The Broken Tradeoff

Wave-particle duality enforces a complementarity relation: path distinguishability and interference visibility cannot both be maximal simultaneously. The more precisely a photon's path is known, the less coherence its interference pattern shows. This tradeoff is quantified by a linear inequality, D² + V² ≤ 1, that has held across every experimental configuration tested since Bohr.

When causal order is itself placed in quantum superposition — using the quantum switch, where the order of two operations is coherently controlled — the standard complementarity relation breaks down. No universal linear relation exists that simultaneously captures path distinguishability, spatial coherence, and causal coherence.

The specific violation: spatial duality (the standard wave-particle tradeoff) and causal coherence (the quantum superposition of temporal orderings) can both be simultaneously maximal. In the standard framework, any form of coherence should trade off against any form of distinguishability. With indefinite causal order, the trading space has more dimensions than the standard inequality accounts for, and the additional dimension — causal coherence — is not constrained by the spatial tradeoff.

The structural observation: complementarity is a consequence of definite causal structure, not a fundamental law. When causal structure becomes quantum, the tradeoff that complementarity enforces acquires additional degrees of freedom that the original inequality does not constrain. The bound D² + V² ≤ 1 remains valid for each definite causal order individually, but the superposition of causal orders allows the system to simultaneously saturate tradeoffs that are mutually exclusive within any single causal order.