  Practical quantum computing requires robust encoding of logical qubits in
physical systems to protect fragile quantum information. Currently, the lack of
scalability limits the logical encoding in most physical systems, and thus the
high scalability of propagating light can be a game changer for realizing a
practical quantum computer. However, propagating light also has a drawback: the
difficulty of logical encoding due to weak nonlinearity. Here, we propose
Gaussian breeding that encodes arbitrary Gottesman-Kitaev-Preskill (GKP) qubits
in propagating light. The key idea is the efficient and iterable generation of
quantum superpositions by photon detectors, which is the most widely used
nonlinear element in quantum propagating light. This formulation makes it
possible to systematically create the desired qubits with minimal resources.
Our simulations show that GKP qubits above a fault-tolerant threshold,
including ``magic states'', can be generated with a high success probability
and with a high fidelity exceeding 0.99. This result fills an important missing
piece toward practical quantum computing.
