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As an example I'm going to start with Google and IBM. Google has the Sycamore processor right now with 53 physical qubits. However I haven't found any info on how many logical qubits it can actually implement (I think they have been experimenting with different amount of logical qubits?). IBM has the Osprey processor with 433 qubits but I'm not sure if they are talking about physical or logical qubits.

My questions are, in general when the news talk about number of qubits are they logical or physical? If it's physical can you generally find out the number of logical qubits they are being able to implement? (I usually find it really hard to find this kind of information). What is the biggest amount of useful logical qubits in a quantum computer globally right now?

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  • $\begingroup$ Unfortunately, we are still in the "NISQ era" of quantum computing. Similarly, unfortunately, everybody means physical qubits when they quote qubit numbers, since nobody has any good logical qubits... $\endgroup$
    – hft
    Nov 13, 2023 at 19:38

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When a quantum computing company or university group speak of qubits, generally they speak of physical qubits. The number of logical qubits depends on the quantum error correction code that you employ, and the code distance. For example: a distance-9 rotated surface code that uses 161 physical qubits encodes 1 logical qubit, while a distance-3 toric code that uses 36 physical qubits encodes 2 logical qubits.

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