists from the Technical University of Munich (TUM), the University of Innsbruck and the Institute of Photonic Sciences (ICFO) in Barcelona have now developed a new protocol to detect entanglement of many-particle quantum states using established measuring methods.

    In quantum theory, interactions between particles create fascinating correlations known as entanglement. They cannot be explained by any means known to the classical world. Entanglement is a consequence of the probabilistic rules of quantum mechanics and seems to permit a peculiar instantaneous connection between particles over long distances that defies the laws of our macroscopic world – a phenomenon that [Albert] Einstein referred to as “spooky action at a distance.”

    Developing protocols to detect and quantify entanglement of many-particle quantum states is a key challenge for current experiments because entanglement becomes very difficult to study when many particles are involved. “We are able to control smaller particle ensembles well, where we can measure entanglement in a relatively straight forward way,” says quantum physicist Philipp Hauke. However, “when we are dealing with a large system of entangled particles, this measurement is extremely complex or rather impossible because the resources required scale exponentially with the system size.

    ”Markus Heyl from the Technical University of Munich, Philipp Hauke and Peter Zoller from the Department of Theoretical Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information (IQOQI) at the Austrian Academy of Sciences in collaboration with Luca Tagliacozzo from the Institute of Photonic Sciences in Barcelona (Spain) have found a new way to detect certain properties of many-particle entanglement independent of the size of the system and by using standard measurement tools.

    Entanglement measurable via susceptibility

    “When dealing with more complex systems, scientists had to carry out a large number of measurements to detect and quantify entanglement between many particles,” says Philipp Hauke. “Our protocol avoids this problem and can also be used for determining entanglement in macroscopic objects, which was nearly impossible until now.”

    Using this new method, physicists can employ tools already well established experimentally. In their study published in Nature Physics the team of researchers gives explicit examples to demonstrate its framework: The entanglement of many-particle systems trapped in optical lattices can be determined using laser spectroscopy while the well-established technique of neutron scattering is utilized for measuring entanglement in solid-state systems.

    The physicists successfully demonstrated that the quantum Fisher information, which can provide reliable proof for genuine multipartite entanglement, is in fact measurable. The researchers emphasize that entanglement can be detected by measuring the dynamic response of a system to a perturbation, which can be determined by comparing individual measurements.

    “For example, when we move a sample through a time-dependent magnetic field, we can determine the system’s susceptibility towards the magnetic field through the measurement data and thereby detect and quantify internal entanglement,” explains Hauke.

    Manifold applications

    Quantum metrology, i.e. measurement techniques with increased precision exploiting quantum mechanics, is not the only important field of application of this protocol. It will also provide new perspectives for quantum simulations, where quantum entanglement is used as a resource for studying properties of quantum systems.

    In solid-state physics, the protocol may be employed to investigate the role of entanglement in many-body systems, thereby providing a deeper understanding of quantum matter. The research work was supported by the European Community, the European Research Council (ERC), the Austrian Science Fund, the Spanish Government and the German National Academy of Sciences Leopoldina.

    Publication:

    Measuring multipartite entanglement via dynamic susceptibilities. Philipp Hauke, Markus Heyl, Luca Tagliacozzo, Peter Zoller. Advanced Online Publication, Nature Physics, on 21 March 2016. – DOI: 10.1038/nphys3700

    See the full article here .

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    Technische Universität München (TUM) is one of Europe’s top universities. It is committed to excellence in research and teaching, interdisciplinary education and the active promotion of promising young scientists. The university also forges strong links with companies and scientific institutions across the world. TUM was one of the first universities in Germany to be named a University of Excellence. Moreover, TUM regularly ranks among the best European universities in international rankings.

     
  • richardmitnick 6:27 pm on January 7, 2016 Permalink | Reply
    Tags: , , , Quantum entanglement,   

    From Physics Today: “Three groups close the loopholes in tests of Bell’s theorem” 

    physicstoday bloc

    Physics Today

    January 2016, page 14
    Johanna L. Miller

    Until now, the quintessential demonstration of quantum entanglement has required extra assumptions.

