Full-scale quantum computers, with all the number crunching, code cracking and jaw-dropping processing power researchers expect them to deliver, remain a mere twinkle in the eye of physicists and computer scientists. It is a twinkle supported by promising experimental and theoretical work, but a twinkle nonetheless—to date only rudimentary quantum processors have been built.
Such computers would harness the physical properties of quantum bits, or qubits, to expand the reach of computation. Whereas ordinary bits can be 0 or 1, a qubit can be in a superposition of 0 and 1 simultaneously—and qubits can be mutually entangled, meaning that their properties are linked.
Along the road to building a full-fledged quantum computer lies a critical experimental milestone that would be a significant achievement in its own right: the construction of a simpler device called a quantum repeater, essentially a relay station for qubits that could enable quantum communication systems, an analogue to the fiber-optic telecommunications already in wide use.
From Scientific American
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