Internals of the simulator

Sparse state vector

QX-simulator internally represents a quantum state using a hash table mapping kets (e.g. $\left| 0010110 \right>$) to their associated complex amplitudes, and omits zero (or near-zero) amplitudes.

You can read about this approach in this paper by Samuel Jaques and Thomas Häner. Note however that QX-simulator was developed independently and the internal implementation differs.

This way to represent a quantum state is, in a lot of cases, very beneficial in terms of simulation runtime and memory usage.