Certifying asymmetry in the configuration of three qubits

Symmetry restrictions limit the types of tasks that can be achieved with a given set of quantum states. Therefore, any breaking of these symmetries could potentially be exploited as a resource for quantum communication. Here we demonstrate this operationally by certifying asymmetry in the configuration of the Bloch vectors of a set of three unknown qubit states within the dimensionally bounded prepare-and-measure scenario. To do this, we construct a linear witness from three simpler witnesses as building blocks, each featuring, along with two binary measurement settings, three preparations; two of them are associated with the certification task, while the third one serves as an auxiliary. The final witness is chosen to self-test some target configuration. We numerically derive a bound $Q_{\text{mirror}}$ for any mirror-symmetric configuration, thereby certifying asymmetry if this bound is exceeded (e.g. experimentally) for the unknown qubit configuration. We also consider the gap $(Q_{\text{max}}-Q_{\text{mirror}})$ between the analytically derived overall quantum maximum $Q_{\text{max}}$ and the mirror-symmetric bound, and use it as a quantifier of asymmetry in the target configuration. Numerical optimization shows that the most asymmetric configuration then forms a right scalene triangle on the unit Bloch sphere. Finally, we implement our protocol on a public quantum processor, where a clear violation of the mirror-symmetric bound certifies asymmetry in the configuration of our experimental triple of qubit states.

💡 Research Summary

The paper investigates how breaking geometric symmetry among quantum states can be turned into a useful resource within a semi‑device‑independent (dimension‑bounded) prepare‑and‑measure (PM) scenario. Specifically, the authors focus on three unknown qubit states whose Bloch vectors form a triangle on the Bloch sphere. If the triangle is mirror‑symmetric (i.e., isosceles or equilateral), the configuration possesses a reflection symmetry; otherwise it is asymmetric (scalene). The goal is to certify asymmetry without any detailed knowledge of the preparation or measurement devices.

The authors start from the well‑known I₃ linear witness introduced by Gallego et al. (Phys. Rev. Lett. 2010) and generalize it by adding a bias parameter ω∈