The S Gate

The S gate is a quarter-turn of phase - a Clifford gate that is cheap in the error-corrected circuits Kronos plans for fusion chemistry.

What it does

S = [[1,0],[0,i]] is the square root of Z; two make a Z. It is a Clifford gate: efficiently simulable classically on its own, but essential inside error-correction circuits.

Where it appears at Kronos

In Kronos's fault-tolerant cost models, Clifford gates like S are the inexpensive operations; the budget is dominated by non-Clifford T gates. Knowing which gates are cheap shapes how a materials circuit is designed.

Honest note

Those cost models describe hardware still maturing toward the ~mid-2030s.