Complementarity Revisited: How Preparation Shapes Incompatibility

Complementarity Revisited: How Preparation Shapes Incompatibility

Seminar author:Kunika Agarwal

Event date and time:12/18/2025 02:30:pm

Event location:Online

Event contact:

Bohr’s principle of complementarity forbids the simultaneous determination of certain quantum properties, manifesting operationally as measurement incompatibility. While ensemble-level incompatibility has recently gained attention, it remains unclear whether it is an intrinsic property of the observables themselves. In this talk, I will show that ensemble incompatibility is in fact configuration-dependent: it hinges crucially on how the underlying quantum systems are prepared. By analyzing joint measurements on ensembles of parallel and antiparallel qubit pairs, we demonstrate that antiparallel preparation uniquely enables exact joint prediction of three mutually orthogonal spin components—a capability fundamentally inaccessible with parallel states. Extending this framework to larger families of spin observables, we uncover a striking No-Comparison Theorem, showing that the relative complementarity of two sets of observables can be reversed solely by altering the state configuration. I will also discuss connections to the mean-King retrodiction task and Bub’s cryptographic protocol, highlighting how the enhanced compatibility of antiparallel ensembles enables efficient device estimation. Finally, I will outline prospects for experimental realization of this enhanced compatibility, examining the robustness of the effect on finite sub-ensembles of states.