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Physical Review Research

Experimentally enhancing high-dimensional subchannel discrimination via optimal local filtering

Yen-An Shih, Wan-Guan Chang, Gelo Noel M. Tabia, Hao‐Cheng Weng, Tsung-Ying Tsai, Chih‐Sung Chuu, Huan-Yu Ku, Costantino Budroni

Published Aug 13, 2025• 0 citations
Publisher

Abstract

The subchannel discrimination task involves distinguishing between different branches of a quantum evolution. The guessing probability of this task can be enhanced using quantum resources such as entanglement and steerability. In this study, we explore how to optimize the task's guessing probability within the one-sided device-independent quantum information processing framework, which is naturally associated with steering, and restricted to one-way local operations and classical communication. From a theoretical standpoint, we demonstrate how to separately optimize local filters and measurements, and provide an optimality condition that directly links the enhancement of guessing probability to the maximum steerability achievable through local filters. We then experimentally demonstrate the enhancement of the discrimination task on a two-ququart photonic system by implementing the optimal local filters.

DOI
10.1103/physrevresearch.7.l032035

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The subchannel discrimination task involves distinguishing between different branches of a quantum evolution. The guessing probability of this task can be enhanced using quantum resources such as entanglement and steerability. In this study, we explore how to optimize the task's guessing probability within the one-sided device-independent quantum information processing framework, which is naturally associated with steering, and restricted to one-way local operations and classical communication. From a theoretical standpoint, we demonstrate how to separately optimize local filters and measurements, and provide an optimality condition that directly links the enhancement of guessing probability to the maximum steerability achievable through local filters. We then experimentally demonstrate the enhancement of the discrimination task on a two-ququart photonic system by implementing the optimal local filters.
Steering is the entanglement-based quantum effect that embodies the "spooky action at a distance" disliked by Einstein and scrutinized by Einstein, Podolsky, and Rosen. Here we provide a necessary and sufficient characterization of steering, based on a quantum information processing task: the discrimination of branches in a quantum evolution, which we dub subchannel discrimination. We prove that, for any bipartite steerable state, there are instances of the quantum subchannel discrimination problem for which this state allows a correct discrimination with strictly higher probability than in the absence of entanglement, even when measurements are restricted to local measurements aided by one-way communication. On the other hand, unsteerable states are useless in such conditions, even when entangled. We also prove that the above steering advantage can be exactly quantified in terms of the steering robustness, which is a natural measure of the steerability exhibited by the state.
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Quantum steering is a central resource for one-sided device-independent quantum information. It is manipulated via one-way local operations and classical communication, such as local filtering on the trusted party. Here, we provide a necessary and sufficient condition for a steering assemblage to be transformable into another via local filtering. We characterize the equivalence classes with respect to filters in terms of the steering equivalent observables (SEO), first proposed to connect the problem of steerability and measurement incompatibility. We provide an efficient method to compute the extractable steerability that is maximal via local filters and show that it coincides with the incompatibility of the SEO. Moreover, we show that there always exists a bipartite state that provides an assemblage with steerability equal to the incompatibility of the measurements on the untrusted party. Finally, we investigate the optimal success probability and rates for transformation protocols (distillation and dilution) in the single-shot scenario together with examples.
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