With mining gaining in popularity every day and the raw power of the ASIC devices increasing at a dramatic rate, I wonder what is the next big jump for miners.

Qubits, or quantum bits, have already shown to be extremely fast at calculations. That makes them seem like a perfect fit for mining bitcoins.

This question is asking if a qubit device can be created for mining bitcoins and, if so, how it would affect the current mining community using standard zeros and ones. To ask this another way...

How much faster would a qubit device mine bitcoins when compared to an existing ASIC device today? Let's compare it to ButterflyLab's Mini Rig which runs at 500 GH/s. Ten, hundreds, thousands, millions times faster? Could quibit devices destroy the bitcoin network by controlling more than 51% of the network?

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The best thing I can imagine is running Grover's algorithm, which speeds up what would be an exhaustive search for a single solution among N input values to take only about the square root of N actual evaluations. Only very special problems, like factoring numbers, have better known quantum algorithms.

If you imagine connecting a hypothetical quantum computer to a mining pool, the impact would be low: The quantum computer can only be run on the share of work it gets, which means finding if any of 2^32 nonce values yield a proof of work. An ASIC miner could probably do that in less than a second using (internally and highly parallelized) something like 2^32 operations, each of which includes several 32-bit additions. The quantum computer could get away with doing about 2^16 equivalent quantum operations, which, after serializing the bit operations of the additions, likely means many thousand times 2^16 consecutive qubit operations. And it will require on the order of a thousand qubits, at least. Sounds to me like in this scheme a quantum computer might, at best, be equivalent to a hundred ASICs or so.

But it might just get interesting if we move more of the mining algorithm onto our hypothetical quantum computer. If the extra nonce can be incorporated into the Grover algorithm, a larger search can be performed, with a correspondingly larger speed-up. Unfortunately, a much larger quantum computer running a much more complicated program would be needed. Or let us say fortunately for whoever might otherwise worry about mining competition!

Using Qubits doesn't automatically make everything faster. There are certain quantum algorithms that can only be run on quantum computers that take advantage of qubits. These algorithm's might be able to solve certain problems "faster" but not necessarily in linear terms -- that is, you wouldn't be able to say it did something ten, or one hundred, or one million times faster, it's more like a "did you get an answer or not?" type of situation.

Another important question might be, Can the bitcoin mining algorithm be converted into such a quantum algorithm?.

I don't know if there is an answer for that, but if we can find such an algorithm, this could theoretically break bitcoin.

Sounds absurd, yes, but consider this -- the difficulty for the entire network is based on how fast blocks are found -- if blocks are found faster than normal, the network permanently increases the difficulty to compensate.

A quantum computer would be able to mine a block pretty much instantly and if introduced into the network, it would cause the difficulty to skyrocket, effectively locking out all other computers on the network from being able to mine simply because it's hard for them.

You might want be tempted to simply build the same network just using quantum computers entirely, but if the aforementioned difficulty reaches the maximum difficulty as defined by the protocol, all quantum computers on the network would mine blocks instantaneously, immediately forking the block-chain at every node. In order to fix this, the entire protocol would have to change in order to avoid any pitfalls brought about by quantum computing.

While technically a separate entity from the bitcoin system, it should be noted that the address system is also potentially vulnerable. Algorithms exist that, if run on a quantum computer, can find a private key that matches a given public address. While the mining may or may not be effected by quantum algorithms, the security definitely is.

So, to answer your question explicitly -- maybe, and if so, it would destroy the mining community.

But let's not forget, that this is all still speculation. It might not be possible to modify the mining algorithm to run on top of qubits. Even if it is possible, it is still debatable as to whether or not a true quantum computer can be built.

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