Science Focus
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I have to admit it: D-Wave is starting to produce some impressive results. For the uninitiated, D-Wave came to our attention by loudly and repeatedly claiming that it had built a quantum computer. Many of us were skeptical. Over time, though, D-Wave has answered its critics in the best way possible: by providing evidence. Now, researchers who actually got inside the black box are reporting some key results that come very close to removing any lingering doubts.
The quantum difference
When we perform computations in an ordinary computer, we have to manipulate each bit individually. Sure, the computer might make this faster through some sort of parallelization, but there's still a set of transistors flipping individual bits for each operation. A quantum computer is different. First, the information is stored in a quantum state (called a qubit), which means that it holds multiple values simultaneously (called superposition states). The value of a qubit is only determined when the result of a computation is read out. An eight-qubit quantum register can therefore hold values from 0-255 simultaneously, but the probability of obtaining a particular value is modified by the computational operations that are performed prior to reading the register out.
That is not the real power of quantum computation, however. The second bit of magic that a quantum system has is called coherence. When a quantum state is in a superposition state, the probability of obtaining a one or a zero changes with time naturally, like a pendulum swinging back and forth. At a particular time, the chance of measuring a one is unity, while some time later, the chance of measuring a zero is unity. In between, the probability of obtaining a one smoothly varies from unity to zero. When two qubits are coherent, this changing probability happens in concert for the two qubits.
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