People have been debating whether a quantum computer presents a realistic threat to the Bitcoin network for over a decade. It was a serious topic of conversation over 13 years ago when I first discovered Bitcoin myself. 

There has been quite a lot of progress, both in terms of theory and real-world engineering, since long ago when I was just a bumbling idiot trying to figure out what was going on here. 

Two major milestones have been reached since then that make a material difference in the likelihood of a viable quantum computer actually being produced sometime in the next decade or so. That doesn’t inherently mean that it will reach a point of ubiquity, or even relative ease of access for those with large amounts of capital. 

But it is very possible that a number of viable machines will be produced in the near future. 

The first major improvement has been in error correction. To account for the inherent noise in working with things at this kind of tiny scale, to get a logical qubit that is useful in computation in practice requires the use of multiple redundant physical qubits. 

The prior state of the art way of doing this was surface codes, a way of bundling multiple physical qubits together in a grid and using some of them as check qubits that periodically “check on” their neighbors to ensure no internal errors in the superposition have occurred (without collapsing the superposition). Each grid’s empty spaces need to be filled with check qubits.

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