Physical vs Logical Qubits
A physical qubit is one real piece of hardware. A logical qubit is one error-protected qubit, stored with backup copies spread across many physical qubits. There is no fixed exchange rate between them. The cost depends on the code, its distance, the raw error rate, and how reliable you need it to be. So a physical count and a logical count answer different questions.
What's the difference between a physical and a logical qubit?
A physical qubit is one real device you can point at. It might be a tiny superconducting circuit, a trapped ion, an atom, or a path for a photon (a particle of light). When a spec sheet says "433 qubits", it almost always means physical qubits.
Physical qubits are noisy. Every step has an error rate: the chance it does the wrong thing. On today's machines, a step that uses two qubits goes wrong somewhere between 1 time in 1,000 and 1 time in 100. A normal computer chip makes mistakes so rarely that engineers usually ignore them.
A logical qubit is one qubit's worth of information spread across many physical qubits. They are set up so that errors can be found and fixed while the job is still running. The recipe for spreading the information is called an error-correcting code.
Think of a sports team. The roster shows how many players you have. The scoreboard shows how well they play together. The two counts answer different questions. Physical count asks, "how much hardware exists?" Logical count asks, "how much reliable work can it do?" Comparing one vendor's physical count with another's logical count mixes up two different kinds of thing.
- How do backup copies fight errors? A worked example
- Run it: encode, break, and still decodeINTERACTIVE
- What did that circuit do?
- Try thisINTERACTIVE
- How many physical qubits make one logical qubit, really?
- On real hardware
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