This week, Google published a paper describing how a quantum computer could theoretically derive a bitcoin private key in 9 minutes, with ramifications extending to Ethereum, other tokens, private banking, and potentially everything in the world.
Quantum computing is easy to confuse with a faster version of a regular computer. But it’s not about a more powerful chip or a bigger server farm. It is a fundamentally different type of machine, different at the level of the atom itself.
A quantum computer starts with a very cold, very small metal loop where particles begin to behave in ways that they don’t behave under normal conditions on Earth, ways that alter what we consider the basic rules of physics.
Understanding what that means, physically, is the difference between reading about the quantum threat and actually understanding it.
How computers and quantum computers really work
Normal computers store information in the form of bits: each is a 0 or a 1. A bit is a small switch. Physically, it is a transistor on a “chip”: a microscopic gate that allows electricity to pass through (1) or not (0).
Every photo, every bitcoin transaction, every word you’ve ever typed is stored as on or off patterns of these switches. There’s nothing mysterious about a bit; is a physical object in one of two defined states.
Each calculation is simply mixing these 0s and 1s together very quickly. A modern chip can do billions of this per second, but it still does it one at a time, in sequence.
Quantum computers use something known as 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 a completely different type of physical object. The most common version, and the one Google uses, is a small loop of superconducting metal cooled to about 0.015 degrees above absolute zero, colder than outer space but here on Earth.
At that temperature, electricity flows through the circuit without any resistance and the current is said to exist in a quantum state.
In the superconducting loop, current can flow either clockwise (let’s call it 0) or counterclockwise (let’s call it 1). But at quantum scales, the current does not have to choose one direction and, in fact, flows in both directions simultaneously.
Don’t confuse it with switching between the two too quickly. The current is measurable, experimental and verifiable in both states simultaneously.
Amazing physics
With us so far? Great, because this is where it gets really weird, because the physics behind how it works isn’t immediately intuitive, and it’s not supposed to be.
Everything someone interacts with in daily life obeys classical physics, which assumes that things are in one place at the same time. But particles don’t behave this way at the subatomic scale.
An electron has no defined position until you look at it. A photon does not have a defined polarization until you measure it. A current in a superconducting circuit does not flow in a defined direction until it is forced to take it.
The reason we don’t experience this in everyday life is decoherence. When a quantum system interacts with its environment, air molecules, heat, vibrations and light, the superposition collapses almost instantaneously.
A soccer ball cannot be in two places at once because it interacts with billions of molecules of air, dust, sound, heat, gravity, etc., every nanosecond. But we isolate a tiny current in a vacuum close to absolute zero, shield it from all possible perturbations, and the quantum behavior survives long enough for calculations to be made.
That’s why it’s so difficult to build quantum computers. People are designing physical environments where the laws of physics that normally prevent this from happening are held in check long enough to run a calculation.
Google machines run in dilution refrigerators the size of enormous rooms, colder than anything in the natural universe, surrounded by layers of protection against electromagnetic noise, vibrations and thermal radiation.
And qubits are fragile even then. They lose their quantum state constantly, which is why “error correction” dominates all conversations about scaling.
Therefore, quantum computing is not a faster version of classical computing. You are exploiting a different set of physical laws that only apply at extremely small scales, extremely low temperatures, and extremely short periods of time.
Now stack that.
Two regular bits can be in one of four states (00, 01, 10, 11), but only one at a time (since current flows in only one direction). Two qubits can represent all four states at once, since current flows in all directions at the same time.
Three qubits represent eight states. Ten qubits represent 1,024. Fifty qubits represent more than a trillion. The number doubles with each qubit added, which is why the scaling is so exponential.
The second trick is something called interlacing. When two qubits are entangled, measuring one instantly tells the observer something about the other, no matter how far away they are. This allows a quantum computer to coordinate all those simultaneous states in a way that normal parallel computing cannot.
And these quantum computers are set up so that incorrect answers cancel each other out (like overlapping waves that flatten out) and correct answers reinforce each other (like waves that stack up higher). At the end of the calculation, the correct answer has the highest probability of being measured.
So it’s not brute force speed. It is a fundamentally different approach to calculation: one that allows nature to explore an exponentially large space of possibilities and then collapses towards the correct answer through physics rather than logic.
A monumental threat to crypto
This mind-blowing physics is why it’s terrifying for encryption.
The mathematics that protects Bitcoin is based on the assumption that checking all possible keys would take longer than the age of the universe.
But a quantum computer doesn’t verify all keys. It scans them all simultaneously and uses interference to find the right one.
That’s where it relates to Bitcoin. Going in one direction, from the private key to the public key, takes milliseconds. Going in the other direction, from public key to private key, would take a classical computer a million years, or even longer than the age of the universe. That asymmetry is the only thing that proves that a person has their coins.
A quantum computer running an algorithm called Shor can go through that trapdoor in reverse. Google’s paper this week showed that it could do so with far fewer resources than anyone had previously estimated, and within a time frame that rivals bitcoin’s own block confirmations.
That’s why the threat of quantum computers breaking blockchain encryption really worries everyone a lot.
How that attack works step by step, what specifically the Google document changed, and what it means for the 6.9 million bitcoins already exposed, is the subject of the next article in this series.
