Single Photon Management for Quantum Computers !!

The quantum computers of tomorrow might use photons, or particles of light, to move around the data they need to make calculations, but photons are tricky to work with. Two new papers* by researchers working at the National Institute of Standards and Technology (NIST) have brought science closer to creating reliable sources of photons for these long-heralded devices.

In principle, quantum computers can perform calculations that are impossible or impractical using conventional computers by taking advantage of the peculiar rules of quantum mechanics. To do this, they need to operate on things that can be manipulated into specific quantum states. Photons are among the leading contenders.

The new NIST papers address one of the many challenges to a practical quantum computer: the need for a device that produces photons in ready quantities, but only one at a time, and only when the computer's processor is ready to receive them. Just as garbled data will confuse a standard computer, an information-bearing photon that enters a quantum processor together with other particles -- or when the processor is not expecting it -- can ruin a calculation.

The single-photon source has been elusive for nearly two decades, in part because no method of producing these particles individually is ideal. "It's a bit like playing a game of whack-a-mole, where solving one problem creates others," says Alan Migdall of NIST's Optical Technology Division. "The best you can do is keep all the issues under control somewhat. You can never get rid of them."

The team's first paper addresses the need to be certain that a photon is indeed coming when the processor is expecting it, and that none show up unexpected. Many kinds of single-photon sources create a pair of photons and send one of them to a detector, which tips off the processor to the fact that the second, information-bearing photon is on its way. But since detectors are not completely accurate, sometimes they miss the "herald" photon -- and its twin zips into the processor, gumming up the works.

The team effort, in collaboration with researchers from the Italian metrology laboratory L'Istituto Nazionale di Ricerca Metrologica (INRIM), handled the issue by building a simple gate into the source. When a herald photon reaches the detector, the gate opens, allowing the second photon past. "You get a photon when you expect one, and you don't get one when you don't," Migdall says. "It was an obvious solution; others proposed it long ago, we were just the first ones to build it. It makes the single photon source better."

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How Much Information Is There in the World !!

Science Express, an electronic journal that provides select Science articles ahead of print, calculates the world's total technological capacity -- how much information humankind is able to store, communicate and compute.

"We live in a world where economies, political freedom and cultural growth increasingly depend on our technological capabilities," said lead author Martin Hilbert of the USC Annenberg School for Communication & Journalism. "This is the first time-series study to quantify humankind's ability to handle information."

So how much information is there in the world? How much has it grown?

Prepare for some big numbers:

* Looking at both digital memory and analog devices, the researchers calculate that humankind is able to store at least 295 exabytes of information. (Yes, that's a number with 20 zeroes in it.)

Put another way, if a single star is a bit of information, that's a galaxy of information for every person in the world. That's 315 times the number of grains of sand in the world. But it's still less than one percent of the information that is stored in all the DNA molecules of a human being.
* 2002 could be considered the beginning of the digital age, the first year worldwide digital storage capacity overtook total analog capacity. As of 2007, almost 94 percent of our memory is in digital form.
* In 2007, humankind successfully sent 1.9 zettabytes of information through broadcast technology such as televisions and GPS. That's equivalent to every person in the world reading 174 newspapers every day.
* On two-way communications technology, such as cell phones, humankind shared 65 exabytes of information through telecommunications in 2007, the equivalent of every person in the world communicating the contents of six newspapers every day.
* In 2007, all the general-purpose computers in the world computed 6.4 x 10^18 instructions per second, in the same general order of magnitude as the number of nerve impulses executed by a single human brain. Doing these instructions by hand would take 2,200 times the period since the Big Bang.
* From 1986 to 2007, the period of time examined in the study, worldwide computing capacity grew 58 percent a year, ten times faster than the United States' GDP.

Telecommunications grew 28 percent annually, and storage capacity grew 23 percent a year.

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