Tom Simonite of the MIT technology review has a first rate piece on how (eight year) startup D-Wave is closing in on quantum computing. (MIT) Machine learning people will appreciate that the D-Wave quantum computer is designed to solve complex optimization problems. For those of you new to optimization and quantum computing, here's a succinct summary:
The processor in every computer you've used is made from silicon and patterned with transistors that create logic gates—switches that are either on (represented by a 1 in the computer's programming) or off (a 0). D-Wave's processors are also made up of elements that switch between 1 and 0, but they are loops of niobium alloy—there are 512 of them in the newest processor. These loops are known as qubits and can trap electrical current, which circles inside the loops either clockwise (signified by a 0) or counterclockwise (1). Smaller superconducting loops called couplers link the qubits so they can interact and even influence one another to flip between 1 and 0.
This delicate setup is designed so that the layout of qubits conforms to an algorithm that solves a particular kind of optimization problem at the core of many tasks difficult to solve on a conventional processor. It's like a specialized machine in a factory able to do one thing really well, on a particular kind of raw material. Performing a calculation on D-Wave's chip requires providing that raw material, in the form of the numbers to be fed into its hard-coded algorithm. It's done by setting the qubits into a pattern of 1s and 0s, and fine-tuning how the couplers allow the qubits to interact. After a wait of less than a second, the qubits settle into new values that represent a lower state of energy for the processor, and reveal a potential solution to the original problem.
What happens during that crucial wait is a kind of quantum mechanical argument. The qubits enter a strange quantum state where they are simultaneously both 1 and 0, like Schrodinger's cat being both dead and alive, and lock into a strange synchronicity known as entanglement, a phenomenon once described by Einstein as "spooky." That allows the system of qubits to explore every possible final configuration in an instant, before settling into on the one that is simplest or very close to it.
For more on Schrodinger's cat and quantum entanglement, see this.