In a stark warning issued on May 18, 2026, analysts at Citigroup highlighted that accelerating advances in quantum computing are compressing the timeline for risks to cryptocurrency and broader internet infrastructure. The report, authored by Will Canny and edited by Nikhilesh De, singles out Bitcoin as particularly exposed due to its slower governance and upgrade process, while noting that Ethereum and other proof-of-stake networks may be able to adapt faster.

Understanding the Quantum Threat

Quantum computing represents a fundamentally different approach to computation, leveraging the principles of quantum mechanics to solve certain problems exponentially faster than classical computers. For the cryptocurrency industry, the most pressing concern is Shor's algorithm, which can efficiently factor large integers and compute discrete logarithms—two mathematical problems that underpin many of today's encryption systems, including the elliptic curve digital signature algorithm (ECDSA) used by Bitcoin.

If a sufficiently powerful quantum computer were built, it could break the cryptographic keys that secure Bitcoin wallets and transaction signatures, potentially allowing an attacker to forge transactions or steal funds. While current quantum computers are far from achieving this capability, the pace of development has accelerated rapidly in recent years, driven in part by massive investments from governments and tech giants like Google, IBM, and Microsoft.

Citi's Specific Concerns

The Citi report emphasizes that quantum breakthroughs are arriving faster than many in the crypto industry had anticipated. The bank's analysts point to recent developments in error correction, qubit coherence times, and the demonstration of quantum advantage in specific tasks as signs that the timeline is compressing. In the report, Citi states: “Quantum breakthroughs are arriving faster than expected, raising risks for crypto security systems.”

Bitcoin is seen as particularly vulnerable because of its decentralized governance structure, which requires broad consensus among miners, developers, and node operators to implement protocol changes. Upgrades to Bitcoin's core infrastructure can take years to be proposed, tested, and adopted, as demonstrated by the lengthy process for SegWit and Taproot. In contrast, Ethereum's governance is more agile, with the ability to implement upgrades through Ethereum Improvement Proposals (EIPs) and a more centralized leadership structure provided by the Ethereum Foundation.

The report notes that Ethereum and other proof-of-stake networks may adapt faster to quantum threats, though they face their own quantum-related risks. The shift to proof-of-stake in Ethereum has already introduced new cryptographic assumptions that may be vulnerable to quantum attacks, but the network's ability to quickly upgrade its consensus layer provides a potential advantage.

AI Accelerates the Timeline

A separate but related article from the same news feed, authored by Margaux Nijkerk and edited by Stephen Alpher, reports that researchers and builders believe artificial intelligence may be accelerating the quantum timeline and forcing a broader rethink of how digital security works. The combination of AI and quantum computing is creating a new cybersecurity paradigm, where AI can be used to design better quantum error correction codes, optimize qubit control, and even simulate quantum circuits that were previously intractable.

Security experts warn that AI is not only advancing quantum hardware but also enabling new types of quantum attacks. For example, machine learning algorithms can be trained to find weak points in cryptographic implementations, potentially reducing the number of qubits required to break a given encryption scheme. This dual acceleration—faster quantum hardware combined with smarter software—means that the crypto industry may have less time to prepare than previously thought.

Background on Bitcoin's Resilience Efforts

The Bitcoin community has not been idle on the quantum threat. Several proposals have been made over the years to implement post-quantum cryptography, such as Lamport signatures, hash-based signatures, or lattice-based cryptography. However, integrating these into Bitcoin's core protocol is a monumental challenge. Any change to the signature scheme would require a hard fork, which is politically difficult and risks splitting the community.

Some developers have suggested using a hybrid approach, where transaction signatures are backed by both ECDSA and a post-quantum scheme, allowing a gradual transition. However, such proposals have not gained traction due to the increased block size and computational overhead. Meanwhile, researchers are actively working on quantum-resistant address formats, such as those based on the BIP-340 Schnorr signatures, which already offer some improvements over ECDSA but are not fully quantum-safe.

