Entanglement swapping between photons that have never coexisted

Delayed-choice entanglement swapping experiments: No evidence for timelike entanglement

In recent years, there has been a growing interest in the possibility of temporal nonlocality, mirroring the spatial nonlocality supposedly evidenced by the Bell correlations. In this context, Glick (2019) has argued that timelike entanglement and temporal nonlocality is demonstrated in delayed-choice entanglement swapping (DCES) experiments, like that of Ma et al. (2012), Megidish et al. (2013) and Hensen et al. (2015). I will argue that a careful analysis of these experiments shows that they in fact display nothing more than "ordinary" spacelike entanglement, and that any purported timelike entanglement is an artefact of selection bias. Regardless any other reason one may have for challenging the assumption of temporal locality, timelike entanglement as evidenced by these experiments is not among them. I conclude by discussing what lessons on the nature of entanglement might be drawn from an examination of DCES experiments.

Insecurity of Quantum Blockchains Based on Entanglement in Time

In this study, the security implications of utilizing the concept of entanglement in time in the quantum representation of a blockchain data structure are investigated. The analysis reveals that the fundamental idea underlying this representation relies on an uncertain interpretation of experimental results. A different perspective is provided by adopting the Copenhagen interpretation, which explains the observed correlations in the experiment without invoking the concept of entanglement in time. According to this interpretation, the qubits responsible for these correlations are not entangled, posing a challenge to the security foundation of the data structure. The study incorporates theoretical analysis, numerical simulations, and experiments using real quantum hardware. By employing a dedicated circuit for detecting genuine entanglement, the existence of entanglement in the process of generating a quantum blockchain is conclusively excluded.

Robust Quantum Memory in a Trapped-Ion Quantum Network Node

We integrate a long-lived memory qubit into a mixed-species trapped-ion quantum network node. Ion-photon entanglement first generated with a network qubit in ^{88}Sr^{+} is transferred to ^{43}Ca^{+} with 0.977(7) fidelity, and mapped to a robust memory qubit. We then entangle the network qubit with a second photon, without affecting the memory qubit. We perform quantum state tomography to show that the fidelity of ion-photon entanglement decays ∼70 times slower on the memory qubit. Dynamical decoupling further extends the storage duration; we measure an ion-photon entanglement fidelity of 0.81(4) after 10 s.

Quantum aspects of the brain-mind relationship: A hypothesis with supporting evidence

If all aspects of the mind-brain relationship were adequately explained by classical physics, then there would be no need to propose alternatives. But faced with possibly unresolvable puzzles like qualia and free will, other approaches are required. In alignment with a suggestion by Heisenberg in 1958, we propose a model whereby the world consists of two elements: Ontologically real Possibles that do not obey Aristotle's law of the excluded middle, and ontologically real Actuals that do. Based on this view, which bears resemblance to von Neumann's 1955 proposal (von Neumann, 1955), and more recently by Stapp and others (Stapp, 2007; Rosenblum and Kuttner, 2006), measurement that is registered by an observer's mind converts Possibles into Actuals. This quantum-oriented approach raises the intriguing prospect that some aspects of mind may be quantum, and that mind may play an active role in the physical world. A body of empirical evidence supports these possibilities, strengthening our proposal that the mind-brain relationship may be partially quantum.

Deterministic Entanglement Swapping with Hybrid Discrete- and Continuous-Variable Systems

The study of entanglement between discrete and continuous variables is an important theoretical and experimental topic in quantum information processing, for which entanglement swapping is one of the interesting elements. Entanglement swapping allows two particles without interacting with each other in any way, to form an entangled state by the action of another pair of entangled particles. In this paper, we propose an experimentally feasible scheme to realize deterministic entanglement swapping in the hybrid system with discrete and continuous variables. The process is achieved by preparing two pairs of entangled states, each is formed by a qubit and two quasi-orthogonal coherent state elements of a cavity, performing a Bell-state analysis through nonlocal operations on the continuous variable states of the two cavities, and projecting the two qubits into a maximally entangled state. The present scheme may be applied to other physical systems sustaining such hybrid discrete and continuous forms, providing a typical paradigm for entanglement manipulation through deterministic swapping operations.

The nonlocal universe

We propose that the universe is nonlocal and that the appropriate worldview or paradigm for this understanding is nonlocal realism. Currently the worldview of local realism guides and frames the understanding and interpretations of science. Local realism was the worldview employed by Einstein in his relativity theories, but the principles of this paradigm have operated as the guiding framework for the rest of classic science for more than a century. This paper points to incoherencies in local realism and to the violation of its principles by recent experiments; it suggests that these negative effects have undermined the credibility and legitimacy of this worldview. We offer a more inclusive worldview for the future of science called nonlocal realism. Unlike local realism, the worldview of nonlocal realism encompasses meaning, mind and universal consciousness.

