Computermaker, embrace the weird!

Interspecies entanglement. It might sound like a biological experiment gone wrong. Actually, it's a dream come true for physicists trying to invent an ultrafast computer.

Welcome to the quaint world of quantum physics, whose weirdness frustrates theorists but whose promise could lead to a machine able to compute in seconds what would take today's supercomputers an eternity to complete.

The key for researchers is to make use of that weirdness, starting with interspecies entanglement.

In the language of quantum physics, different "species" means different particles. That could be, say, an electrically charged atom and a photon of light.

"Entanglement" means that certain properties of those two particles - such as the spin of the atom and the polarization of the photon - are correlated. They remain linked however far apart the particles are.

Moreover, what you do to one member of the pair can determine the state of the other member. Detecting the photon's polarization determines which way the electrically charged atom (called an ion) spins. It happens instantly, even if they are at opposite ends of the universe.

Various experiments have demonstrated entanglement between quantum objects of the same species. The prize for developers of the ultracomputer has been to demonstrate entanglement between ions and light.

It's called a quantum computer because the states of quantum entities, such as the direction of an ion's spin, can represent the zeros and ones of the binary math that computers use.

That's useful, but to make a working computer, those zeros and ones have to travel. That's where the photons come in.

Photons entangled with those ions theoretically could carry that information to other parts of a computing network. B.B. Blinov and colleagues at the University of Michigan in Ann Arbor now have made that kind of entanglement work between a cadmium ion and a photon.

That's a significant step in "the spectacular progress" being made toward building a quantum computer, writes Eugene Polzik with the Niels Bohr Institute in Copenhagen in a recent issue of Nature. "The real breakthrough ... is that, for the first time, entanglement has been observed between a stationary computational [unit] (a trapped ion) and a 'flying' communication [unit] (an optical photon)."

While that's good news for quantum computer developers, it makes a larger point. Working physicists have to master the so-called quantum weirdness that they have wished they could live without.

Quantum theory's counterintuitive predictions were a background theme as 20th-century physics unfolded. They predicted such oddities as groups of atoms in which each atom is everywhere at once. Physicists could tie themselves in knots trying to understand such things. Einstein hated the notion of what he called "spooky" entanglement.

The mantra for practical researchers was "calculate, don't speculate." They used the theory's mathematical tools to develop modern chemistry and produce such wonders as computer chips and lasers. Now they must embrace its physical weirdness.

No one knows when a practical quantum computer will be developed or exactly what it will be used for.

But to quote from an overview of quantum entanglement published last year in Physics Today: "Researchers now treat [spooky] entanglement not simply as a paradoxical feature of quantum mechanics, but as a physical resource."

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