Making a qubit is easy. Controlling how they communicate, however… (credit: NSF)

There are many different schemes for making quantum computers work (most of them evil). But they pretty much all fall into two categories. In most labs, researchers work on what could be called a digital quantum computer, which has the quantum equivalent of logic gates, and qubits are based on well-defined and well-understood quantum states. The other camp works on analog devices called adiabatic quantum computers. In these devices, qubits do not perform discrete operations, but continuously evolve from some easily understood initial state to a final state that provides the answer to some problem. In general, the analog and digital camps don’t really mix. Until now, that is.

The adiabatic computer is simpler than a quantum computer in many ways, and it is easier to scale. But an adiabatic computer can only be generalized to any type of problem if every qubit is connected to every other qubit. This kind of connectivity is usually impractical, so most people build quantum annealers with reduced connectivity. These are not universal and cannot, even in principle, compute solutions to all problems that might be thrown at it.

The issues with adiabatic quantum computers don’t end there. Adiabatic quantum computers are inherently analog devices: each qubit is driven by how strongly it is coupled to every other qubit. Computation is performed by continuously adjusting these couplings between some starting and final value. Tiny errors in the coupling—due to environmental effects, for instance—tend to build up and throw off the final value.