This week, Google published a paper describing how a quantum computer could theoretically derive a Bitcoin private key in 9 minutes, with ramifications that extend to Ethereum, other tokens, private banking, and potentially everything in the world.
Quantum computing is easy to confuse with a faster version of a classical computer. But it’s not about a more powerful chip or a bigger server farm. This is a fundamentally different type of machine, different at the level of the atom itself.
A quantum computer starts with a very small, very cold metal loop in which particles begin to behave in ways that they don’t behave under normal conditions on Earth, ways that change what we think of as the basic rules of physics.
Understanding what this means, physically, is the difference between reading about the quantum threat and actually grasping it.
How computers and quantum computers actually work
Ordinary computers store information in the form of 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 electricity through (1) or not (0).
Every photo, every Bitcoin transaction, every word you’ve typed is stored as patterns of turning these switches on or off. There is nothing mysterious about a bit; it is a physical object in one of two defined states.
Each calculation is simply a matter of mixing these 0s and 1s together very quickly. A modern chip can do billions of them per second, but it always does them one at a time, in order.
Quantum computers use what are called qubits instead of bits. A qubit can be 0, 1 or – and this is the weird thing – 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 used by Google, is a tiny loop of superconducting metal cooled to about 0.015 degrees above absolute zero, colder than outer space but here on Earth.
At this temperature, electricity flows through the loop without any resistance and the current would exist in a quantum state.
In the superconducting loop, current can flow clockwise (let’s call it 0) or counterclockwise (let’s call it 1). But on the quantum scale, current does not need to choose a direction and flows in both directions simultaneously.
Don’t confuse it with very fast switching between the two. 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, nor is it meant to be.
Everything a person interacts with in daily life obeys classical physics, which assumes that things are in the same place at a time. But particles don’t behave this way on the subatomic scale.
An electron doesn’t have a definite position until you look at it. A photon has no definite polarization until you measure it. A current in a superconducting loop doesn’t flow in a set direction until you force it to choose.
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 instantly.
A soccer ball can’t be in two places at once because it interacts with billions of molecules of air, dust, sound, heat, gravity, etc., every nanosecond. But isolate a tiny current in a vacuum near absolute zero, protect it from all possible disturbances, and the quantum behavior will survive long enough to allow calculation.
This is why quantum computers are so difficult to build. People design physical environments in which the laws of physics that normally prevent this from happening are held at bay for just long enough to perform 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 protection against electromagnetic noise, vibration and thermal radiation.
And qubits are fragile even then. They are constantly losing their quantum state, which is why “error correction” dominates all conversations about scaling.
Quantum computing is therefore not a faster version of classical computing. It exploits a different set of physical laws that only apply on extremely small scales, at extremely low temperatures, and over extremely short time frames.
Now stack that.
Two normal 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, because current flows in all directions at the same time.
Three qubits represent eight states. Ten qubits is 1,024. Fifty qubits is more than a quadrillion. The number doubles with each added qubit, which is why the scaling is so exponential.
The second trick is called entanglement. When two qubits are entangled, the measurement of one instantly informs the observer about the other, regardless of the distance separating them. This allows a quantum computer to coordinate all of these simultaneous states in a way that classical parallel computing cannot.
And these quantum computers are set up so that wrong answers cancel each other out (like overlapping waves that get flatter) and right 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. This is a fundamentally different approach to computation: one that lets nature explore an exponentially large space of possibilities, then collapses to find the right answer using physics rather than logic.
A monumental threat to crypto
This mind-blowing physics explains why it is terrifying for encryption.
The calculations protecting Bitcoin rely on the assumption that verifying all possible keys would take longer than the age of the universe.
But a quantum computer doesn’t check every key. He explores them all simultaneously and uses interference to bring out the right one.
This is where it ties into Bitcoin. Going in one direction, from private key to public key, takes milliseconds. Going the other way, from public key to private key, would take a classical computer a million years, or even longer than the age of the universe. This asymmetry is the only thing that proves a person owns their coins.
A quantum computer running an algorithm called Shor can go through this trapdoor in reverse. Google’s paper this week showed it could do so with far fewer resources than previously estimated, and in a time frame that flies in the face of Bitcoin’s own blocking confirmations.
This is why the threat of quantum computers breaking blockchain encryption truly worries everyone.
How this attack works step by step, what Google’s document specifically changed, and what this means for the 6.9 million bitcoins already exposed, are the subject of the next article in this series.
