This week, Google published a paper that described how a quantum computer could theoretically derive a bitcoin private key in 9 minutes, with implications that extend to Ethereum, other tokens, private banking and potentially everything in the world.
Quantum computing is easily mistaken for a faster version of a regular computer. But it’s not a more powerful chip or a bigger server farm. It is a fundamentally different machine, different at the level of the atom itself.
A quantum computer starts with a very cold, very small loop of metal, where particles begin to behave in ways they don’t behave under normal conditions on Earth, ways that change what we think of as the basic rules of physics.
Understanding what it physically means is the difference between reading about the quantum threat and actually understanding it.
How computers and quantum computers actually work
Ordinary computers store information as bits – each is either 0 or 1. A bit is a small switch. Physically, it’s a transistor on a “chip” – a microscopic gate that either lets electricity through (1) or doesn’t (0).
Every picture, every bitcoin transaction, every word you’ve ever typed is stored as patterns of these switches being turned on or off. There is nothing mysterious about a little; it is a physical object in one of the two definite states.
Each calculation just shuffles these 0s and 1s around really fast. A modern chip can do billions of these per second, but it still does them 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 kind 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 loop without any resistance, and the current is said to exist in a quantum state.
In the superconducting loop, current can flow clockwise (call it 0) or counterclockwise (call it 1). But at quantum scales, the current does not have to choose one direction and actually flows in both directions simultaneously.
Don’t confuse it with switching between the two really quickly. The current is measurable, experimental and verifiable in both modes simultaneously.
Mind-bending 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, nor should it be.
Everything that anyone interacts with in daily life obeys classical physics, which assumes that things are in one place at a time. But particles do not behave this way on the subatomic scale.
An electron does not have a definite position until you look at it. A photon does not have a definite polarization until you measure it. A current in a superconducting loop does not flow in a particular direction until you force it to pick.
The reason we don’t experience it in everyday life is incoherence. When a quantum system interacts with its surroundings, air molecules, heat, vibrations and light, the superposition collapses almost instantly.
A soccer ball cannot be in two places at once because it interacts with trillions of air molecules, dust, sound, heat, gravity, etc., every nanosecond. But isolate a small current in a near-absolute-zero vacuum, shield it from any possible disturbance, and the quantum behavior survives long enough to calculate it.
This is why quantum computers are so hard to build. People develop physical environments where the physical laws that normally prevent these things from happening are held in check just long enough to run a calculation.
Google’s machines operate in dilution refrigerators the size of huge rooms, colder than anything in the natural universe, surrounded by layers of shielding against electromagnetic noise, vibration and thermal radiation.
And qubits are fragile even then. They constantly lose their quantum state, which is why “error correction” dominates any conversation about scaling up.
So quantum computing is not a faster version of classical computing. It exploits a different set of physical laws that only apply at extremely small scales, extremely low temperatures, and extremely short time frames.
Now stack it up.
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 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 over a quadrillion. The number doubles for each qubit added, which is why the scaling is so exponential.
The second trick is something called entanglement. When two qubits are entangled, measuring one instantly tells an observer something about the other, no matter how far apart they are. This lets a quantum computer coordinate across all the concurrent states in a way that ordinary parallel computing cannot.
And these quantum computers are set up so that wrong answers cancel each other out (like overlapping waves that flatten) and right answers reinforce each other (like waves that stack 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’s a fundamentally different approach to computation—one that lets nature explore an exponentially large space of possibilities and then collapses to the right answer through physics rather than logic.
A monumental threat to cryptography
This mind-boggling physics is why it’s scary for encryption.
The math that protects bitcoin relies on the assumption that checking all possible keys would take longer than the age of the universe.
But a quantum computer does not check all keys. It explores them all simultaneously and uses interference to show the right one.
That’s where it ties in with Bitcoin. Going one way, from private key to public key, takes milliseconds. Going the other way, from public key back 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 a person is holding their coins.
A quantum computer running an algorithm called Shor’s can go through that trapdoor in reverse order. Google’s paper this week showed it could do so with far fewer resources than anyone had previously estimated, and in a time frame that rivals bitcoin’s own block confirmations.
This is why the threat of quantum computers breaking blockchain encryption really has everyone very worried.
How that attack works step by step, what Google’s paper specifically changed, and what it means for the 6.9 million bitcoins already exposed is the subject of the next piece in this series.
