Many stable logical qubits built from many more physical qubits.
Quantum Risk Institute
What Is a Cryptographically Relevant Quantum Computer?
Short definitionCRQC
A cryptographically relevant quantum computer is a quantum computer powerful and reliable enough to break deployed public-key cryptography in practice, not just in a small lab demonstration.
Why the word relevant matters
Quantum computers already exist, but that does not mean they can break modern cryptography. A cryptographically relevant quantum computer must combine enough logical qubits, low enough error rates, enough runtime, and enough control quality to execute very large quantum circuits.
What a CRQC would need
Reliable computation over long circuit depths.
Operations must stay accurate through many repeated steps.
Control systems must coordinate correction and computation fast enough.
What it would affect first
The first major concern is public-key cryptography: RSA, Diffie-Hellman, ECDSA, EdDSA, and related systems. These are used for signatures, key exchange, certificates, wallets, software packages, device identity, and financial infrastructure.
What it would not mean
A CRQC would not mean all cryptography fails in the same way at the same time. Symmetric cryptography and hash functions are affected differently. Bitcoin's most specific concern is signatures and exposed public keys, not the sudden collapse of SHA-256.
Technology still needed
- Lower physical qubit error rates.
- More efficient quantum error-correcting codes.
- Scalable manufacturing of stable qubits.
- Modular interconnects between chips or cryostats.
- Fast decoding and classical feedback loops.
- Reliable logical gates and magic-state preparation.
- Thermal, power, packaging, and control-system engineering at much larger scale.
How QRI uses the term
Quantum Risk Institute uses CRQC as a practical threshold concept. The Bitcoin Quantum Index remains low because public systems have not crossed the evidence threshold for a Bitcoin-relevant CRQC.