Crosstalk & Isolation

Checks for always-on ZZ, neighbour isolation, leakage, package modes, and unintended electromagnetic coupling.

#	VER ID	Parameter	Severity	Design Rule / Constraint	Ideal / Optimal Value	Acceptable Range	Good	Bad	Why It Matters
35	HFSS-C-001	ZZ Coupling ζ (idle)	Critical	ζ_ZZ/2π < 10 kHz between non-coupled pairs (idle)	< 1 kHz	1 – 100 kHz	< 1 kHz: conditional phase < 0.01 rad in 10 µs; negligible idle ZZ error	> 1 MHz: > 1 rad conditional phase per µs; circuit depth severely limited	Always-on ZZ interaction from dispersive coupling. Even static ZZ causes phase errors that accumulate, limiting circuit depth.
36	HFSS-C-002	Nearest Neighbour Isolation	High	S_ij (nearest neighbour) ≤ −40 dB	< −40 dB	−30 to −50 dB	< −40 dB: driven rotation on neighbour qubit < 10⁻⁴ rad during single-qubit gate	> −20 dB: significant driven rotation on neighbours; simultaneous single-qubit gates not independent	EM isolation between nearest-neighbour qubits. Insufficient isolation causes unwanted rotations during single-qubit gates.
37	HFSS-C-003	Next-Nearest Isolation	High	S_ij (next-nearest) ≤ −60 dB	< −60 dB	−50 to −70 dB	< −60 dB: long-range crosstalk < 10⁻⁶ rad; 2D grid operation fully independent	> −40 dB: long-range coupling; frequency collisions compound with nearest-neighbour	Isolation beyond nearest neighbour. Critical for scalable multi-qubit processors; long-range EM leakage compounds errors.
38	HFSS-C-004	Leakage to |2⟩ (L₁)	Critical	L₁ < 0.01% per gate	< 0.01 %	0.01 – 0.1 %	< 0.01%: seepage outside qubit subspace negligible; DRAG pulse sufficient	> 1%: leakage accumulates over circuit; error correction cannot track non-qubit states	Population leaked to |2⟩ non-computational state during gates. DRAG pulses mitigate but require sufficient anharmonicity.
39	HFSS-C-005	Spurious Mode Gap Δf_spur	High	Δf_spur ≥ 1 GHz from nearest spurious EM mode	> 1 GHz gap	0.5 – 2 GHz gap	> 1 GHz gap: no spurious mode within pulse bandwidth; gate calibration stable	< 0.2 GHz: mode within pulse bandwidth; state leakage and parametric drive of spurious modes	Frequency distance to nearest unintended EM mode. Modes within drive bandwidth cause state leakage and gate calibration drift.
40	HFSS-C-006	Package Mode Density	Medium	< 1 spurious package mode per GHz near qubit band	< 0.5 modes/GHz	< 5 modes/GHz	< 0.5/GHz: low probability of accidental hybridisation across 64-qubit chip	> 10/GHz: dense mode spectrum; unavoidable hybridisation; package must be redesigned	Density of package/enclosure modes near qubit frequency. High density increases probability of accidental mode hybridisation.