Sneha Suresh | Searcharxiv

arXiv2026

Global versus Local Discrimination of Locally Implementable Multipartite Unitaries

We study single-shot distinguishability of locally implementable multipartite unitaries under Local Operations and Classical Communication (LOCC) and global operations. As unitary discrimination depends on both the choice of probing states and the measurements on the evolved states, we classify LOCC and global distinguishability into two categories: adaptive strategies, where probing states are chosen based on measurement outcomes from other subsystems, and restricted strategies, where probing states remain fixed. Our findings uncover three surprising features in the bipartite setting and establish new structural limits for unitary discrimination: (i) Certain pairs of unitaries are globally distinguishable with restricted strategies but indistinguishable under LOCC, even with adaptive strategies. (ii) There exist sets of four unitaries that are distinguishable via LOCC, yet remain globally indistinguishable with restricted strategies. (iii) Some sets of unitaries are globally indistinguishable under adaptive strategies, when probed with separable states, but become distinguishable via LOCC.

quant-ph↗

arXiv2025

Single-shot Distinguishability and Anti-distinguishability of Quantum Measurements

Among the surprising features of quantum measurements, the problem of distinguishing and antidistinguishing general quantum measurements is fundamentally appealing. Unlike classical systems, quantum theory offers entangled states and peculiar state update rule of the post-measurement state, which gives rise to four distinct scenarios: (i) probing single systems and without access to the Post-measurement States (PMS), (ii) probing entangled systems and without access to the PMS, (iii) probing single systems with access to the PMS, and (iv) probing entangled systems with access to the PMS. We study the probability of distinguishing (and antidistinguishing) quantum measurements sampled from a given set in the single-shot regime. For some scenarios, we provide the analytical expressions of distinguishability (and antidistinguishability) for qubit projective measurements. We show that the distinguishability of any pair of qubit projective measurements in scenario (iii) is always greater than its value in scenario (ii). Interestingly, certain pairs of non-projective qubit measurements achieve optimal distinguishability in scenario (ii) with a non-maximally entangled state. In general, for any set of measurements, distinguishability (and antidistinguishability) in scenario (i) never exceeds that in any other scenario, while it reaches its highest possible value in scenario (iv). We establish that there is no hierarchical relation between scenarios (ii) and (iii). In particular, we introduce different variants of the well-known `trine' qubit measurement to construct pairs (and triples) of qubit quantum measurements such that they are perfectly distinguishable (and antidistinguishable) in scenario (ii) but not in scenario (iii), and vice versa. Additionally, we present qubit measurements that are perfectly distinguishable (and antidistinguishable) in scenario (iv) but not in any other scenarios.

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