A simple explainer on what quantum computing actually is, and why it is terrifying for bitcoin

Most simplifies the complex process of quantum computing as "it can be 0 and 1 at the same time." That is not an explanation for why it threatens Bitcoin. This is.

What to know:

• Google has published research suggesting a future quantum computer could theoretically derive a bitcoin private key from its public key in about nine minutes, threatening the security of Bitcoin and other cryptographic systems.
• Unlike classical computers, which process bits as either 0 or 1, quantum computers use qubits that can exist in multiple states simultaneously and exploit phenomena like superposition and entanglement to explore many possibilities at once.
• This fundamentally different form of computation could undermine the mathematical assumptions behind current encryption, raising urgent concerns about the safety of existing blockchain assets and digital security more broadly.

This week, Google published a paper describing how a quantum computer could theoretically derive a bitcoin private key in 9 minutes, with ramifications that stretch to Ethereum, other tokens, private banking, and potentially everything in the world.

Quantum computing is easy to mistake for a faster version of a regular computer. But it is not a more powerful chip or a bigger server farm. It is a fundamentally different kind of 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 do not behave under normal conditions on Earth, ways that alter what we think of as the basic rules of physics.

Understanding what that means, physically, is the difference between reading about the quantum threat and actually grasping it.

How computers and quantum computers actually work

Regular computers store information as bits — each is either a 0 or a 1. A bit is a tiny switch. Physically, it's a transistor on a “chip” — a microscopic gate that either lets electricity through (1) or doesn't (0).

Every photo, every bitcoin transaction, every word you've ever typed is stored as patterns of these switches being on or off. There is nothing mysterious about a bit; it is a physical object in one of the two definite states.

Every calculation is just shuffling 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 as a qubit is a completely different kind of physical object. The most common version, and the one Google uses, 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 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 that 0) or counterclockwise (call that 1). But at quantum scales, the current does not have to pick one direction and actually flows in both directions simultaneously.

Don't mistake it for switching between the two really fast. The current is measurably, experimentally and verifiably in both states simultaneously.

Mind-bending physics

With us so far? Great, because here's where it gets genuinely strange, because the physics behind how it works isn't immediately intuitive, and it is not supposed to be.

Everything someone interacts with in daily life obeys classical physics, which assumes that things are in one place at one time. But particles do not behave this way at 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 definite direction until you force it to pick.

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 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 tiny current in a near-absolute-zero vacuum, shield it from every possible disturbance, and the quantum behavior survives long enough to compute with.

That's why quantum computers are so hard to build. People are engineering physical environments where the laws of physics that normally prevent this stuff from happening are held at bay for 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 the qubits are fragile even then. They lose their quantum state constantly, which is why "error correction" dominates every conversation about scaling up.

So quantum computing is not a faster version of classical computing. It is exploiting a different set of physical laws that only apply at extremely small scales, extremely low temperatures, and extremely short timeframes.

Now stack that up.

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, as the current is flowing 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 with every qubit that is 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 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) and right answers reinforce each other (like waves that stack higher). By the end of the computation, the correct answer has the highest probability of being measured.

So it's not brute-force speed. It's a fundamentally different approach to calculation — 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-bending physics is why it is terrifying for encryption.

The math protecting bitcoin relies on the assumption that checking every possible key would take longer than the age of the universe.

But a quantum computer doesn't check every key. It explores all of them simultaneously and uses interference to surface the right one.

That is where it ties into Bitcoin. Going one direction, from private key to public key, takes milliseconds. Going the other direction, 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 proving that a person is holding their coins.

A quantum computer running an algorithm called Shor's can go through that trapdoor in reverse. Google's paper this week showed it could do so with far fewer resources than anyone previously estimated, and within a timeframe that races against bitcoin's own block confirmations.

This is why the threat of quantum computers breaking blockchain encryption is genuinely making 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 bitcoin already exposed, is the subject of the next piece in this series.

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