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Comment by dr_zoidberg | Hacker News Reader

Decoherence is the answer. Veeeeery long story short, qubits have to be isolated from the world as much as possible to maintain their "magical quantum properties". Decoherence is when those ideal conditions break, and the qubits no longer behave as needed for the quantum computer to work. The more qubits you have, thet harder it gets to fend off the inteference from the outside world that decoheres the qubits.

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Ok, so it gets N times harder, but you get M times more compute power. Is there a relation between N and M? Is at least M larger than N? Will N get lower with more qubits?

Thinking about things being N times harder is probably the wrong approach at the current time.

We’re so early in the engineering of these computers that it’s “we don’t know how to make more than X usable qbits in this configuration at any cost” rather than “X+1 qbits is Y dollars more expensive”.

Worth noting that the value of X depends heavily on the configuration and the things holding the qbits; IIRC the D-wave design is scalable but also not a “universal” quantum computer. I’m unclear on the specifics of how and why.

The D-Wave design is for quantum annealing, which is still super interesting and solves practical problems, in particular optimization problems. You can read a few summaries of the different systems and how they were compared to classical computers at launch in Wikipedia[0].

For a quick tl;dr: D-Wave computers can speed up certain optimization problems, but more than a few times it's been in doubt if they provide an advantage over classical computers or not (notice that a few times they compared against single threaded systems).

[0] https://en.wikipedia.org/wiki/D-Wave_Systems#Computer_system...

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