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Loopy photons clarify 'spookiness' of quantum physics
March 19, 2008Researchers at the National Institute of Standards and Technology (NIST) and the Joint Quantum Institute (NIST/University of Maryland) have developed a new method for creating pairs of entangled photons, particles of light whose properties are interlinked in a very unusual way dictated by the rules of quantum physics. The researchers used the photons to test fundamental concepts in quantum theory.
In the experiment, the researchers send a pulse of light into both ends of a twisted loop of optical fiber. Pairs of photons of the same color traveling in either direction will, every so often, interact in a process known as "four-wave mixing," converting into two new, entangled photons, one that is redder and the other that is bluer than the originals.
Although the fiber's twist means that pairs emerging from one end are vertically polarized (having electric fields that vibrate up and down) while pairs from the other end are horizontally polarized (vibrating side to side), the setup makes it impossible to determine which path the newly created photon pairs took. Since the paths are indistinguishable, the weird rules of quantum physics say that the photon pairs actually will be in both states-horizontal and vertical polarization-at the same time. Until someone measures one, at which time both photons must chose one specific, and identical, state.
This "spooky action at a distance" is what caused Einstein to consider quantum mechanics to be incomplete, prompting debate for the past 73 years over the concepts of "locality" and "realism." Decades of experiments have demonstrated that measurements on pairs of entangled particles don't agree with the predictions made by "local realism," the concept that processes occurring at one place have no immediate effect on processes at another place (locality) and that the particles have definite, preexisting properties (called "hidden variables") even without being measured (realism).
Experiments so far have ruled out locality and realism as a combination. But could a theory assuming only one of them be correct" Nonlocal hidden variables (NLHV) theories would allow for the possibility of hidden variables but would concede nonlocality, the idea that a measurement on a particle at one location may have an immediate effect on a particle at a separate location.
Measuring the polarizations of the pairs of entangled particles in their setup, the researchers showed that the results did not agree with the predictions of certain NLHV theories but did agree with the predictions of quantum mechanics. In this way, they were able to rule out certain NLHV theories. Their results agree with other groups that have performed similar experiments.
National Institute of Standards and Technology (NIST)
This "spooky action at a distance" is what caused Einstein to consider quantum mechanics to be incomplete, prompting debate for the past 73 years over the concepts of "locality" and "realism." Decades of experiments have demonstrated that measurements on pairs of entangled particles don't agree with the predictions made by "local realism," the concept that processes occurring at one place have no immediate effect on processes at another place (locality) and that the particles have definite, preexisting properties (called "hidden variables") even without being measured (realism).
Experiments so far have ruled out locality and realism as a combination. But could a theory assuming only one of them be correct" Nonlocal hidden variables (NLHV) theories would allow for the possibility of hidden variables but would concede nonlocality, the idea that a measurement on a particle at one location may have an immediate effect on a particle at a separate location.
Measuring the polarizations of the pairs of entangled particles in their setup, the researchers showed that the results did not agree with the predictions of certain NLHV theories but did agree with the predictions of quantum mechanics. In this way, they were able to rule out certain NLHV theories. Their results agree with other groups that have performed similar experiments.
National Institute of Standards and Technology (NIST)
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Quantum Physics Current Events and Quantum Physics News RSSNIST physicists demonstrate quantum entanglement in mechanical system
Physicists at the National Institute of Standards and Technology (NIST) have demonstrated entanglement-a phenomenon peculiar to the atomic-scale quantum world-in a mechanical system similar to those in the macroscopic everyday world.
Evidence of macroscopic quantum tunneling detected in nanowires
A team of researchers at the University of Illinois has demonstrated that, counter to classical Newtonian mechanics, an entire collection of superconducting electrons in an ultrathin superconducting wire is able to "tunnel" as a pack from a state with a higher electrical current to one with a notably lower current, providing more evidence of the phenomenon of macroscopic quantum tunneling.
MIT reels in RNA surprise with microbial ocean catch
An ingenious new method of obtaining marine microbe samples while preserving the microbes' natural gene expression has yielded an unexpected boon: the presence of many varieties of small RNAs - snippets of RNA that act as switches to regulate gene expression in these single-celled creatures.
Caltech physicists detect entanglement of one photon shared among four locations
Scientists at the California Institute of Technology (Caltech) have developed an efficient method to detect entanglement shared among multiple parts of an optical system.
Quantum ghosts are helpful
The idea that far distant particles can somehow 'talk' to each other worried Einstein so much that he called it 'spooky action at a distance'.
Quantum computers will require complex software to manage errors
Highlighting another challenge to the development of quantum computers, theorists at the National Institute of Standards and Technology (NIST) have shown* that a type of software operation, proposed as a solution to fundamental problems with the computers' hardware, will not function as some designers had hoped.
Quantum paradox directly observed -- a milestone in quantum mechanics
In quantum mechanics, a vanguard of physics where science often merges into philosophy, much of our understanding is based on conjecture and probabilities, but a group of researchers in Japan has moved one of the fundamental paradoxes in quantum mechanics into the lab for experimentation and observed some of the 'spooky action of quantum mechanics' directly.
Quantum dance: Discovery led by Princeton researchers could revolutionize computing
An international team of scientists, led by a Princeton University group, has observed an exciting and strange behavior in electrons' spin within a new material that could be harnessed to transform computing and electronics.
U of T physicists are first to 'squeeze' light to quantum limit
A team of University of Toronto physicists have demonstrated a new technique to squeeze light to the fundamental quantum limit, a finding that has potential applications for high-precision measurement, next-generation atomic clocks, novel quantum computing and our most fundamental understanding of the universe.
McGill physicists find a new state of matter in a 'transistor'
McGill University researchers have discovered a new state of matter, a quasi-three- dimensional electron crystal, in a material very much like those used in the fabrication of modern transistors.
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