This week, Google published a paper describing how a quantum computer could theoretically derive a Bitcoin private key in nine minutes, with implications that extend to Ethereum, other tokens, private banking, and potentially everything in the world.
Quantum computing can easily be mistaken for a faster version of a regular computer. But it’s not more powerful chips or larger server farms. It is a fundamentally different kind of machine, different at the level of the atoms themselves.
Quantum computers start with a very cold, very small metal ring in which particles begin to behave in different ways than under normal conditions on Earth, ways that change what we consider the fundamental rules of physics.
Understanding what this means physically is the difference between reading about the quantum threat and actually mastering it.
How computers and quantum computers actually work
Ordinary computers store information as bits—each bit is either a 0 or a 1. A bit is a tiny switch. Physically speaking, it’s a transistor on a “chip” – a microscopic gate that either lets current flow through (1) or doesn’t (0).
Every photo, every Bitcoin transaction, every word you type is stored as a pattern of these switches being on or off. There’s nothing mysterious about it at all. It is a physical object in one of two definite states.
Each calculation just quickly scrambles these 0’s and 1’s. Modern chips can do this billions of times per second, but it still does it sequentially one at a time.
Quantum computers use something called qubits instead of bits. A qubit can be 0, 1, or—and here’s the weird part—both 0 and 1 at the same time!
This is possible because qubits are an entirely different type of physical object. The most common version (the one used by Google) is a tiny ring of superconducting metal cooled to about 0.015 degrees above absolute zero, which is colder than outer space, but here on Earth.
At this temperature, current flows through the loop without any resistance, and the current is thought to exist in a quantum state.
In a superconducting loop, current can flow clockwise (called 0) or counterclockwise (called 1). But at the quantum scale, current doesn’t have to choose one direction and can actually flow in two directions at once.
Don’t be fooled into thinking that switching between the two is very fast. The current is in two states at the same time and can be measured experimentally and verified.
mind-bending physics
Are we still together so far? That’s great, because that’s what’s really weird about it, because the physics behind how it works isn’t immediately intuitive, nor should it be.
Everything people come into contact with in daily life follows classical physics, which assumes that things are in one place at the same time. But that’s not how particles behave at the subatomic scale.
Electrons have no definite location until you observe them. Before measurement, photons do not have a clear polarization. Electric current in a superconducting loop does not flow in a certain direction unless you force it to choose.
The reason we don’t experience this in our daily lives is decoherence. When a quantum system interacts with its environment, air molecules, heat, vibrations and light, the superposition collapses almost immediately.
A football can’t be in two places at once because it interacts with trillions of air molecules, dust, sound, heat, gravity, etc. every nanosecond. But isolating a tiny current in a vacuum close to absolute zero shields it from all possible interference, and the quantum behavior can be preserved long enough to allow for calculations.
This is why quantum computers are so difficult to build. People are designing physical environments where the laws of physics that normally prevent this from happening are constrained long enough to run computations.
Google’s machines operate in a dilution refrigerator that’s the size of a large room, colder than anything in the natural universe, and surrounded by multiple layers of shielding to prevent electromagnetic noise, vibration, and thermal radiation.
And qubits are fragile even so. They keep losing quantum states, which is why “error correction” dominates every discussion of amplification.
Therefore, quantum computing is not a faster version of classical computing. It’s taking advantage of a different set of physical laws that only apply to extremely small scales, extremely low temperatures, and extremely short time scales.
Now fold it up.
Two regular bits can be in one of four states (00, 01, 10, 11), but only one state at a time (because current only flows in one direction). Two qubits can represent all four states at the same time because the current flows in all directions at the same time.
Three qubits represent eight states. 10 qubits represent 1,024. Fifty qubits represent over a quadrillion. This number doubles with every qubit added, which is why the scaling is so exponential.
The second technique is called entanglement. When two qubits are entangled, measuring one qubit instantly tells an observer about the other, no matter how far apart they are. This allows quantum computers to coordinate all of these simultaneous states in a way that conventional parallel computing cannot.
These quantum computers are set up so that wrong answers cancel each other out (like overlapping waves flattening out), while right answers reinforce each other (like waves stacked 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 computing – letting nature explore an exponentially larger space of possibilities and arriving at the right answer through physics rather than logic.
A huge threat to cryptography
This mind-bending physics is what makes crypto so scary.
The math behind securing Bitcoin is based on the assumption that checking every possible key would take longer than the age of the universe.
But quantum computers don’t check every key. It explores all of them simultaneously and uses interference to find the correct one.
This is how it relates to Bitcoin. One direction from private key to public key takes several milliseconds. In contrast, going from a public key back to a private key would take a classical computer a million years, longer than the age of the universe. This asymmetry is the only evidence that a person holds a coin.
A quantum computer running a method called Shor’s algorithm can travel through the trapdoor in reverse. Google’s paper this week shows that it can do this with far fewer resources than anyone had previously estimated, and in a timeframe that rivals Bitcoin’s own block confirmations.
This is why the threat of quantum computers breaking blockchain encryption is really worrying everyone.
How this attack was carried out step by step, what exactly Google’s paper changed, and what this means for the 6.9 million Bitcoins that have been exposed, are the topics of the next article in this series.