This week, Google published a paper detailing how a quantum computer could theoretically obtain a Bitcoin private key in 9 minutes, which has implications for Ethereum, other tokens, private banking, and potentially everything in the world.
It’s easy to think of quantum computing as a faster version of a regular computer. But it’s not a more powerful chip or a larger server farm. This is a fundamentally different type of machine, different at the atomic level.
A quantum computer starts with a very cold, very small loop of metal where particles begin to behave in ways that they don’t behave under normal circumstances on Earth, ways that change the way we think about the basic laws of physics.
Understanding what this means physically is the difference between reading about quantum hazard and actually understanding it.
How Computers and Quantum Computers Really Work
Regular computers store information as bits – each one being either a 0 or a 1. A bit is a small switch. Physically, it is a transistor on a “chip” – a microscopic gate that either lets power through (1) or not through (0).
Every photo, every Bitcoin transaction, every word you type is stored as a pattern of these switches turning on or off. There is nothing mysterious in the slightest; It is a physical object in one of two definite states.
Every calculation is just moving these 0’s and 1’s around very rapidly. A modern chip can do billions of these tasks per second, but it still does them one at a time, in sequence.
Quantum computers use something called qubits instead of bits. A qubit can be 0, 1, or – and this is the weird part – both at the same time!
This is possible because a qubit is an entirely different kind of physical object. The most common version, and the one Google uses, is a small loop of superconducting metal that is cooled to about 0.015 degrees above absolute zero, which is colder than in outer space but here on Earth.
At that temperature, electricity flows through the loop without any resistance, and the current is said to exist in a quantum state.
In a superconducting loop, current can flow clockwise (call 0) or counterclockwise (call 1). But at the quantum scale, the current does not have to choose a direction and actually flows in both directions simultaneously.
Don’t make the mistake of switching between the two really fast. Current measurement takes place simultaneously in both experimental and verification stages.
mind-bending physics
Still with us? Great, because this is where it gets really weird, because the physics behind how this works isn’t immediately intuitive, and it shouldn’t be.
Everyone we interact with in daily life adheres to classical physics, which assumes that things happen at the same place at the same time. But particles do not behave this way on the subatomic scale.
An electron has no definite position until you see it. A photon has no definite polarization until you measure it. Current in a superconducting loop doesn’t flow in a certain direction unless you force it to choose it.
The reason we don’t experience this in everyday life is inconsistency. When a quantum system interacts with its environment, air molecules, heat, vibrations, and light, the superposition collapses almost instantly.
A football cannot be in two places at once because it is interacting with trillions of air molecules, dust, sound, heat, gravity, etc. every nanosecond. But isolate a small flow in a near-absolute vacuum, protect it from every possible perturbation, and the quantum behavior will survive long enough to be calculated.
That’s why building a quantum computer is so difficult. People are engineering physical environments where the laws of physics that would normally prevent this thing from happening are kept at bay just long enough to run the calculations.
Google’s machines operate in giant room-sized refrigerators, colder than anything in the natural universe, surrounded by layers of shielding against electromagnetic noise, vibration, and thermal radiation.
And qubits are fragile still. They constantly lose their quantum state, which is why “error correction” dominates every conversation about scaling.
So quantum computing is not a faster version of classical computing. It is using a different set of physical rules that apply only on extremely small scales, extremely low temperatures, and extremely short time frames.
Now stack it.
Two regular bits can be in one of four states (00, 01, 10, 11), but only one at a time (since current only flows in one direction). Two qubits can represent all four states simultaneously, because current is flowing in all directions at the same time.
Three qubits represent eight states. Ten qubits represent 1,024. Fifty qubits represent more than a quadrillion. The number doubles as each qubit is added, which is why the scaling is so exponential.
The second trick is something called entanglement. When two cubes are entangled, measuring one immediately tells the observer something about the other, no matter how far apart they are. This allows a quantum computer to coordinate across all those simultaneous states in a way that regular parallel computing cannot.
And these quantum computers are set up so that wrong answers cancel each other out (like overlapping waves that flatten out) and correct answers reinforce each other (like waves that flatten out). By the end of the calculation, the correct answer is most likely to be measured.
So this is not brute speed. This is a fundamentally different approach to computation – one that lets nature explore an increasingly larger space of possibilities and then arrive at the correct answer through physics rather than logic.
A major threat to cryptography
This mind-bending physics is catastrophic for encryption.
The mathematics that underpin Bitcoin’s security rely on the assumption that examining every possible key would take more time than the age of the universe.
But a quantum computer doesn’t check every key. It explores them all together and uses interference to bring the right ones to the surface.
This is where it connects to Bitcoin. It takes milliseconds to go from private key to public key in one direction. Going in the other direction, going from the public key back to the private key, would take a classical computer more than a million years, or the age of the universe. That oddity is the only thing that proves a person is holding his coins.
A quantum computer running an algorithm called Shores can go through that trap in the opposite way. Google’s paper this week showed it could do so with far fewer resources than previously estimated and within a deadline racing against Bitcoin’s own block confirmations.
That’s why the threat of quantum computers breaking blockchain encryption is really worrying everyone.
How that attack worked step-by-step, what specifically changed in Google’s paper, and what it means for the 6.9 million Bitcoins already exposed is the subject of the next part of this series.