arXiv subjects

Andrea Pompermaier

Publications and source records attributed to Andrea Pompermaier.

3 recordsLinked to original sources

arXiv2026

Postselection-loophole-free Bell test under strict spacetime constraints

Entanglement gives rise to correlations between distant quantum systems that cannot be explained by local realistic theories. Bell inequality violations provide a direct way to reveal these correlations and certify nonlocality, especially when the relevant experimental loopholes are closed. Time-bin encoding, in which quantum information is encoded into well-defined temporal modes, is a commonly used platform for distributing photonic entanglement in optical fibers. Yet loophole-free Bell tests with time-bin entanglement have received comparatively little attention, owing in part to the postselection loophole introduced by conventional interferometric measurements. Here, we demonstrate a fiber-based platform for Bell tests with time-bin entanglement that simultaneously closes the locality, freedom-of-choice, and postselection loopholes. We observe a CHSH violation of $S=2.583 \pm 0.002$, exceeding the local-realistic bound by over 265 standard deviations. Notably, this rigorous certification of nonlocality is achieved at a separation distance of $49.0 \pm 0.7$ m, substantially shorter than previous photonic Bell tests addressing comparable space-time constraints. Beyond its foundational significance, our results demonstrate time-bin entanglement as a viable route towards practical device-independent quantum communication and a future quantum internet.

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arXiv2026

GHz-rate all-fiber active polarization state analyzer for quantum protocols

Active selection of the measurement basis underpins quantum protocols including device-independent quantum key distribution, quantum teleportation with active feed-forward, and Bell tests. High-speed operation is crucial to minimize the latency between consecutive measurement choices, enabling faster protocol execution and higher achievable communication rates. Here, we demonstrate a GHz-rate all-fiber polarization state analyzer enabling active, trial-by-trial reconfiguration of the measurement basis, which we validate by performing a CHSH Bell test. The state analyzer is based on a fiber Sagnac interferometer incorporating a lithium niobate electro-optic phase modulator, built entirely using off-the-shelf fiber-optic components. Operating at a nominal repetition rate of 1 GHz, the system performs dynamic polarization measurements for the CHSH Bell test, achieving polarization visibilities up to 99% and a violation of $S = 2.6975 \pm 0.0005$, certifying entanglement at an unprecedented rate. Furthermore, the system demonstrates excellent long-term stability, preserving the Bell-inequality violation for more than 6 hours without realignment. These results establish the proposed state analyzer as a versatile, scalable platform for quantum communication protocols requiring fast, reconfigurable polarization-state measurements.

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arXiv2026

Quantum bounds and device-independent security with rank-one qubit measurements

Device-independent (DI) quantum protocols exploit Bell inequality violations to ensure security or certify quantum properties without making assumptions about the internal workings of the devices. In this work, we study the role of rank-one qubit positive operator-valued measures (POVMs) in DI scenarios. This class includes all qubit extremal POVMs, i.e., those measurements that cannot be realized by randomly choosing among others, as well as part of non-extremal POVMs, which have recently been shown to be useful for security applications in sequential quantum protocols. We demonstrate that any rank-one POVM can generate correlations in bipartite scenarios that saturate a Tsirelson inequality, i.e., a quantum bound on linear combinations of outcome statistics, when the two parties share an arbitrary entangled two-qubit state and some other self-tested measurements are performed. For extremal POVMs, such saturation allows for an explicit calculation of the guessing probability and the worst-case conditional von Neumann entropy. From the Tsirelson inequality, we establish a randomness certification method that facilitates numerical simulations and noise analysis. To test its feasibility, we performed a proof-of-concept experiment employing a three-outcome POVM on tilted entangled states under experimental non-idealities. We further explore the case of non-extremal POVMs, providing insights into their role in DI protocols.

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