A preregistered multi-lab replication of Maier et al. (2014, Exp. 4) testing retroactive avoidance

The term “retroactive avoidance” refers to a special class of effects of future stimulus presentations on past behavioral responses. Specifically, it refers to the anticipatory avoidance of aversive stimuli that were unpredictable through random selection after the response. This phenomenon is supposed to challenge the common view of the arrow of time and the direction of causality. Preliminary evidence of “retroactive avoidance” has been published in mainstream psychological journals and started a heated debate about the robustness and the true existence of this effect. A series of seven experiments published in 2014 in the Journal of Consciousness Studies (Maier et al., 2014) tested the influence of randomly drawn future negative picture presentations on avoidance responses based on key presses preceding them. The final study in that series used a sophisticated quantum-based random stimulus selection procedure and implemented the most severe test of retroactive avoidance within this series. Evidence for the effect, though significant, was meager and anecdotal, Bayes factor (BF₁₀) = 2. The research presented here represents an attempt to exactly replicate the original effect with a high-power (N = 2004) preregistered multi-lab study. The results indicate that the data favored the null effect (i.e., absence of retroactive avoidance) with a BF₀₁ = 4.38. Given the empirical strengths of the study, namely its preregistration, multi-lab approach, high power, and Bayesian analysis used, this failed replication questions the validity and robustness of the original findings. Not reaching a decisive level of Bayesian evidence and not including skeptical researchers may be considered limitations of this study. Exploratory analyses of the change in evidence for the effect across time, performed on a post-hoc basis, revealed several potentially interesting anomalies in the data that might guide future research in this area.

Entanglement swapping with autonomous polarization-entangled photon pairs from a warm atomic ensemble

Entanglement swapping forms a key concept in the realization of scalable quantum networks and large-scale quantum communication. For the practical implementation of entanglement swapping, completely autonomous entanglement sources and a joint Bell-state measurement (BSM) between two independent photons are essential. Here, we experimentally demonstrate entanglement swapping between two independent polarization-entangled photon-pair sources obtained via spontaneous four-wave mixing (SFWM) in a Doppler-broadened atomic ensemble of ${^{87}}{\rm Rb}$⁸⁷Rb atoms. From the joint BSM, we estimate the ${\rm S}$S parameter in the Clauser-Horne-Shimony-Holt (CHSH) form of Bell's inequality and confirm the violation of the Bell-CHSH inequality by ${\rm S}={2.32} \pm {0.07}$S=2.32±0.07 with 4.5 standard deviations. We believe that this work is an important step toward realizing practical scalable quantum networks.

Entanglement Swapping with Semiconductor-Generated Photons Violates Bell's Inequality

Transferring entangled states between photon pairs is essential in quantum communication. Semiconductor quantum dots are the leading candidate for generating polarization-entangled photons deterministically. Here we show for the first time swapping of entangled states between two pairs of photons emitted by a single dot. A joint Bell measurement heralds the successful generation of the Bell state Ψ^{+}, yielding a fidelity of 0.81±0.04 and violating the CHSH and Bell inequalities. Our photon source matches atomic quantum memory frequencies, facilitating implementation of hybrid quantum repeaters.

Informationally symmetrical Bell state preparation and measurement

Bell state measurement (BSM) plays crucial roles in photonic quantum information processing. The standard linear optical BSM is based on Hong-Ou-Mandel interference where two photons meet and interfere at a beamsplitter (BS). However, a generalized two-photon interference is not based on photon-photon interaction, but interference between two-photon probability amplitudes. Therefore, it might be possible to implement BSM without interfering photons at a BS. Here, we investigate a linear optical BSM scheme which does not require two photon overlapping at a BS. By unleashing the two photon coexistence condition, it can be symmetrically divided into two parties. The symmetrically dividable property suggests an informationally symmetrical BSM between remote parties without a third party. We also present that our BSM scheme can be used for Bell state preparation between remote parties without a third party. Since our BSM scheme can be easily extended to multiple photons, it can be useful for various quantum communication applications.

Counterfactual entanglement swapping enables high-efficiency entanglement distribution

We propose an alternative entanglement swapping scheme based on the principle of the counterfactual quantum communication, which demonstrates nonlocal entanglement swapping can be achieved by the operations of a third party. During the whole process, it is not needed to transmit any physical particles among the participants. Furthermore, all the entangled particles are not destroyed in the counterfactual entanglement swapping process, which means we can obtain two pairs of nonlocal entanglement at the same time, thus achieve high-efficiency entanglement distribution. The numerical analysis about the performance of the presented scheme shows that this counterfactual protocol can be implemented with high success probability and fidelity in the ideal asymptotic limit. The scheme may be meaningful for large-scale quantum communication network and quantum repeater.

