The numbers come from quantum error correction and the goal to build a logical qubit (error protected qubit made of many actual qubits). In the surface code (p

The numbers come from quantum error correction and the goal to build a logical qubit (error protected qubit made of many actual qubits). In the surface code (planar version of a toric code), a single logical qubit protected with a code distance d = 3 corresponds to 17 physical qubits, and a distance d = 5 corresponds to 49 physical qubits. [1] https://en.wikipedia.org/wiki/Toric_code https://en.wikipedia.org/wiki/Toric_code [2] https://www.nature.com/articles/s41534-016-0004-0 https://www.nature.com/articles/s41534-016-0004-0