    The predictions of quantum mechanics are often difficult to reconcile with intuitions about the classical world. Whereas classical particles have well-defined positions and momenta, quantum wavefunctions give only the probability distributions of those quantities. What’s more, quantum theory posits that when two systems are entangled, a measurement on one instantly changes the wavefunction of the other, no matter how distant.

    Might those counterintuitive effects be illusory? Perhaps quantum theory could be supplemented by a system of hidden variables that restore local realism, so every measurement’s outcome depends only on events in its past light cone. In a 1964 theorem John Bell showed that the question is not merely philosophical: By looking at the correlations in a series of measurements on widely separated systems, one can distinguish quantum mechanics from any local-realist theory. (See the article by Reinhold Bertlmann, Physics Today, July 2015, page 40.) Such Bell tests in the laboratory have come down on the side of quantum mechanics. But until recently, their experimental limitations have left open two important loopholes that require additional assumptions to definitively rule out local realism.

    Now three groups have reported experiments that close both loopholes simultaneously. First, Ronald Hanson, Bas Hensen (both pictured in figure 1), and their colleagues at Delft University of Technology performed a loophole-free Bell test using a novel entanglement-swapping scheme.1 More recently, two groups—one led by Sae Woo Nam and Krister Shalm of NIST,2 the other by Anton Zeilinger and Marissa Giustina of the University of Vienna3—used a more conventional setup with pairs of entangled photons generated at a central source.

    Temp 1
    Figure 1. Bas Hensen (left) and Ronald Hanson in one of the three labs they used for their Bell test. FRANK AUPERLE

    The results fulfill a long-standing goal, not so much to squelch any remaining doubts that quantum mechanics is real and complete, but to develop new capabilities in quantum information and security. A loophole-free Bell test demonstrates not only that particles can be entangled at all but also that a particular source of entangled particles is working as intended and hasn’t been tampered with. Applications include perfectly secure quantum key distribution and unhackable sources of truly random numbers.

    In a typical Bell test trial, Alice and Bob each possess one of a pair of entangled particles, such as polarization-entangled photons or spin-entangled electrons. Each of them makes a random and independent choice of a basis—a direction in which to measure the particle’s polarization or spin—and performs the corresponding measurement. Under quantum mechanics, the results of Alice’s and Bob’s measurements over repeated trials can be highly correlated—even though their individual outcomes can’t be foreknown. In contrast, local-realist theories posit that only local variables, such as the state of the particle, can influence the outcome of a measurement. Under any such theory, the correlation between Alice’s and Bob’s measurements is much less.

    But what if some hidden signal informs Bob’s experiment about Alice’s choice of basis, or vice versa? If such a signal can change the state of Bob’s particle, it can create quantum-like correlations in a system without actual quantum entanglement. That possibility is at the heart of the so-called locality loophole. The loophole can be closed by arranging the experiment, as shown in figure 2, so that no light-speed signal with information about Alice’s choice of basis can reach Bob until after his measurement is complete.

    Temp 2
    Figure 2. The locality loophole arises from the possibility that hidden signals between Alice and Bob can influence the results of their measurements. This space–time diagram represents an entangled-photon experiment for which the loophole is closed. The diagonal lines denote light-speed trajectories: The paths of the entangled photons are shown in red, and the forward light cones of the measurement-basis choices are shown in blue. Note that Bob cannot receive information about Alice’s chosen basis until after his measurement is complete, and vice versa.

    In practice, under that arrangement, for Alice and Bob to have enough time to choose their bases and make their measurements, they must be positioned at least tens of meters apart. That requirement typically means that the experiments are done with entangled photons, which can be transported over such distances without much damage to their quantum state. But the inefficiencies in handling and detecting single photons introduce another loophole, called the fair-sampling or detection loophole: If too many trials go undetected by Alice, Bob, or both, it’s possible for the detected trials to display quantum-like correlations even when the set of all trials does not.

