Quantum computers have the potential to be the next big leap forward for humankind. However, there has long been speculation that quantum computers could become so powerful that they could break through Bitcoin’s cryptography — and the cryptography of all other cryptocurrencies and blockchains for that matter. If this happens, hackers with quantum computers could obtain Bitcoin private keys from the public keys, compromising Bitcoin’s security and value.
This article will explore if there is there any truth to this scenario, and if so, how many years from now would this sort of quantum computer attack be possible?
Classical computers utilize bits which have only 2 states, 0 and 1, i.e. binary. With every bit added to a computer, the computer’s processing power increases linearly. On the other hand, quantum computers use qubits which can have the states 0 and 1 in addition to any quantum superposition of those two states. Due to quantum superposition, as the number of qubits increases the number of possible states increases according to the formula 2^N, where N is the number of qubits. Therefore a quantum computer with 2 qubits has 4 possible states, 3 qubits has 8 possible states, 10 qubits has 1,024 possible states, and so on. With every qubit added to a quantum computer, the processing power of that quantum computer doubles, i.e. the processing power of a quantum computer increases exponentially with the number of qubits.
It is estimated that a quantum computer with 1,500 qubits would be able to crack a Bitcoin private key from a public key via Shor’s algorithm. Indeed, a sufficiently large quantum computer using Shor’s algorithm could break any public key cryptography system. Therefore, at some point in the future, it is likely that quantum computers will become powerful enough to compromise the current cryptography of Bitcoin (BTC).
Currently, the quantum computer with the most qubits is the D-Wave, which has 2,000 qubits and plans to expand to 5,000 qubits. However, the D-Wave is a quantum annealing computer, which finds the most probable outcome in a series of possible solutions and does not correct for any errors. Apparently, quantum annealing computers like the D-Wave are incapable of attacking Bitcoin (BTC).
A major problem hindering the advancement of quantum computers is noise. A review of the D-Wave’s specifications reveals the great lengths that companies go to reduce environmental noise in a quantum computer. The D-Wave is refrigerated to just a fraction of a degree above absolute zero, shielded from Earth’s magnetic field, and in a vacuum with pressure 10 billion times lower than the atmosphere.
Essentially any noise from heat, magnetism, or even air molecules is enough to render a quantum computer useless. Even with all the protection from the environment, the D-Wave is still inhibited by noise.
Another example of a cutting-edge quantum computer is the IBM Q System One that launched in January and uses integrated circuits. The IBM Q System One has 20 qubits. Google has developed an integrated circuit quantum computer that has 72 qubits. However, integrated circuit quantum computers suffer from noise despite the best attempts to seal them off from the environment.
A different type of quantum computer uses ions as qubits and uses lasers to interact with the ions. IonQ has developed a 160 qubit quantum computer with this method, but it is just as prone to being affected by noise as the other quantum computers.
It is estimated that, for a quantum computer to surpass a classical computer, it would need 1,000 qubits that are error-free, which would be called a logical qubit. However, the record for most qubits without errors is 10.
A type of theoretical quantum computer that would circumvent the noise problem would use topological qubits, where an electron would be split in half to produce two Majorana fermion quasi-particles. Apparently, these qubits will be much less prone to noise error. However, no working topological qubits have been produced yet.
Quantum computers are still in their infancy. There are two metrics to determine the advancement of quantum computers, called quantum supremacy and quantum advantage. Quantum supremacy is when a quantum computer performs a specific task better than a classical computer, and the quantum advantage is when a quantum computer performs a useful task better than a classical computer. Neither of these benchmarks has been reached yet.
In other words, at this time, the most powerful classical supercomputers are still more powerful than the most advanced quantum computer, and the most powerful supercomputers in the world cannot compromise Bitcoin’s cryptography in any practical time frame. Therefore, at this time, quantum computers are not a threat to Bitcoin (BTC).
However, it is important to put the evolution of quantum computers in perspective before dismissing the quantum threat to Bitcoin (BTC). In 1995, the first quantum logic gate was developed, the basis of an integrated circuit quantum computer. In 1998, a working quantum computer with two qubits was demonstrated. In 2001, Shor’s algorithm, the algorithm that could crack any public key cryptography system with sufficient power was demonstrated by a quantum computer for the first time. In 2006 a quantum computer exceeded 10 qubits for the first time. In 2017 Microsoft unveiled a quantum programming language. In late 2018, the president of the United States signed the National Quantum Initiative Act to accelerate the development of quantum computers.
Essentially, quantum computers have evolved from not even existing to being as powerful as some supercomputers within the span of two decades. Google’s 72-qubit quantum computer called Bristlecone is aiming to attain quantum supremacy, the point at which a quantum computer is more efficient than the most powerful classical supercomputer at solving a particular task.
It appears that, sometime in the near future, quantum supremacy will be achieved, whether it be Google’s Bristlecone or a different quantum computer. We must remember, though, that quantum computers scale exponentially with increasing numbers of qubits, so in coming years, quantum computers have the potential to become exponentially more powerful once these early quantum computers are perfected.
So although there are no quantum computers at this moment that can compromise Bitcoin (BTC), in the not so distant future, a quantum computer will likely be created that can use Shor’s algorithm to compromise any public key cryptography system, at which point Bitcoin’s current cryptography would become obsolete.
But this would not be the end of Bitcoin (BTC).
One possible solution is to use Lamport Signatures that can be implemented with a soft fork and are quantum resistant. Beyond Lamport signatures, researchers have time to develop other quantum resistant cryptography systems.
Thus, the crypto community and developers should pay attention to the quantum threat, and develop proper quantum resistance in the coming years. If the quantum threat is ignored completely it would likely be catastrophic for Bitcoin (BTC), but if proper preparations are undertaken then Bitcoin can survive into the quantum era.