Quantum computation is a steadily-improving technology with vast promise that is arguably poised to take off in the next decade or two and potentially revolutionize a number of fields.
Quantum computers exploit quantum effects of entanglement and superposition in systems acting as "qubits" (quantum bits) to efficiently perform calculations that are inefficient using classical computers. Most famous is the ability to factor very large numbers, the difficulty of which underlies many current cryptography systems. But there is much more: quantum computers could, for example, efficiently simulate quantum systems in chemistry and biology, leading to dramatically more powerful tools for designing materials, drugs, etc.
The basic theory behind quantum computers is well understood, and computers with a handful of qubits have been created as of 2016. It is generally (though not universally) believed that only technological barriers exist to creating large-scale quantum computers.
An interesting threshold is 50 qubits, at which quantum computers could outpace contemporary supercomputers in at least some applications. Google has recently stated a goal of a 49 (7x7 array) qubit device in 2018, and IBM has announced a similar goal.
These goals may be "ambitious" but suggest that we may be years, rather than decades, away from general-purpose quantum computers. We ask:
When will the first general-purpose quantum computer with > 48 qubits be constructed and operated to solve a computational problem?
Resolution will occur via credible media or company report. Non-universal quantum computing systems (such as D-Wave's device) do not satisfy the criteria. The problem (e.g. prime factorization) need not be solved faster than any given classical computer, but must use the full 49+ qubit apparatus and be arguably nontrivial, e.g. use an algorithm that was developed specifically for quantum computation.