    In Bell tests that are implemented honestly, there’s little reason to think that the detected trials are anything other than a representative sample of all trials. But one can exploit the detection loophole to fool the test on purpose by causing trials to go undetected for reasons other than random chance. For example, manifestly classical states of light can mimic single photons in one basis but go entirely undetected in another (see Physics Today, December 2011, page 20). Furthermore, similar tricks can be used for hacking quantum cryptography systems. The only way to guarantee that a hacker is not present is to close the loopholes.

    Instead of the usual entangled photons, the Delft group based their experiment on entangled diamond nitrogen–vacancy (NV) centers, electron spins associated with point defects in the diamond’s crystal lattice and prized for their long quantum coherence times. The scheme is sketched in figure 3: Each NV center is first entangled with a photon, then the photons are sent to a central location and jointly measured. A successful joint measurement, which transfers the entanglement to the two NV centers, signals Alice and Bob that the Bell test trial is ready to proceed.

    Temp 3
    Figure 3. Entanglement swapping between diamond nitrogen–vacancy (NV) centers. Alice and Bob entangle their NV spins with photons, then transmit the photons to a central location to be jointly measured. After a successful joint measurement, which signals that the NV spins are entangled with each other, each spin is measured in a basis chosen by a random-number generator (RNG). (Adapted from ref. 1.)

    n 2013 the team carried out a version of that experiment4 with the NV spins separated by 3 m. “It was at that moment,” says Hanson, “that I realized that we could do a loophole-free Bell test—and also that we could be the first.” A 3-m separation is not enough to close the locality loophole, so the researchers set about relocating the NV-center equipment to two separate labs 1.3 km apart and fiber-optically linking them to the joint-measurement apparatus at a third lab in between.

    A crucial aspect of the entanglement-swapping scheme is that the Bell test trial doesn’t begin until the joint measurement is made. As far as the detection loophole is concerned, attempted trials without a successful joint measurement don’t count. That’s fortunate, because the joint measurement succeeds in just one out of every 156 million attempts—a little more than once per hour.

    That inefficiency stems from two main sources. First, the initial spin–photon entanglement succeeds just 3% of the time at each end. Second, photon loss in the optical fibers is substantial: The photons entangled with the NV centers have a wavelength of 637 nm, well outside the so-called telecom band, 1520–1610 nm, where optical fibers work best. In contrast, once the NV spins are entangled, they can be measured efficiently and accurately. So of the Bell test trials that the researchers are able to perform, none are lost to nondetection.

    Early in the summer of 2015, Hanson and colleagues ran their experiment for 220 hours over 18 days and obtained 245 useful trials. They saw clear evidence of quantum correlations—although with so few trials, the likelihood of a nonquantum system producing the same correlations by chance is as much as 4%.

    The Delft researchers are working on improving their system by converting their photons into the telecom band. Hanson estimates that they could then extend the separation between the NV centers from 1.3 km up to 100 km. That distance makes feasible a number of quantum network applications, such as quantum key distribution.

    In quantum key distribution—as in a Bell test—Alice and Bob perform independently chosen measurements on a series of entangled particles. On trials for which Alice and Bob have fortuitously chosen to measure their particles in the same basis, their results are perfectly correlated. By conferring publicly to determine which trials those were, then looking privately at their measurement results for those trials, they can obtain a secret string of ones and zeros that only they know. (See article by Daniel Gottesman and Hoi-Kwong Lo, Physics Today, November 2000, page 22.)

