Bad Qubits — Learn Quantum by Doing

Multiple choice

In the Raussendorf-Harrington-Goyal topological (3D cluster) approach to fault-tolerant MBQC, what role does the third dimension of the three-dimensional cluster state play?

It plays the role of time — successive rounds of stabilizer measurement — while the other two dimensions hold the 2D surface code
It is a redundant spatial dimension with no computational role
It encodes the classical decoder's output directly in the lattice
It stores a second, independent copy of the logical qubit for backup