# What would a quantum hacker need to crack bitcoin?

Scientists have pinned down a quantum revolution timescale by turning to a task that is impossible for classical computers to complete.

Computational programming has progressed steadily over the previous decade, reaching into the quantum realm, resulting in mind-bending machines that promise unimaginable levels of power.

Chinese scientists, for example, used a quantum computer in 2020 to solve a math problem that would have taken a traditional supercomputer 2.5 billion years to answer. It took the quantum machine 200 seconds to solve it.

However, the excitement extends beyond beyond superhero calculations. Quantum computing has the ability to change our relationship with nature.

It has the potential to speed up drug development by rapidly sifting through molecular structures, a task IBM is working on with Cleveland Clinic. It has attracted the attention of the US Department of Energy because it has the potential to improve internet security to near-unhackability. Even manufacturing giants, such as BMW, have entered the quantum game because it has the potential to perfect materials science and rewrite the artificial intelligence framework.

We may be on the approach of a quantum revolution, in which scientists will be able to manufacture medication with breakneck speed, predict weather with extraordinary accuracy, and discover new physics theories.

### However, there's a snag.

Quantum computers in prototype form are presently only capable of working on a tiny scale. The power of a quantum computer is driven by qubits, the basic units of the quantum version of computer language. The largest quantum processor currently on the market, created by IBM, has 127 qubits. For quantum breakthroughs, these figures are much insufficient.

What would it be, though? Mark Webber, a quantum architect at English firm Universal Quantum, and his team computed the number of qubits required to break the tough security mechanism used by bitcoin, the decentralized digital currency, in an attempt to determine how far along the quantum timeline we are today.

A quick response? Several times more than IBM's meager $1 billion.

Is there a quick answer? Several millions more than IBM's 127-qubit processor, which was the first to illuminate the way.

### The quantum flaw in Bitcoin

Bitcoin's security system is thought to be ultra-secure against conventional computers, making it an excellent tool for estimating quantum computing power. It's a complicated topic, but here's all you need to know for now.

### When a transaction is completed, two critical things occur.

A public key is generated that is visible to everyone, and a secure private key is generated that is only visible to the spender. This key combination is then digitally "recorded" onto the system's ledger of monetary transactions, known as a blockchain.

The transaction then "locks," preventing anyone from doing anything with the funds associated with it. "When someone conducts a transaction with bitcoin, it's disclosed to the world," Webber explained, "but it's not entirely secure until it's merged into the blockchain."

In other words, there is a vulnerability window between the public declaration of a transaction and the integration. Technically, the funds can be altered inside that window. I say theoretically because that would necessitate algorithms so sophisticated that even the most powerful supercomputers wouldn't be able to perform them — and forget about humans manually attempting to do so. Quantum computers may be able to do so in the future.

"If you had a quantum computer that could run swiftly enough, you could hypothetically apply it to transactions on a regular basis to re-divert [them] to a different address," Webber explained.

Webber argues the window's finiteness makes it a particularly good test for "We've got a target runtime, how many qubits do we need?" even though the window's approximate ballpark runs from 10 minutes to a day.

But, first, let's talk about where all of this qubit power comes from. Superposition and entanglement, two stunning quantum properties you won't believe aren't science fiction, are to thank.

### Qubit-land is a short trip away.

"Is it heads or tails?" I ask after spinning a coin on a table. "What?" you'd probably ask, because my query isn't very clear. Before settling on a side, the coin is essentially both alternatives at the same time. Consider this perplexing coin to be "superposed."

You can't restore the exact state of limbo if you interrupt its superposition to investigate its fate — that is, stop the coin spinning. Superposition is irreversibly broken once it is broken.

Let's change the scenario so that two coins spin adjacent to each other. I've added a condition this time: If coin A lands on heads, so will coin B. In a sense, these coins have become intertwined. Each

Let's change the scenario so that two coins are spinning adjacent to each other. I've added a condition this time: if coin A lands on heads, so will coin B. These coins are now, in a sense, interconnected. The superposition of each coin is "entangled" with the superposition of the other.

Changes to coin A's superposition have an immediate impact on coin B's. Even if only coin A stops spinning, you acquire information about coin B, breaking its superposition as well. Even if the coins are on opposite sides of the universe, this holds true.

Okay, you're probably thinking: These analogies are somewhat dependent on the observer's thoughts. You are correct. That is, however, due to the fact that we are discussing currencies. These things happen physically with quantum particles like electrons and photons.

Superposition determines the state of a bit in the quantum computing universe. Classical bits are either 0 or 1, while qubits, which are made up of quantum particles, can be in superposition — that is, they can be both 0 and 1. The most crucial thing is that they retrieve data while in that state.

Qubits, as you might expect, race through calculations at unfathomable speeds, testing multiple iterations at once and entangling with other qubits to relay data instantly. That's the gist of it.

For perspective, Google and IBM quantum computers use superconducting quantum technology to equally distribute qubits on a grid. Qubits that are close together can entangle and exchange information. Webber's startup focuses on trapped ion circuitry, which allows qubits to freely travel about a grid and interact. More qubits, in any case, equals exponentially more processing power.

But, in order to take advantage of bitcoin's vulnerability window, how many of these qubits must be in sync?

### The challenge has been accepted: hack bitcoin.

So far, here's what we know: Bitcoin transactions are vulnerable to quantum computers for a limited period of time, but not to conventional computers or people. This is because quantum systems are densely packed with qubits, which fire at speeds that the human brain can hardly fathom.

Webber used external research to figure out how many qubits are required to get through that window, and he came up with some accurate estimates. But keep in mind the delicate nature of qubits. If something goes wrong in a quantum computer, superposition is disrupted, and all of the valuable quantum data is lost for good. And then things start to go wrong.

Quantum programmers do something fairly simple to avoid this calamity. They simply employ more qubits. Quantum error correction is the term for it.

To increase the probability of right data, they throw an army of qubits at every computation for the sake of simplification. It'd be reasonable to say that if 9/10 qubits supplied the same solution, it's correct.

"It's something like 1,000 physical qubits for one relatively high-quality, logical qubit — it's not perfect, but it's good," Webber said. To achieve a final answer, he increased his initial estimate by 1,000.

To hack bitcoin in one hour, you'd need around 317 million qubits. "It would just be a greater number" if you're looking at a 10-minute span, he said. "I'm guessing six times more."

"It requires less qubits overall if you want to break it more slowly," Webber explained, "so something like 13 million to break it in one day."

"Look at the movement of classical computing from vacuum tubes of 10 bits, or however many they had early on, to the extremes that we have now," Webber encourages, despite the fact that we're still a long way from a 13-million-qubit processor.

"Quantum computing will undoubtedly undergo a similar transformation."