    The NIST and Vienna groups both performed their experiments with photons, and both used single-photon detectors developed by Nam and his NIST colleagues. The Vienna group used so-called transition-edge sensors that are more than 98% efficient;5 the NIST group used superconducting nanowire single-photon detectors (SNSPDs), which are not as efficient but have far better timing resolution. Previous SNSPDs had been limited to 70% efficiency at telecom wavelengths—in part because the polycrystalline superconductor of choice doesn’t couple well to other optical elements. By switching to an amorphous superconducting material, Nam and company increased the detection efficiency to more than 90%.6

    Shalm realized that the new SNSPDs might be good enough for a loophole-free Bell test. “We had the detectors that worked at telecom wavelengths, so we had to generate entangled photons at the same wavelengths,” he says. “That was a big engineering challenge.” Another challenge was to boost the efficiency of the mazes of optics that carry the entangled photons from the source to the detector. “Normally, every time photons enter or exit an optical fiber, the coupling is only about 80% efficient,” explains Shalm. “We needed to get that up to 95%. We were worrying about every quarter of a percent.”

    In September 2015 the NIST group conducted its experiment between two laboratory rooms separated by 185 m. The Vienna researchers positioned their detectors 60 m apart in the subbasement of the Vienna Hofburg Castle. Both groups had refined their overall system efficiencies so that each detector registered 75% or more of the photons created by the source—enough to close the detection loophole.

    In contrast to the Delft group’s rate of one trial per hour, the NIST and Vienna groups were able to conduct thousands of trials per second; they each collected enough data in less than one hour to eliminate any possibility that their correlations could have arisen from random chance.

    It’s not currently feasible to extend the entangled-photon experiments into large-scale quantum networks. Even at telecom wavelengths, photons traversing the optical fibers are lost at a nonnegligible rate, so lengthening the fibers would lower the fraction of detected trials and reopen the detection loophole. The NIST group is working on using its experiment for quantum random-number generation, which doesn’t require the photons to be conveyed over such vast distances.

    Random numbers are widely used in security applications. For example, one common system of public-key cryptography involves choosing at random two large prime numbers, keeping them private, but making their product public. Messages can be encrypted by anyone who knows the product, but they can be decrypted only by someone who knows the two prime factors.

    The scheme is secure because factorizing a large number is a computationally hard problem. But it loses that security if the process used to choose the prime numbers can be predicted or reproduced. Numbers chosen by computer are at best pseudorandom because computers can run only deterministic algorithms. But numbers derived from the measurement of quantum states—whose quantum nature is verified through a loophole-free Bell test—can be truly random and unpredictable.

    The NIST researchers plan to make their random-number stream publicly available to everyone, so it can’t be used for encryption keys that need to be kept private. But a verified public source of tamperproof random numbers has other uses, such as choosing unpredictable samples of voters for opinion polling, taxpayers for audits, or products for safety testing.

    REFERENCES

    B. Hensen et al., Nature 526, 682 (2015). http://dx.doi.org/10.1038/nature15759
    L. K. Shalm et al., Phys. Rev. Lett. (in press), http://arxiv.org/abs/1511.03189.
    M. Giustina et al., Phys. Rev. Lett. (in press), http://arxiv.org/abs/1511.03190.
    H. Bernien et al., Nature 497, 86 (2013). http://dx.doi.org/10.1038/nature12016
    A. E. Lita, A. J. Miller, S. W. Nam, Opt. Express 16, 3032 (2008). http://dx.doi.org/10.1364/OE.16.003032
    F. Marsili et al., Nat. Photonics 7, 210 (2013).http://dx.doi.org/10.1038/nphoton.2013.13

    © 2016 American Institute of Physics
    DOI: http://dx.doi.org/10.1063/PT.3.3039

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  • richardmitnick 5:32 pm on January 2, 2016 Permalink | Reply
    Tags: , , Quantum entanglement,   

    From ETH Zürich: “Faster entanglement of distant quantum dots” 

    ETH Zurich bloc

    ETH Zürich

    21.12.2015
    Oliver Morsch

    Entanglement between distant quantum objects is an important ingredient for future information technologies. Researchers at the ETH have now developed a method with which such states can be created a thousand times faster than before.

