Will quantum computers destroy bitcoin?

Will a quantum computer one day emerge and use its greater computing power to mine all Bitcoin and empty all wallets? While some believe this could happen in the long run, others disagree and claim that there is no fear for very long.

Let’s discuss an article in which two Canadian computer science professors analyze whether there is a threat from quantum miners, and if so, what it is.

The way forward

The following idea serves as the basis for the authors formulating their question: If a miner becomes too strong, the network may be at risk. He can run 51% of attacks if he has an absolute majority, but even if he doesn’t, he can still deal damage by doing “aggressive” or “selfish” mining.

What then happens if a miner uses quantum computing to such an extent that his share of the hashrate increases disproportionately?

All of this is really basic and has happened before: mining is accelerated by technological advances.

It often happens quickly rather than gradually.

Leading processors were replaced by graphics cards from 2011, and Asics stopped making graphics cards in 2013.

In these times, efficiency grows quadratically or exponentially rather than linearly.

Since conventional processors have largely reached their limits, quantum computers may represent the next big step.

This should not be a problem in itself, as the game-theoretic principles of bitcoin encourage rational players to be trustworthy and keep each other in line.

However, a technological leap can be challenging, and it can open doors for opposition forces that work irrationally to undermine Bitcoin.

Knowing when it can happen is therefore sensible.

What must happen for traditional Asics to be replaced by quantum computers in Bitcoin mining?

Unsorted database search

It is possible to see miners creating a lot using their computational power when discussing bitcoin mining.

They produce random hashes, and if a hash satisfies a set of standards for scarcity, the miner discovers a block.

Another term for it would be a brute force attack against the SHA256 hash algorithm.

The miners are trying to partially reverse a cryptographic hash function, according to the two professors.

Doing this “partial inversion of a hash function” is “equivalent to looking for a marked element in an unordered list of things (an unstructured search).”

Although it seems like a small problem, everything else depends on it.

Since quantum computers are limited in what they can achieve, finding a specific element in an unordered list is one of the few tasks where quantum computers have been shown to be superior to conventional computers.

A traditional computer must go through each entry one at a time while performing a brute force attack or scanning an unsorted database.

It can be compared to a two-dimensional pointer that navigates between objects.

The probability of a hit approaches 50% when it has seen half of the listings.

A traditional computer therefore requires on average N/2 operations, where N is the total number of objects that can be processed.

The advantage of quantum computers is as follows: Several qubits can simultaneously represent all imaginable variations since a qubit can be both 0 and 1.

It can be compared to a pointer that points in N dimensions. The answer is already there in the qubits if they are in this “superposition”.

But as soon as you measure, the solution is broken.

This is the infamous quantum paradox: if you count, you force the quantum into a certain but random state.

The quantum computer therefore knows the answer, but in a cruel twist, when you go to retrieve it, it devalues ​​it.

Grover’s formula squares realistic acceleration

Grover’s algorithm, which was created by Lov Grover in 1996, is a technique for verifying the outcome.

The qubits identify false results and inhibit them by combining several “quantum gates”, which are the operations of quantum computers.

With each iteration, or so-called Grover iteration, the probability of reaching the correct answer increases.

The level of complexity throughout is enormous.

But one thing is certain: the Grover method can significantly speed up such searches if the right amount of repetitions is used.

Grover requires only n attempts to find a particular element in an unsorted list.

As a result, it is almost four times faster.

Two cases: Both conventional and quantum computers need two attempts if there are four objects.

In contrast, a quantum computer is discovered after 2,280 attempts when there are 5,198,400 pieces, but a normal computer has to run more than two million times.

This difference is significant, especially for activities with very high N or that are very tough. The so-called quantum advantage is this difference.

One of those jumps that can completely destroy an ecosystem. at least conceptually.

The quantum advantage disappears

In reality, a quantum miner faces a specific problem: He cannot find a block until he measures the outcome, which forces him to stop the operation.

He must therefore plan how many iterations he will perform in advance.

The question is challenging. because there are disadvantages to both having too many and too few.

More iterations increase the danger that another miner will be faster and the probability that the correct answer will be found.

Conversely, fewer repetitions reduce the probability of a legitimate result and, as a result, the quantum advantage.

A quantum computer could fully utilize the quantum advantage if it had infinite time. However, mining prohibits this. A balance must be struck between too few and too many iterations.

The researchers created a Markov chain containing all potential outcomes to find the best trade-off.

A mathematical representation of potential, mostly random or partially unexpected sequences is called a Markov chain.

Such a chain shows which route through the maze of probability, or the best Grover algorithm configuration, often leads to the best results.

This would take, surprisingly, 16 minutes.

Two outstanding finds

Let’s say it takes a quantum miner 16 minutes to read Grover’s algorithm output. Compared to the long-term disadvantages, the advantage over traditional mining is at its greatest.

The researchers claim that this advantage is there regardless of the challenge.

Because it can be used, the result is quite impressive. Two serious outcomes can be seen here:

First, by using such a strategy, the miner excludes himself from about 80% of the blocks. This is a result of things being discovered in less than 16 minutes.

With the remaining 20%, he increases the chances of success.

The total mining power that quantum computers should be able to reach should not exceed this without compromising efficiency.

Second, the time between blocks is often shorter for cryptocurrencies. Ethereum and Ripple only have a few seconds, while Dogecoin and Litecoin have a few minutes.

With these blockchains, the quantum advantage does not hold, therefore quantum miners get a bloody nose. In mining, they are already quantum secure.

Quantum computer parallelization also appears to be a dead end.

The Grover method makes this possible, but the authors’ calculations show that it only improves performance by a factor of m.

The element is m for traditional computers, making it squarely larger.

It is therefore doubtful that quantum computers will ever be useful for mining.

Megahash: 78

These calculations already significantly reduce the threat posed by quantum computers.

But the most important question remains unanswered: What must happen before quantum miners have the upper hand over traditional miners?

When, if ever, will it become more affordable to use a quantum computer to discover a block?

The cost per grover iteration and the proportion of hashes needed for a block to grover iterations needed are the two crucial elements of this.

The authors make this calculation using the example of a quantum computer that is currently widespread and has a “gate speed” of 66.7 MegaHertz.

The gates, or quantum processes, are the gates.

According to the researchers’ calculations, this quantum computer could perform 224 Grovers every second.

Importance? A hash rate of 78 mega-hashes per second is equivalent to 224 Grover iterations.

It makes up a small fraction of the Bitcoin hash rate and is far less than that achieved by modern Asics. It would be absurd to perceive any threat here.

Possibly future versions that are more energy efficient

But are quantum miners at least more productive if they don’t represent a threat? So, is it possible to move to quantum mining, albeit gradually? Also, when?

The energy cost of a Grover iteration should only be 3.49 × 105 times that of a conventional hash to be more efficient.

A quantum computer would need an efficiency better than 3.49 x 105 × 10-10, or around ten J each Grover iteration, to be as energy efficient as traditional miners, which have an energy efficiency of 10-10 Jules per hash, perhaps to and with 2240 J/s.

It seems really degrading. However, quantum computers need relatively little energy.

The quantum bits transform into a superconductor after the system is cooled to 15 millikelvin, or almost absolute zero, and require almost no electricity and generate almost no heat.

A quantum computer is still uneconomical at the moment due to cooling compared to electricity.

However, as technology advances, this should change.

In conclusion, Bitcoin users should rest easy and know that they are a little smarter and can no longer imagine the terror of a world run by quantum computers.