Quantum Risk Institute
Logical Qubits vs Physical Qubits
Physical qubits are the hardware pieces. Logical qubits are the protected, error-corrected units needed for long, useful quantum computations.
Physical qubits
A physical qubit is an actual quantum system used by hardware: a superconducting circuit, trapped ion, photon, neutral atom, spin system, or another platform. Physical qubits are noisy. They lose information, drift, and make errors.
Logical qubits
A logical qubit is built by encoding information across many physical qubits with an error-correcting code. The goal is not merely to store a qubit, but to run reliable operations long enough for useful algorithms.
Why this matters for cryptography
Cryptographic attacks are not short demos. Breaking meaningful public keys requires deep circuits and many operations. Without error correction, small errors accumulate until the result is useless.
| Question | Physical qubits | Logical qubits |
|---|---|---|
| What are they? | Raw hardware qubits. | Error-corrected units built from physical qubits. |
| Why reported? | Easier headline number. | Better security-relevant capability signal. |
| Main weakness | Noise and short coherence. | Expensive overhead and engineering complexity. |
| QRI weight | Secondary signal. | Primary signal. |
The overhead problem
One logical qubit can require many physical qubits. The exact ratio depends on the hardware, error rates, code family, target reliability, and required operations. This overhead is one reason public qubit-count headlines can dramatically overstate cryptographic risk.
What progress would be meaningful
- Stable logical memories lasting longer than physical qubits.
- Logical operations with error rates below physical operations.
- Logical computation, not only storage.
- Scalable error correction that works across many logical qubits.
- Architectures that can connect modules without destroying fidelity.