Quantum Supremacy Achieved: A Long Term Threat to ‘Classical’ Cryptocurrencies

September 26, 2019 / by Zachary Mashiach

Google and NASA have jointly developed the most powerful quantum computer in history and have achieved the long sought after goal of quantum supremacy. Apparently the paper published about this development was not supposed to be posted yet and was quickly deleted, but fortunately, it was saved by internet users before being taken offline.

This quantum computer is codenamed Sycamore and has 53 qubits, meaning it has 2^53 states, or equivalently 9 X 10^15 states. Compare this to classical computers that use binary and just have 2 states, 0 and 1.

The researchers who developed Sycamore compared the efficiency of the quantum computer versus a classical computer in sampling a pseudo-random quantum circuit. This circuit entangles a set of qubits, and for a classical computer sampling, this circuit becomes exponentially harder with an increasing number of qubits and gate cycles.

Ultimately, Sycamore was able to perform the same calculations in 200 seconds that the most powerful classical supercomputer in the world, Summit, could perform in 10,000 years.

This is called quantum supremacy, meaning a quantum computer can outperform the most powerful classical computer at a specific but not necessarily useful task. In fact, the task that Sycamore completed can only be completed by a quantum computer since a classical computer is simply too slow to handle it.

The next goal to hit is called quantum advantage, where a quantum computer is better than the most powerful computer at a useful task.

The researchers who developed Sycamore expect that quantum computing power will continue to grow at a double exponential rate, essentially the quantum equivalent of Moore’s Law.

For classic computers, Moore’s Law states that the number of transistors on a microchip doubles every two years, but the cost is halved.

Also, the researchers say that they are “only one creative algorithm away from valuable near-term applications”.

Specifically, the researchers mention that if quantum error processing is focused on, it will be possible to run Shor’s and Grover’s algorithm on a quantum computer.

Shor’s and Grover’s algorithm can be used by a sufficiently powerful quantum computer to attack Bitcoin’s (BTC) cryptography, and the cryptography of all other cryptocurrencies.

Fortunately, it is estimated that a quantum computer would have to be 1,500 qubits to attack Bitcoin (BTC), and Sycamore is only 53 qubits and still too prone to error to run Shor’s or Grover’s algorithm. 

So it seems Bitcoin (BTC) and other cryptocurrencies are safe for now, but the quantum version of Moore’s law suggests that, eventually, quantum computers will be powerful enough to compromise Bitcoin (BTC) and all other cryptocurrencies, and all classical cryptographic algorithms for that matter. 

Even if that happens there is hope for cryptocurrency thanks to quantum cryptography. Due to the nature of quantum cryptography, where an encrypted message is destroyed, if anyone intercepts it, not even the most powerful quantum computer could compromise it. There have already been some experiments in developing a primitive quantum cryptocurrency.

That being said, in order to use a quantum cryptocurrency, users would need quantum computers, and it may be many years before such computers are available to the public. It is unknown if quantum computers will be powerful enough to attack classical cryptocurrencies before that happens.