Two-photon interference of temporally separated photons

We present experimental demonstrations of two-photon interference involving temporally separated photons within two types of interferometers: a Mach-Zehnder interferometer and a polarization-based Michelson interferometer. The two-photon states are probabilistically prepared in a symmetrically superposed state within the two interferometer arms by introducing a large time delay between two input photons; this state is composed of two temporally separated photons, which are in two different or the same spatial modes. We then observe two-photon interference fringes involving both the Hong-Ou-Mandel interference effect and the interference of path-entangled two-photon states simultaneously in a single interferometric setup. The observed two-photon interference fringes provide simultaneous observation of the interferometric properties of the single-photon and two-photon wavepackets. The observations can also facilitate a more comprehensive understanding of the origins of the interference phenomena arising from spatially bunched/anti-bunched two-photon states comprised of two temporally separated photons within the interferometer arms.

Active stabilization of a fiber-optic two-photon interferometer using continuous optical length control

The practical realization of long-distance entanglement-based quantum communication systems strongly rely on the observation of highly stable quantum interference between correlated single photons. This task must accompany active stabilization of the optical path lengths within the single-photon coherence length. Here, we provide two-step interferometer stabilization methods employing continuous optical length control and experimentally demonstrate two-photon quantum interference using an actively stabilized 6-km-long fiber-optic Hong-Ou-Mandel interferometer. The two-step active control techniques are applied for measuring highly stable two-photon interference fringes by scanning the optical path-length difference. The obtained two-photon interference visibilities with and without accidental subtraction are found to be approximately 90.7% and 65.4%, respectively.

Fundamental awareness: A framework for integrating science, philosophy and metaphysics

The ontologic framework of Fundamental Awareness proposed here assumes that non-dual Awareness is foundational to the universe, not arising from the interactions or structures of higher level phenomena. The framework allows comparison and integration of views from the three investigative domains concerned with understanding the nature of consciousness: science, philosophy, and metaphysics. In this framework, Awareness is the underlying reality, not reducible to anything else. Awareness and existence are the same. As such, the universe is non-material, self-organizing throughout, a holarchy of complementary, process driven, recursive interactions. The universe is both its own first observer and subject. Considering the world to be non-material and comprised, a priori, of Awareness is to privilege information over materiality, action over agency and to understand that qualia are not a “hard problem,” but the foundational elements of all existence. These views fully reflect main stream Western philosophical traditions, insights from culturally diverse contemplative and mystical traditions, and are in keeping with current scientific thinking, expressible mathematically.

Quantifying Non-Markovianity with Temporal Steering

Einstein-Podolsky-Rosen (EPR) steering is a type of quantum correlation which allows one to remotely prepare, or steer, the state of a distant quantum system. While EPR steering can be thought of as a purely spatial correlation, there does exist a temporal analogue, in the form of single-system temporal steering. However, a precise quantification of such temporal steering has been lacking. Here, we show that it can be measured, via semidefinite programing, with a temporal steerable weight, in direct analogy to the recently proposed EPR steerable weight. We find a useful property of the temporal steerable weight in that it is a nonincreasing function under completely positive trace-preserving maps and can be used to define a sufficient and practical measure of strong non-Markovianity.

Generalized quantum interference of correlated photon pairs

Superposition and indistinguishablility between probability amplitudes have played an essential role in observing quantum interference effects of correlated photons. The Hong-Ou-Mandel interference and interferences of the path-entangled photon number state are of special interest in the field of quantum information technologies. However, a fully generalized two-photon quantum interferometric scheme accounting for the Hong-Ou-Mandel scheme and path-entangled photon number states has not yet been proposed. Here we report the experimental demonstrations of the generalized two-photon interferometry with both the interferometric properties of the Hong-Ou-Mandel effect and the fully unfolded version of the path-entangled photon number state using photon-pair sources, which are independently generated by spontaneous parametric down-conversion. Our experimental scheme explains two-photon interference fringes revealing single- and two-photon coherence properties in a single interferometer setup. Using the proposed interferometric measurement, it is possible to directly estimate the joint spectral intensity of a photon pair source.

Frieden wave-function representations via an Einstein-Podolsky-Rosen-Bohm experiment

The appearance of the spin-1/2 and spin-1 representations in the Frieden-Soffer extreme physical information (EPI) statistical approach to the Einstein-Podolsky-Rosen-Bohm (EPR-Bohm) experiment is shown. In order to obtain the EPR-Bohm result, in addition to the observed structural and variational information principles of the EPI method, the condition of the regularity of the probability distribution is used. The observed structural information principle is obtained from the analyticity of the logarithm of the likelihood function. It is suggested that, due to the self-consistent analysis of both information principles, quantum mechanics is covered by the statistical information theory. The estimation of the angle between the analyzers in the EPR-Bohm experiment is discussed.