Arquivo da tag: Laboratório

Photons Run out of Loopholes: Quantum World Really Is in Conflict With Our Everyday Experience (Science Daily)

Apr. 15, 2013 — A team led by the Austrian physicist Anton Zeilinger has now carried out an experiment with photons in which they have closed an important loophole. The researchers have thus provided the most complete experimental proof that the quantum world is in conflict with our everyday experience.

Lab IQOQI, Vienna 2012. (Credit: Copyright: Jacqueline Godany)

The results of this study appear this week in the journal Nature (Advance Online Publication/AOP).

When we observe an object, we make a number of intuitive assumptions, among them that the unique properties of the object have been determined prior to the observation and that these properties are independent of the state of other, distant objects. In everyday life, these assumptions are fully justified, but things are different at the quantum level. In the past 30 years, a number of experiments have shown that the behaviour of quantum particles — such as atoms, electrons or photons — can be in conflict with our basic intuition. However, these experiments have never delivered definite answers. Each previous experiment has left open the possibility, at least in principle, that the observed particles ‘exploited’ a weakness of the experimental setup.

Quantum physics is an exquisitely precise tool for understanding the world around us at a very fundamental level. At the same time, it is a basis for modern technology: semiconductors (and therefore computers), lasers, MRI scanners, and numerous other devices are based on quantum-physical effects. However, even after more than a century of intensive research, fundamental aspects of quantum theory are not yet fully understood. On a regular basis, laboratories worldwide report results that seem at odds with our everyday intuition but that can be explained within the framework of quantum theory.

On the trail of the quantum entanglement mystery

The physicists in Vienna report not a new effect, but a deep investigation into one of the most fundamental phenomena of quantum physics, known as ‘entanglement.’ The effect of quantum entanglement is amazing: when measuring a quantum object that has an entangled partner, the state of the one particle depends on measurements performed on the partner. Quantum theory describes entanglement as independent of any physical separation between the particles. That is, entanglement should also be observed when the two particles are sufficiently far apart from each other that, even in principle, no information can be exchanged between them (the speed of communication is fundamentally limited by the speed of light). Testing such predictions regarding the correlations between entangled quantum particles is, however, a major experimental challenge.

Towards a definitive answer

The young academics in Anton Zeilinger’s group including Marissa Giustina, Alexandra Mech, Rupert Ursin, Sven Ramelow and Bernhard Wittmann, in an international collaboration with the National Institute of Standards and Technology/NIST (USA), the Physikalisch-Technische Bundesanstalt (Germany), and the Max-Planck-Institute of Quantum Optics (Germany), have now achieved an important step towards delivering definitive experimental evidence that quantum particles can indeed do things that classical physics does not allow them to do. For their experiment, the team built one of the best sources for entangled photon pairs worldwide and employed highly efficient photon detectors designed by experts at NIST. These technological advances together with a suitable measurement protocol enabled the researchers to detect entangled photons with unprecedented efficiency. In a nutshell: “Our photons can no longer duck out of being measured,” says Zeilinger.

This kind of tight monitoring is important as it closes an important loophole. In previous experiments on photons, there has always been the possibility that although the measured photons do violate the laws of classical physics, such non-classical behaviour would not have been observed if all photons involved in the experiment could have been measured. In the new experiment, this loophole is now closed. “Perhaps the greatest weakness of photons as a platform for quantum experiments is their vulnerability to loss — but we have just demonstrated that this weakness need not be prohibitive,” explains Marissa Giustina, lead author of the paper.

Now one last step

Although the new experiment makes photons the first quantum particles for which, in several separate experiments, every possible loophole has been closed, the grand finale is yet to come, namely, a single experiment in which the photons are deprived of all possibilities of displaying their counterintuitive behaviour through means of classical physics. Such an experiment would also be of fundamental significance for an important practical application: ‘quantum cryptography,’ which relies on quantum mechanical principles and is considered to be absolutely secure against eavesdropping. Eavesdropping is still theoretically possible, however, as long as there are loopholes. Only when all of these are closed is a completely secure exchange of messages possible.

An experiment without any loopholes, says Zeilinger, “is a big challenge, which attracts groups worldwide.” These experiments are not limited to photons, but also involve atoms, electrons, and other systems that display quantum mechanical behaviour. The experiment of the Austrian physicists highlights the photons’ potential. Thanks to these latest advances, the photon is running out of places to hide, and quantum physicists are closer than ever to conclusive experimental proof that quantum physics defies our intuition and everyday experience to the degree suggested by research of the past decades.

This work was completed in a collaboration including the following institutions: Institute for Quantum Optics and Quantum Information — Vienna / IQOQI Vienna (Austrian Academy of Sciences), Quantum Optics, Quantum Nanophysics and Quantum Information, Department of Physics (University of Vienna), Max-Planck-Institute of Quantum Optics, National Institute of Standards and Technology / NIST, Physikalisch-Technische Bundesanstalt, Berlin.