    Temp 1
    In two entangled quantum objects the spins are in a superposition of the states “up/down” and “down/up”. Researchers at the ETH have created such states in quantum dots that are five meters apart. (Visualisations: ETH Zürich / Aymeric Delteil)

    In many future information and telecommunication technologies, a remarkable quantum effect called entanglement will likely play an important role. The entanglement of two quantum objects means that measurements on one of the objects instantaneously determine the properties of the other one – without any exchange of information between them.

    Disapprovingly, Albert Einstein called this strange non-locality “spooky action at a distance”. In the meantime, physicists have warmed to it and are now trying to put it to good use, for instance in order to make data transmission immune to eavesdropping. To that end, the creation of entanglement between spatially distant quantum particles is indispensable. That, however, is not easy and typically works rather slowly. A group of physicists led by Atac Imamoglu, a professor at the Institute for Quantum Electronics at the ETH in Zurich, have now demonstrated a method that allows the creation of a thousand times more entangled states per second than was possible before.

    Distant quantum dots

    In their experiments, the young researchers Aymeric Delteil, Zhe Sun und Wei-bo Gao used two so-called quantum dots that were placed five metres apart in the laboratory. Quantum dots are tiny structures, measuring only a few nanometres, inside a semiconductor material and in which electrons are trapped in a sort of cage. The quantum mechanical energy states of those electrons can be represented by spins, i.e., little arrows pointing up or down. When the spin states are entangled, it is possible to deduce from a measurement performed on one of the quantum dots which state the other one will be found in. If the spin of the first quantum dot points up, the other one points down, and vice versa. Before the measurement, however, the directions of the two spins are both unknown: they are in a quantum mechanical superposition of both spin combinations.

    Entanglement by scattershot

    In order to entangle the two quantum dots with each other the researchers at ETH used the principle of heralding. “Unfortunately, at the moment it is practically impossible to entangle quantum objects that are far apart with certainty and on demand”, explains Imamoglu. Instead, it is necessary to create the entangled states using a scattershot approach in which the quantum dots are constantly bombarded with light particles, which are then scattered back. Every so often this will result in a fluke: one of the scattered light particles makes a detector click, and the resulting spin states are actually entangled.

    Imamoglu and his colleagues make use of this trick. They send laser pulses simultaneously to the two quantum dots and measure the light particles subsequently emitted by them. Before doing so, they carefully eliminated any possibility to find out which quantum dot the light particles originated from. The click in the light detector then “heralds” the actual entanglement of the quantum dots and signals that they can now be used further, e.g., for transmitting quantum information.

    Possible improvements

    The researchers tested their method by continuously shooting around ten million laser pulses per second at the quantum dots. This high repetition rate was possible because the spin states of quantum dots can be controlled within just a few nanoseconds. The measurements showed that in this way 2300 entangled states were produced per second.

    “That’s already a good start”, says Imamoglu, adding that the method certainly has room for improvement. Entangling quantum dots that are more than five metres apart, for instance, would require an enhancement of their coherence time. This time indicates how long a quantum state survives before it is destroyed through the influence of its environment (such as electric or magnetic fields). If the heralding light particle takes longer than one coherence time to fly to the detector, then a click no longer heralds entanglement. In future experiments the physicists want, therefore, to replace the quantum dots by so-called quantum dot molecules, whose coherence time are a hundred times longer. Furthermore, improvements of the detection probability of the light particles could lead to an even higher entanglement yield.
    Reference

    Delteil A, Sun Z, Gao W, Togan E, Faelt S, Imamoglu A: Generation of heralded entanglement between distant hole spins, Nature Physics, 21 December 2015, doi: 10.1038/nphys3605

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    ETH Zurich is one of the leading international universities for technology and the natural sciences. It is well known for its excellent education, ground-breaking fundamental research and for implementing its results directly into practice.

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    Twenty-one Nobel Laureates have studied, taught or conducted research at ETH Zurich, underlining the excellent reputation of the university.

     
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