Ethereum's Quantum Strategy

Ethereum's path to quantum resistance is arguably more straightforward due to its more flexible architecture. The network already supports multiple signature schemes through its account abstraction model, and the upcoming EIP-4844 (proto-danksharding) and subsequent upgrades could include quantum-resistant primitives. The Ethereum Foundation has funded research into post-quantum cryptography and is actively monitoring developments in quantum computing.

However, Ethereum also faces unique quantum risks. The proof-of-stake consensus mechanism relies on a random beacon that could be vulnerable to quantum attacks if the underlying Verifiable Delay Function (VDF) or randomness beacon is broken. Additionally, the smart contract ecosystem is complex, and many existing contracts may be vulnerable to quantum attacks once a practical quantum computer exists.

Broader Internet Infrastructure at Risk

The Citi report also highlights that the quantum threat extends far beyond cryptocurrencies. The entire internet infrastructure relies on public-key cryptography for secure communications (TLS/SSL), digital signatures, and public-key infrastructure (PKI). If quantum computers become capable of breaking RSA or ECC, online banking, email, and virtually all e-commerce would be compromised.

The National Institute of Standards and Technology (NIST) has been leading an effort to standardize post-quantum cryptographic algorithms, with several candidates selected in 2022 and final standards expected by 2024. However, deploying these standards across the internet will take years, and the transition is expected to be one of the largest cryptography upgrades in history. Cryptocurrencies, being newer and more nimble in some respects, may have an opportunity to lead the transition if they act quickly.

Recent Quantum Milestones

In recent years, several milestones have demonstrated the accelerating pace of quantum computing. In 2023, IBM announced a 1,121-qubit processor, Osprey, followed by plans for a 4,000+ qubit system by 2025. Google's Sycamore processor achieved quantum supremacy in 2019, and its successor, Willow, demonstrated error correction improvements. In 2024, researchers at the University of Science and Technology of China created a photonic quantum computer that solved a sampling problem in seconds that would take a classical supercomputer billions of years.

More recently, in early 2026, a team from MIT and Harvard demonstrated a fault-tolerant logical qubit that operated reliably for extended periods, a critical step toward building a practical quantum computer. Citi's analysts note that these breakthroughs are compressing the timeline for quantum risk from decades to perhaps just a few years.

Market Implications

The quantum threat has implications for cryptocurrency markets as well. A credible threat to Bitcoin's security could undermine confidence in the asset, potentially leading to a sell-off. However, some analysts argue that the market already discounts the risk, and that any actual quantum attack would be quickly mitigated by a hard fork. The Citi report does not make a specific price prediction but emphasizes that investors should be aware of the risk.

Some projects are already working on quantum-resistant blockchains. For example, the QRL (Quantum Resistant Ledger) uses hash-based signatures to provide long-term security. Other projects, such as IOTA and Cardano, have quantum-resistant features in their research pipelines. However, these projects have yet to gain significant adoption compared to Bitcoin and Ethereum.

Regulatory and Industry Response

Regulators are also beginning to take notice. The U.S. National Cyber Security Center (NCSC) and the European Union Agency for Cybersecurity (ENISA) have both issued guidance on preparing for the quantum transition. The SEC has not specifically addressed quantum threats to crypto, but the broader regulatory push for cybersecurity includes provisions for emerging threats.

Industry groups such as the Crypto Council for Innovation and the Blockchain Association have called for collaboration between crypto developers and quantum researchers to ensure a smooth transition. Some have proposed a “quantum readiness” certification for blockchain projects, similar to the NIST cryptographic standards.

The combination of faster-than-expected quantum breakthroughs, the accelerating role of AI, and the inherent vulnerabilities in current crypto infrastructure means that the clock is ticking for the industry to act. As Citi's report concludes, the timeline is compressing, and Bitcoin's slow governance could be its Achilles' heel.

Source: Coindesk News