Can the strongest spear, quantum computers, penetrate the strongest shield, cryptocurrency?

Millions of qubits are needed to crack cryptocurrencies.

Over the past decade, computational programming has steadily advanced into the quantum realm, producing mind-bending devices that promise incredible computational efficiency.

For example, in 2020, Chinese scientists used a quantum computer to run a math problem that would have taken a typical supercomputer 2.5 billion years to solve, but the quantum machine solved the problem in 200 seconds.

Quantum computing is changing the way we interact with nature. It could super-fast-track drug discovery by rapidly screening molecular structures, a feat IBM is exploring in partnership with the Cleveland Clinic. It could push internet security to a point of near-unhackability, catching the attention of the U.S. Department of Energy. Even manufacturing companies, such as auto giant BMW, are getting into the quantum game because it could perfect materials science and rewrite the framework of artificial intelligence.

We may be on the brink of a quantum revolution, where scientists could develop drugs at record speed, predict the weather with incredible certainty, and discover new angles in physics.

At this point, the author thought of an interesting question: Can the strongest spear, quantum computers, penetrate the strongest shield, cryptocurrency?

To gauge how far along the quantum timeline we are, Mark Webber, a quantum architect at the British startup Universal Quantum, and his team calculated the number of qubits needed to theoretically crack the powerful security system used by Bitcoin, the decentralized digital currency that has long been a volatile investment, attracted the attention of Elon Musk and become a symbol of an imminent financial revolution. The answer? Millions more qubits than IBM's 127-qubit processor would allow.

Quantum Weakness of Cryptocurrencies

The security system of cryptocurrencies based on blockchain technology, such as the famous Bitcoin, is considered to be ultra-secure against classical computers, which is why it provides an excellent way to measure the power of quantum computing.

Every time a cryptocurrency transaction is made, a public key available to everyone and a secure private key visible only to the user are generated. This key combination is then digitally "written" to the currency transaction ledger within the system, known as the blockchain.

After that, the transaction is “locked” again, preventing anyone from doing anything with the associated funds. But there’s a blind spot: “When someone makes a transaction with Bitcoin, it’s announced to the world, but until it’s integrated into the blockchain, it’s not completely secure,” Weber said.

In other words, there is a window of vulnerability between the public announcement and integration of a transaction. Technically, within this window, funds can be manipulated. Technically, because this requires very complex algorithms, and even the most powerful supercomputers do not have enough computing power to execute them, unless quantum computers are used.

“If you did have a quantum computer and it could run fast enough, you could theoretically apply it to transactions regularly, for example, to re-move them to different addresses,” Weber said.

While the general range of the window ranges from 10 minutes to a day, Weber said its finite nature makes it a particularly good test because it asks “how many qubits do we need if we have an ideal runtime?”

Before we go any further, let’s discuss the origins of all this qubit power, thanks to two dazzling quantum features, superposition and entanglement.

The amazing quantum computer

Suppose I spin a coin on a table and ask, "Is it heads or tails?" You might say, "What?" because my question doesn't make much sense. Before the coin lands on its side, it essentially exists as two options at the same time, and this dazzling coin is in what is called a "superposition" during its spin.

If you interrupt its superposition to check its state, that is, stop the coin from spinning, you cannot recover the exact indeterminate state; once you break the superposition, it is broken forever.

Now let's modify the case and spin the two coins together. This time, I have a condition. If coin A lands on heads, coin B will also land on heads, and these coins can now be said to be dependent on each other. The superposition of each coin is "entangled" with the other coin.

Adjusting the superposition of coin A will immediately affect coin B, even if the coins are at opposite ends of the universe. For example, if even only coin A stops spinning, you gain relevant information about coin B, breaking its superposition as well, which also sounds correct.

Well, you might be thinking: these analogies depend on the mind of the observer. And you'd be right. But that's because we're talking about coins. For quantum particles like electrons and photons, these things really, physically happen.

Back to the world of quantum computing, superposition determines the state of quantum bits, or qubits. Classical bits exist as either 0 or 1, but qubits, which are made up of quantum particles, can be in a superposition state of 0 and 1 at the same time, and most importantly, they retrieve data while still in that state.

You can imagine qubits performing calculations at unfathomable speeds, testing several iterations simultaneously, and becoming entangled with other qubits to transfer information instantaneously. That’s the general gist.

For context, Google and IBM quantum computers use so-called superconducting quantum hardware to evenly distribute qubits on a grid. Adjacent qubits can be entangled to transfer information. Webber's company focuses on trapped ion hardware, which allows qubits to move freely and collaborate anywhere on the grid. Either way, however, more qubits equals exponentially more computing power.

But how many of these qubits must be in sync to exploit Bitcoin’s vulnerability window?

Making quantum computers hackers

Bitcoin transactions have a window during which they are vulnerable to attack by quantum computers, but not classical computers, and definitely not people. That’s because quantum systems are filled with qubits, which fire and perform calculations at speeds that are barely comprehensible to the human brain.

Using outside research, Weber tabulated how many qubits were needed to penetrate this window and found some reliable calculations. But recall that if anything goes wrong with the quantum computer, the superposition breaks down and all that precious quantum data could be lost forever.

To prevent this catastrophe, quantum programmers do something fairly intuitive. They just use more qubits. This is called quantum error correction.

To simplify, they throw a bunch of qubits at each calculation to increase the chances of getting the right data: if 9 out of 10 qubits provide the same solution, for example, then it's safe to say it's the right one.

"It's not easy to have a reasonably high-quality logical qubit, it's almost like picking the best one out of 1,000 physical qubits," Weber said. So he multiplied his initial estimate by 1,000 to get the final answer. If you want to do it in 10 minutes, the number of qubits required will be a much larger number, he said. "Maybe six times as many." This would bring the number of qubits to billions, and human quantum computers currently have not yet reached a fraction of it.

"If you want to crack it more slowly, it takes fewer qubits overall, about 13 million qubits if you want to crack it in a day," Weber added.

Weber isn’t the only one considering how quantum computing could circumvent cryptocurrency security. For example, the National Institute of Standards and Technology is looking for quantum-proof cryptographic algorithms to secure cryptocurrency, while the Ethereum Foundation is studying the concept of quantum resistance.

But we still have a long way to go before we can really hack into cryptocurrencies through quantum computers. But just as classical computers have gone through a path before: it only took half a century to go from 10-bit vacuum tubes to more than 50 billion calculations per second, its computing power will grow exponentially every year.

Original text: "Quantum hackers could break bitcoin in minutes, but don't panic just yet"

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