A quantum thought experiment

[ap334]

As I understand it (and my understanding may be very noobish), quantum computing isn't a thread unless you re-use addresses and even where it is, the relevant hash algorithms can be swapped out in the future for quantum resistant ones.

However, what if someone has already developed a qc without anyone knowing? What sorts of analyses can we be doing now to assess the likelihood that such an attack has already happened?

Just a thought experiment; I think we're likely far out, but I also think that if any capitalist is developing a qc, he/she isn't incentivized to share his/her status, so it's probably worth having this conversation early.

[franky1]

first you need to understand what a QC can do in plain english.. without out the mystical multiuniverse mumbo jumbo they try to spew out to hide their technologies secret ingredient..

and so, the fundamentals..
normal computers work by understanding that if it stores or transmits a voltage that is considered a binary 1.. no voltage is considered binary 0

QC is NOT about bending time/space or entering alternate universes, it is simply that the voltage is variable allowing more possibilities

so instead of a byte only having 256 possibilities of 00000000 to 11111111
a qubit can have more. 00000000 to 33333333(using a 4 position qubit) which is 65536 possibilities for the same 8bit of voltage variants.
all because instead of just 0volts or 1 volt. it can be measured that
0volts=0
0.33volts =1..
0.66volts=2
1volts=3
..
1 binary bit has 2 "super positions" 0-1
1 qubit has 4 "super positions" 0-1-2-3 in the example above(though some facilities are only playing with 3 super positions 0-1-2)
..when translating a single qubit position into binary can be seen as different things(i wont waffle into that aspect)

now with that said. although there are now about a dozen different facilities playing around with QC.. they are still looking at what can be done with it.

things they do know is that if they put in a binary logic problem they are limited to the efficiency of QC to about 2x.
this is because the result they need also needs to be binary so that a binary system can accept the problem is solved. and as such they can only play within the constraints of binary. and thus the only thing they can do is use half a qubyte (0000), for the systems with 4 positions to do what a binary system needs a full byte(00000000) for

so first things first.
binary logic problems such as block hashing is not a worry. firstly because not only are QC limited to a efficiency of 2x per operation.. but also the power of the QC systems are no where near as powerful as the network hashing rate.

for instance the largest binary super computer is only able to do a few terrahashes of bitcoin mining a second..(1 quadrillion ops a second)

 yet the bitcoin network is 24million terrahashes.(24sextillion ops a second)
yep 24million binary supercomputer facilities are needed just to match bitcoin mining

and a QC systems are no where near even capable of doing the same amount of operations per second that the largest binary supercomputers can right now..
so its not a threat.
yep it will require far more than just a dozen QC systems just to match ONE binary super computer and then 24million times that again to match bitcoins hash power. so relax, nothing to worry about any time soon
bitcoin ASICS will exceed the hashpower of QC for a long long while

...
now thats all said.
QC are not great at binary logic problems when it comes to efficiencies. but where they do prosper is in the realm of 3D. basically vectors
which is something that legacy bitcoin private/public key conversion does rely on.

which is where a 4super position bit can using 8 bytes have 65536 possible variations instead of just 256.. meaning that its an efficiency of 256 times better at brute forcing a privkey but only 2 times better at brute forcing a block hash

and yes.. if your re-using addresses which give away parts of the answer after use. a QC computer can brute force the answer 256 times faster than a comparable binary pc.

[ap334]

Thanks franky1 for the patient response; this is super useful for my understanding! In particular, really helps my intuition of why re-using addresses is dangerous.