This work was supported by: ERC (Advanced Grant), Austrian Science Fund (FWF), grant Q-ESSENCE, Marie Curie Research Training Network EMALI, and John Templeton Foundation. This work was also supported by NIST Quantum Information Science Initiative (QISI).

Journal Reference:

  1. Marissa Giustina, Alexandra Mech, Sven Ramelow, Bernhard Wittmann, Johannes Kofler, Jörn Beyer, Adriana Lita, Brice Calkins, Thomas Gerrits, Sae Woo Nam, Rupert Ursin, Anton Zeilinger. Bell violation using entangled photons without the fair-sampling assumptionNature, 2013; DOI: 10.1038/nature12012
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Language is shaped by brain’s desire for clarity and ease (University of Rochester)

Public release date: 15-Oct-2012
By Susan Hagen
University of Rochester

 VIDEO: Translation: “Referee statue pick up. ” Above is one of the 80 animated video clips used to teach an artificial language to study participants. Cognitive scientists are just beginning to use…

Cognitive scientists have good news for linguistic purists terrified about the corruption of their mother tongue.

Using an artificial language in a carefully controlled laboratory experiment, a team from the University of Rochester and Georgetown University has found that many changes to language are simply the brain’s way of ensuring that communication is as precise and concise as possible.

“Our research shows that humans choose to reshape language when the structure is either overly redundant or confusing,” says T. Florian Jaeger, the Wilmot Assistant Professor of the Sciences at Rochester and co-author of a study published in the Proceedings of the National Academy of SciencesOct. 15. “This study suggests that we prefer languages that on average convey information efficiently, striking a balance between effort and clarity.”

The brain’s tendency toward efficient communication may also be an underlying reason that many human languages are structurally similar, says lead author Maryia Fedzechkina, a doctoral candidate at Rochester. Over and over, linguists have identified nearly identical grammatical conventions in seemingly unrelated languages scattered throughout the globe. For decades, linguists have debated the meaning of such similarities: are recurrent structures artifacts of distant common origins, are they simply random accidents, or do they reflect fundamental aspects of human cognition?

This study supports the latter, says co-author Elissa L. Newport, professor of neurology and director of the Center for Brain Plasticity and Recovery at Georgetown, and the former George Eastman Professor of Brain and Cognitive Sciences at Rochester. “The bias language learners have toward efficiency and clarity acts as a filter as languages are transmitted from one generation of learners to another,” she says. Alterations to language are introduced through many avenues, including the influence of other languages and changes in accents or pronunciation. “But this research finds that learners shift the language in ways that make it better – easier to use and more suitable for communication,” says Newport. That process also leads to the recurrent patterns across languages.

To observe the language acquisition process, the team created two miniature artificial languages that use suffixes on nouns to indicate subject or object. These “case markers” are common to Spanish, Russian, and other languages, but not English. In two experiments, 40 undergraduates, whose only language was English, learned the eight verbs, 15 nouns, and grammatical structure of the artificial languages. The training was spaced over four 45-minute sessions and consisted of computer images, short animated clips, and audio recordings. Then participants were asked to describe a novel action clip using their newly learned language.

 VIDEO: Translation: “Singer hunter chop. ” Unlike English, the artificial languages used in the study have free word order. When the subject and object could be easily confused, participants chose to reshape…

When faced with sentence constructions that could be confusing or ambiguous, the language learners in both experiments chose to alter the rules of the language they were taught in order to make their meaning clearer. They used case markers more often when the meaning of the subject and object might otherwise have caused unintended interpretations. So for example, a sentence like “Man hits wall,” is typical because the subject is a person and the object is a thing. But the sentence “Wall hits man,” as when a wall falls on top of a man, is atypical and confusing since the subject is a thing and the object is a person.

The results, write the authors, provide evidence that humans seek a balance between clarity and ease. Participants could have chosen to be maximally clear by always providing the case markers. Alternatively, they could have chosen to be maximally succinct by never providing the case markers. They did neither. Instead, they provided case-markers more often for those sentences that would otherwise have been more likely to be confused.

The findings also support the idea that language learners introduce common patterns, also known as linguistic universals, conclude the authors. The optional case marking that participants introduced in this experiment closely mirrors naturally occurring patterns in Japanese and Korean—when animate objects and inanimate subjects are more likely to receive case markings.

The history of English itself might reflect these deep principles of how we learn language, says Jaeger. Old English had cases and relatively free word order, as is still true for German. But at some point pronunciation changes began to obscure the case endings, creating ambiguity. In contemporary English, word order has become the primary signal by which speakers could decode the meaning, he says.

“Language acquisition can repair changes in languages to insure they don’t undermine communication,” says Fedzechkina. In light of these findings, new generations can perhaps be seen as renewing language, rather than corrupting it, she adds.

By the same token, says Jaeger, many elements of informal speech can be interpreted as rising from the brain’s bias toward efficiency. “When people turn ‘automobile’ into ‘auto,’ use informal contractions, swallow syllables, or take other linguistic shortcuts, the same principles are at work,” he says. Recent research has shown that these types of shortcuts appear only when their meaning is easily inferable from the context, he adds.