Quantum computers cannot currently break Bitcoin’s cryptography, but recent advances from Google and IBM have accelerated progress toward systems that might eventually do so. This has intensified concerns over “Q-Day”—the point when quantum computers could exploit Bitcoin’s vulnerability, especially older addresses with exposed public keys, threatening over $700 billion in funds. Transitioning Bitcoin to quantum-resistant signatures is complex and may take years, yet the timeline for a quantum breakthrough is uncertain. Many at-risk coins are in abandoned or lost wallets, and if their private keys remain unrecoverable, these funds will remain unprotected. Effective defense includes migrating vulnerable funds, adopting post-quantum addresses, and managing the data storage challenges of larger post-quantum signatures, which can be 10-100 times larger than current ones. Multiple proposals for quantum-resilient upgrades exist, but strong decentralization makes broad adoption difficult and slow. Most current Bitcoin users face minimal immediate risk but can reduce long-term exposure by not reusing addresses and using modern wallets. Experts disagree on when quantum computers may become a genuine threat, with estimates ranging from five years to well into the 2030s.

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