What is the required isolation between qubit control and readout signal paths?

The required isolation between qubit control and readout signal paths in a quantum computing system must be sufficient to prevent: control signals from leaking into the readout chain (which would appear as spurious measurement signals and cause readout errors), readout signals from leaking into the control path (which would drive unwanted qubit transitions), and cross-talk between different qubit control lines (which would cause correlated errors between qubits). Typical isolation requirements are: control-to-readout isolation: > 60-80 dB (the qubit drive signal at approximately -120 dBm at the chip is approximately 60 dB above the single-photon readout signal at approximately -180 dBm; any leakage that is comparable to the readout signal level will corrupt the measurement), readout-to-control isolation: > 40-60 dB (the readout drive signal at approximately -130 dBm must not drive the qubit; the qubit responds to fields as weak as a few photons, so the leakage must be below the single-photon level), and control-to-control isolation between adjacent qubit lines: > 40-60 dB (to prevent a drive pulse on qubit A from causing a rotation on qubit B; the cross-talk-induced rotation angle must be below the error correction threshold, typically < 0.1%). Isolation is achieved through: physical separation of signal paths on the qubit chip and in the cryostat, careful microwave packaging (superconducting enclosures, wire bonding techniques, and flip-chip bonding that minimize electromagnetic coupling), frequency separation (qubit and readout frequencies are typically 1-2 GHz apart), and filtering (Purcell filters, bandpass filters, and directional couplers that separate the signal paths).