Comment by dlcarrier
16 hours ago
Coherency
To get useful results, a quantum computer needs all of its qbits to stay entangled with each other, until the entire group collapses into the result. With current technology, it is very difficult for a reasonable sized group of qbits to stay coherently entangled, so it can only solve problems that are also relatively easy to solve on classical computers.
If someone today were to figure out how to keep large numbers of bits entangled, then quantum computing would instantly be able to break any encryption that isn't quantum safe. It's not something that we are slowly working toward; it's a breakthrough that we can't predict when, or even if, it will happen.
> instantly
Shor's and Grover's still are algorithm that require a massive amount of steps...
I don't think they meant "in O(1) steps", I think they meant "the day someone figures out how to keep many thousands of qubits entangled while operating on them with gates will be the same day we have the first QC that can start breaking encryption in reasonable time". Where, of course, same day is also an exaggeration. But the general point is that we need a single breakthrough to achieve this, and it's very hard to estimate how long a breakthrough might take to appear.