Quantum computers are special machines that can perform certain calculations significantly faster than everyday computers – known as “classical computers” – and represent the next frontier in computation technology.

In recent years, there have been growing fears these superior computers could eventually be directed at crunching crypto mining computations required to generate new blocks.

This piece is part of CoinDesk's Mining Week.

If that were to happen, the concern is that those wielding quantum computers could, in theory, gain a significant advantage over every other miner in the blockchain network, threatening the decentralization and security of proof-of-work blockchains like Bitcoin and Litecoin. Not to mention, earning a vast majority of the remaining block rewards.

To understand how quantum computers work, you first need to understand that classical computers – like the one you have at home or use for work – represent all bits of data as being one of two states, either a 0 or a 1. This is known as binary code.

By stringing together 0s and 1s, it becomes possible to run more complex computations and store more complex data. But even with stringing 0s and 1s together, classical computers are still limited in their processing capabilities and can run only one computation at a time.

Quantum computers, on the other hand, can run simultaneous computations thanks to the use of quantum bits, also known as “qubits.” Instead of representing data in two states – either a 0 or 1 – qubits can represent data in three states: 0, 1 or both. That's known as a "superposition."

Remember Schrödinger's cat? That’s one of the most popular examples of a quantum superposition, where a cat left in a box can neither be dead nor alive. It’s considered both.

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By providing a higher number of states, quantum computers have the ability to perform exponentially larger computations. But there are a number of significant caveats to this technology, which we shall explore further down.

A recently published academic paper in AVS Quantum Science entitled “The impact of hardware specifications on reaching quantum advantage in the fault tolerant regime” outlined two key threats posed by quantum computing to crypto mining, specifically bitcoin (BTC) mining, and the wider ecosystem.

The proof-of-work consensus mechanism refers to the special system certain blockchains employ to select honest participants to perform the important role of proposing new blocks of transaction data to be added to the blockchain. Because there is no single authority governing a blockchain, it must rely on an automated system coded into the protocol to filter out dishonest users who might attempt to corrupt the blockchain with invalid transactions.

Quantum computers have the capacity to perform higher calculations than other types of specialized machines, and so the obvious concern is they could dominate the mining-based competition. According to the paper’s authors, however, that threat is considered to be minimal because of the nature of the considerably slower clock cycle time of quantum computers versus application-specific integrated circuit (ASIC) miners

“The algorithmic speed-up is unlikely to make up for the considerably slower clock cycle times relative to state of the art classical computing for the foreseeable future,” according to the paper.

But how can quantum computers have slower clock cycle times but process more calculations than classical computers? According to Macauley Coggins, founder of Quantum Computing UK, it has to do with a quantum computer’s ability to process calculations simultaneously:

In fact, computer scientists in another academic paper entitled “Vulnerability of blockchain technologies to quantum attacks,” which was published in ScienceDirect, suggested it may take as long as to the year 2028 before quantum computers are sophisticated enough to outcompete current ASIC chip technology and perform a majority attack on a blockchain network. That’s not taking into consideration any future improvements to ASIC chip technology by that time.

“Our own calculations based on current ASIC technology, as well as that of other authors [2,3], put the earliest likely date that this type of attack will be possible at 2028. However, advances in ASIC technology are likely to push back this date much farther,” according to the study in ScienceDirect.

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Both papers concurred that the largest threat posed by quantum computers to crypto is not to mining but by breaking the "Elliptic Curve Digital Signature Algorithm," or ECDSA, which is used by bitcoin and a vast majority of other leading cryptocurrencies.

ECDSA is the cryptographic system used to generate mathematically linked public-private keys – the digital tools needed to send and receive cryptocurrency as well as prove who owns the assets held within a crypto wallet.

Breaking this form of encryption would mean a person could ascertain someone's private key from that person's public key, which is freely broadcasted to the entire network each time that wallet makes a transaction. Accessing a private key is like identifying a person’s password and would give the attacker complete control over any funds held in the wallet address.

“If the same public/private key pair is used to hold the users' bitcoin after the public key becomes public knowledge, then all funds secured by the key pair will be vulnerable. However, it must also be considered that bitcoin wallets tend to not repeatedly use the same key pairs,” according to the paper in AVS Quantum Science.

So how many qubits would it take to break the elliptic curve algorithm? According to the AVS Quantum Science paper, quite a lot:

“... It would require 317 × 106 physical qubits to break the encryption within one hour with a code cycle time of 1 μs. To break it within 10 min with the same code cycle time, it would require 1.9 × 109 physical qubits, whereas to break it within 1 day, it would require only 13 × 106 physical qubits.”

While quantum computers are already a thing, the technology is still very much in its infancy.

IBM's quantum processor, dubbed “Eagle,” is considered the world’s most powerful quantum computing system to date – containing 127 qubits. A long way off from the estimated 1.9 billion qubits required to break ECDSA within 10 minutes.

Adding more qubits is by no means as straightforward as it sounds, either. It all comes down to a hugely limiting factor known as “quantum noise.” The term refers to any type of subtle environmental change that can affect the performance of a qubit. In fact, the most minor of vibrations or fluctuations in temperate or electromagnetic waves can cause something known as “decoherence,” rendering qubits unable to perform a single calculation. The problem becomes increasingly more persistent the more qubits are involved.

It’s this sensitivity to external factors that significantly inhibits the progression of quantum computers and means they are unlikely to become a major threat to cryptocurrency mining or to the underlying cryptography that secures transactions until this issue is addressed.

Efforts are being directed toward creating hybridized quantum-classical computers as well as creating software to minimize the disturbance caused by quantum noise. But that doesn’t address another critical issue faced by quantum computers.

Unlike with classical computers, it’s incredibly difficult to remove errors when performing calculations on a quantum computer because of the linear nature of quantum computations. Checking qubits for errors can potentially disrupt their state or superposition, skewing results.

There have been, however, a number of advancements in quantum error correction, namely something called the Bacon-Shor code developed by physicist Christopher Monroe and a number of researchers from the University of Maryland. But again, this type of error correction is estimated to require a quantum computer boasting at least 1,300 qubits – more than 10 times the number of qubits present in IBM’s Eagle processor.

As it stands, while quantum computers may one day possess the ability to severely undermine crypto mining and the integrity of blockchain-based networks, the current technology is far from being sophisticated enough to cause any serious concern.

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