Reading out a qubit means sending a weak probe into its resonator and sending the reflected, state-dependent signal out to a sensitive amplifier — both directions use the same wire. A plain 2-port isolator that just blocks one direction can't do this; it would block the useful signal too.
A circulator is nonreciprocal across three ports arranged in a fixed rotation: signal in at port 1 exits only at port 2, in at port 2 exits only at port 3, in at port 3 exits only at port 1 — never backward around the loop.
- Qubit → port 1, amplifier → port 2, cold termination → port 3.
- Probe signal: qubit (1) → amplifier (2). Forward, always allowed, in both toggle states here.
- Amplifier noise trying to go backward enters at port 2. With the circulator in, port-2 input can only exit at port 3 — the termination — where it is absorbed, never reaching port 1 (the qubit).
- With the circulator bypassed, the chain becomes an ordinary reciprocal wire: amplifier noise entering from the amplifier side travels straight back to the qubit, degrading its coherence before the measurement finishes.