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The Sciences

You'll Know Less if You Read This Story

You'll Know Less If You Read This Story

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Ever feel that the more you learn about atomic science, the less you know? Recent research indicates that the old adage may be literally true. Physicists have found quantum interactions can impart negative information—acquiring it means you know less.

How can that be? Imagine that a boy and a girl named Paul and Marie are shown two playing cards—the ace of hearts and the ace of spades. Each draws one of the cards blindly from an envelope but can immediately deduce the other's card simply by looking at his or her own. Now suppose that Paul gives Marie the ace he has drawn before either of them looks at the cards. The correlation is broken: Neither can know who originally had Paul's ace. Knowledge has been reduced.

In physics, this game can be played using pairs of electrons in place of cards. Electrons can spin in what are called singlets. One spins in one direction; the other spins in the opposite. They are said to be "entangled" because, in the bizarre world of quantum mechanics, neither electron has a definite spin until one of them is measured. At that instant both particles immediately settle into their respective spins. Put another way, each electron has an equal chance of being measured while spinning in one direction or the other. Unlike playing cards, which can be glanced at and remembered, the electron pairs cannot be observed without nullifying their delicate duality. So if Paul and Marie each have half a singlet, and Paul gives away his electron, the correlation is irrevocably lost. The fragile, solipsistic nature of quantum states allows for negative information.

Theoretician Andreas Winter of the University of Bristol, along with physicists Jonathan Oppenheim of Cambridge University and Michal Horodecki of Gdánsk University, came across this concept while trying to numerically measure quantum information, a key step in creating a computer based on systems of subatomic particles rather than conventional microchips. "What is most surprising," says Oppenheim, "is that in quantum theory that number can be negative."

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"You'll Know Less if You Read This Story." Cambridge University physicist Jonathan Oppenheim provides an in-depth explanation of his and his colleagues' work on quantum information theory, as well as links to other interesting physics topics: www.damtp.cam.ac.uk/user/jono.

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