Experimental Certification of Sustained Entanglement and Nonlocality after Sequential Measurements

Foletto, Giulio; Calderaro, Luca; Tavakoli, Armin; Schiavon, Matteo; Picciariello, Francesco; Cabello, Adán; Villoresi, Paolo; Vallone, Giuseppe

doi:10.1103/physrevapplied.13.044008

Cited by 49 publications

(39 citation statements)

References 39 publications

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“…Such a setting would open up the possibility of the Bobs using an adaptive strategy. In [29] some steps were made in this direction but the authors' strategy required that the inputs and outputs of the Bobs were also sent to Alice before she made her measurement. Whilst our work here implies that it is also possible in the classically-assisted setting to achieve unbounded violations, it would be interesting to know if the classical communication could be used to produce larger violations and more noise-tolerant strategies.…”

Section: Discussionmentioning

confidence: 99%

Arbitrarily Many Independent Observers can Share the Nonlocality of a Single Maximally Entangled Qubit Pair

2020

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Alice and Bob each have half of a pair of entangled qubits. Bob measures his half and then passes his qubit to a second Bob who measures again and so on. The goal is to maximize the number of Bobs that can have an expected violation of the Clauser-Horne-Shimony-Holt (CHSH) Bell inequality with the single Alice. This scenario was introduced in [Phys. Rev. Lett. 114, 250401 (2015)] where the authors mentioned evidence that when the Bobs act independently and with unbiased inputs then at most two of them can expect to violate the CHSH inequality with Alice. Here we show that, contrary to this evidence, arbitrarily many independent Bobs can have an expected CHSH violation with the single Alice. Our proof is constructive and our measurement strategies can be generalized to work with a larger class of two-qubit states that includes all pure entangled two-qubit states. Since violation of a Bell inequality is necessary for device-independent tasks, our work represents a step towards an eventual understanding of the limitations on how much device-independent randomness can be robustly generated from a single pair of qubits.

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Section: Discussionmentioning

confidence: 99%

Arbitrarily Many Independent Observers can Share the Nonlocality of a Single Maximally Entangled Qubit Pair

2020

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“…Recently, it has been exploited for sequential protocols, in which a system undergoes multiple measurements without ever completely collapsing or losing its useful quantum features, which can be harvested repeatedly. In this manner, Bell inequalities can be violated more times [36][37][38][39][40], quantum random access codes can be used by two parties [41,42], and quantum instruments can be tested [43]. More important for this work is using weak measurements to produce random bits from the same physical system repeatedly [44,45].…”

Section: Introductionmentioning

confidence: 99%

Experimental test of sequential weak measurements for certified quantum randomness extraction

et al. 2021

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Quantum nonlocality offers a secure way to produce random numbers: Their unpredictability is intrinsic and can be certified just by observing the statistic of the measurement outcomes, without assumptions on how they are produced. To do this, entangled pairs are generated and measured to violate a Bell inequality with the outcome statistics. However, after a projective quantum measurement, entanglement is entirely destroyed and cannot be used again. This fact poses an upper bound to the amount of randomness that can be produced from each quantum state when projective measurements are employed. Instead, by using weak measurements, some entanglement can be maintained and reutilized, and a sequence of weak measurements can extract an unbounded amount of randomness from a single state as predicted in Phys. Rev. A 95, 020102(R) (2017). We study the feasibility of these weak measurements, analyze the robustness to imperfections in the quantum state they are applied to, and then test them using an optical setup based on polarization-entangled photon pairs. We show that the weak measurements are realizable, but can improve the performance of randomness generation only in close-to-ideal conditions.

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“…For other theoretical works in the direction of recyclability of entangled and nonlocal correlations, see [27][28][29][30][31][32][33][34][35][36][37][38][39][40]. For experimental works, see [41][42][43][44][45].…”

Section: Introductionmentioning

confidence: 99%

Recycled entanglement detection by arbitrarily many sequential and independent pairs of observers

Pandit,

Srivastava,

Sen

2022

Preprint

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We investigate the witnessing of two-qubit entangled states by sequential and independent pairs of observers, with both observers of each pair acting independently on their part of the shared state from spatially separated laboratories, and subsequently passing their qubits to the next pair in the sequence. It has previously been conjectured that not more than one pair of observers can detect Clauser-Horne-Shimony-Holt "Bell-nonlocal" correlations in a similar set-up. This is intriguing since it is possible to have an arbitrarily long sequence of Bell-nonlocal correlations when only a single observer is allowed to share a bipartite state with multiple observers at the other end. It is therefore interesting to ask whether such restrictions are also present when entangled correlations are considered in the scenario of multiple pairs of observers. We find that a two-qubit entangled state can be used to witness entanglement arbitrarily many times, by pairs of observers, acting sequentially and independently. We prove the statement to be true when the initial pair of observers in the sequence share any pure entangled state or when they share a state from a class of mixed entangled states. We demonstrate that the phenomenon can also be observed for a certain class of entangled states in which an arbitrarily long sequence of observer pairs witnessing entanglement is reached in the limit of the initial entanglement content tending to a vanishing amount.

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Experimental Certification of Sustained Entanglement and Nonlocality after Sequential Measurements

Cited by 49 publications

References 39 publications

Arbitrarily Many Independent Observers can Share the Nonlocality of a Single Maximally Entangled Qubit Pair

Arbitrarily Many Independent Observers can Share the Nonlocality of a Single Maximally Entangled Qubit Pair

Experimental test of sequential weak measurements for certified quantum randomness extraction

Recycled entanglement detection by arbitrarily many sequential and independent pairs of observers

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