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06-06-12, 06:30 PM
http://cdn.arstechnica.net/wp-content/uploads/2012/06/20060825laser2-640x426.jpg A titanium sapphire crystal, used as an optical element in a laser.
unl.edu (http://newsroom.unl.edu/releases/downloadables/photo/20060825laser2.jpg)


Quantum computers come in many different shapes and forms, but the granddaddy of them all is based on light. This is because it is very easy to create the basic computational unit, called a qubit, from light. The big problem is the memory unit. Light has a pesky habit of traveling quite fast, so by the time you are ready to use your carefully prepared qubit, it is halfway to the Moon, never to return.

A pair of research (http://dx.doi.org/10.1103/PhysRevLett.108.190505) groups (http://dx.doi.org/10.1103/PhysRevLett.108.190504), working independently, showed an effective and reliable memory for light-based qubits. Looking back, a number of competing research groups will be hitting themselves on the forehead, saying, "why didn't we think of that?" Yes, the idea is that simpleā??though I should caution that there is some distance between an idea and its implementation.

There are three key elements that make a quantum computer special: superposition, coherence, and entanglement. A qubit that is in a superposition state does not represent a logic one or zero; instead, it represents the probability of measuring it as a logic one. Coherence is essentially how the superposition state of different qubits change synchronously with each other. Entanglement takes two or more particles and makes them (mathematically speaking) a single particle. So when we are choosing a qubit system, we must choose physical properties that can be entangled between different particles and remain coherent.

Read more (http://arstechnica.com/science/2012/06/trapping-flying-qubits-in-a-crystal-and-getting-them-back-out/) | Comments (http://arstechnica.com/science/2012/06/trapping-flying-qubits-in-a-crystal-and-getting-them-back-out/?comments=1#comments-bar)



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