earlier threads on Bitcoin and quantum computing:

https://bt.irlbtc.com/view/133425.0

https://bt.irlbtc.com/view/78693.0

https://news.ycombinator.com/item?id=2602570

The last thing somebody is going to do with a quantum computer is mess with bitcoin. I can think of almost infinite things they should be doing first like bioinformatics and physics calculations for new trillion dollar alternative power methods.

Bruce Schneier has written about QC numerous times on how useless it is for practical cryptography, just look up quantum computing on his blog, go through all the posts, deflate the hype

I feel like we talk about this every few months.

EVERYBODY PANIC

Interesting paper.

According to it, preimage searches can be faster on quantum computers than on classic computers.  BTC mining can be expressed as preimage search, so the extended Grovers algorithm could be applied.  The paper sais it is not clear what search complexity is required to reach the tip-over point at which a quantum computer is more efficient.  Therefore it isnt clear on which side BTC sits.  It may or may not be more efficient.

If BTC mining was more efficient on quantum computers, it wouldnt necessarily be the end of BTC.  As long as the length of SHA2 hashes permit, quantum mining rigs will be dealt with by difficulty adjustments.  Just like GPU, FPGA, ASIC technology each is so much more efficient than the previous generations.  The network has compensated for all of them and is still running as designed.

There is a maximum possible difficulty though.  Only when the network hashing power gets beyond that point, BTC mining is broken.  Until then: business as usual.

Other areas of BTC (such as the public key crypto) are probably much more vulnerable to quantum computers than the mining process.

To me if there is a computer powerful enough to attack bitcoin then it is powerful enough to attack traditional banking institutions as well.  All monetary systems would be at risk not just bitcoin.

Indeed all that I read about the D-wave device was that it can outperform classical computers for certain optimisation problems, but it is not a "general purpose quantum computer" which could either run Grover's algorithm for sqrt(N) searches in unsorted lists, or Shor's algorithm for integer factoring and discrete logarithms.  Note that in my opinion, mining SHA-2 is not the crucial point if a quantum computer is ever dangerous to bitcoin.  Instead it is the public key cryptography and ECDSA (although I don't know how susceptible ECDSA is to quantum computing).

The question is, will only the bad guys have quantum computers? What will be the computational power of personal q.c. versus large q.c.? If reasonable quantum computing power is available to everyone, nothing will change - people will use small q.c. for encryption and the NSA will have big ones that take trillions of years to brute force.

Only if normal people are stuck with classical computers, and the bad guys have q.c. then we're fucked.

Prediction: The next Pirateat40 will be advertising a quantum miner. He'll make a fortune and delivery will be pending for years.