Abstract
Unsharp measurements are increasingly important for foundational insights in quantum theory and quantum information applications. Here, we report an experimental implementation of unsharp qubit measurements in a sequential communication protocol, based on a quantum random access code. The protocol involves three parties; the first party prepares a qubit system, the second party performs operations that return both a classical and quantum outcome, and the latter is measured by the third party. We demonstrate a nearly optimal sequential quantum random access code that outperforms both the best possible classical protocol and any quantum protocol that utilizes only projective measurements. Furthermore, while only assuming that the involved devices operate on qubits and that detected events constitute a fair sample, we demonstrate the noise-robust characterization of unsharp measurements based on the sequential quantum random access code. We apply this characterization towards quantifying the degree of incompatibility of two sequential pairs of quantum measurements.
Highlights
Introduction.—Textbook measurements in quantum theory are represented by complete sets of orthogonal projectors
We demonstrate a nearly optimal sequential quantum random access code that outperforms both the best possible classical protocol and any quantum protocol that utilizes only projective measurements
While only assuming that the involved devices operate on qubits and that detected events constitute a fair sample, we demonstrate the noise-robust characterization of unsharp measurements based on the sequential quantum random access code
Summary
Encountered communication task known as a quantum random access code (QRAC) [40,41,42]. Unsharp measurements become indispensable: in order for both QRACs to achieve a high success rate, Bob must interact with the incoming system in such a way that sufficient information is extracted to power his guess of xy, while simultaneously the disturbance is limited to allow Charlie to accurately guess xz It was shown [38,39] that sequential QRACs can serve as certification tools for characterizing the unsharpness of Bob’s operations while only assuming that the states are qubits. In this Letter, we report experimental implementation of sequential QRACs using measurements of tunable unsharpness and demonstrate nearly optimal quantum correlations that outperform both all classical protocols as well as all quantum protocols based only on projective qubit measurements.
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