Entangled Particles Research Articles

Measurements in QFT: Weakly coupled local particle detectors and entanglement harvesting

We present a comparison of the AQFT-based Fewster-Verch framework with the Unruh-DeWitt particle detector models commonly employed in relativistic quantum information and QFT in curved space. We use this comparison to respond to a recent paper [arXiv:2103.13400] in which it was argued that the Reeh-Schlieder theorem prevents weakly coupled local particle detectors from harvesting vacuum entanglement from a quantum field. Their claim can be traced back to the mixedness that a local particle detector cannot escape from because of the entanglement that it must have with the quantum fields outside of the localization area. We argue that for any realistic scale of localization for physical particle detectors the effect of that mixedness is negligible, and that weakly coupled localized particle detectors are not impeded from harvesting vacuum entanglement.

Single particle entanglement in the mid- and ultra-relativistic regime

In this work we analyse the amount of entanglement associated with the spin and momentum degrees of freedom of a single massive spin- particle from a relativistic perspective. The effect of a Lorentz boost introduces a Wigner rotation that correlates the spin and momentum degrees of freedom. We show that the natural basis to discuss the geometrical effects of the boost are the helicity eigenstates in the rest frame. In the mid-relativistic regime (where the Wigner rotation angle is limited by ) we prove for states with equal helicity that the entanglement with respect to the Wigner rotation angle is monotonically decreasing, however, in the ultra-relativistic regime () the entanglement is increasing. If the states are prepared as a superposition of unequal helicity eigenstates, the monotonic behaviour is inverted. This implies that in the ultra-relativistic regime a geometrical setup can be found such that the amount of entanglement exhibits local maxima or minima. This shows a counter-intuitive behaviour of the relative amount of entanglement, an effect due to the internal and external geometrical configuration space, and points towards the difficulties in achieving a Lorentz invariant formulation of entanglement in general.

Excluded area of superellipse sector particles.

Superellipse sector particles (SeSPs) are segments of superelliptical curves that form a tunable set of hard-particle shapes for granular and colloidal systems. SeSPs allow for continuous parametrization of corner sharpness, aspect ratio, and particle curvature; rods, circles, rectangles, and staples are examples of shapes SeSPs can model. We investigate the space of allowable (nonoverlapping) configurations of two SeSPs, which depends on both the center-of-mass separation and relative orientation. Radial correlation plots of the allowed configurations reveal circular regions centered at each of the particle's two end points that indicate configurations of mutually entangled particle interactions. Simultaneous entanglement with both end points is geometrically impossible; the overlap of these two regions therefore represents an excluded area in which no particles can be placed regardless of orientation. The regions' distinct boundaries indicate a translational frustration with implications for the dynamics of particle rearrangements (e.g., under shear). Representing translational and rotational degrees of freedom as a hypervolume, we find a topological change that suggests geometric frustration arises from a phase transition in this space. The excluded area is a straightforward integration over excluded states; for arbitrary relative orientation this decreases sigmoidally with increasing opening aperture, with sharper SeSP corners resulting in a sharper decrease. Together, this work offers a path towards a unified theory for particle shape control of bulk material properties.

Entanglement and U(D)-spin squeezing in symmetric multi-quDit systems and applications to quantum phase transitions in Lipkin\u2013Meshkov\u2013Glick D-level atom models

Collective spin operators for symmetric multi-quDit (namely identical D-level atom) systems generate a U(D) symmetry. We explore generalizations to arbitrary D of SU(2)-spin coherent states and their adaptation to parity (multi-component Schrödinger cats), together with multi-mode extensions of NOON states. We write level, one- and two-quDit reduced density matrices of symmetric N-quDit states, expressed in the last two cases in terms of collective U(D)-spin operator expectation values. Then, we evaluate level and particle entanglement for symmetric multi-quDit states with linear and von Neumann entropies of the corresponding reduced density matrices. In particular, we analyze the numerical and variational ground state of Lipkin–Meshkov–Glick models of 3-level identical atoms. We also propose an extension of the concept of SU(2)-spin squeezing to SU(D) and relate it to pairwise D-level atom entanglement. Squeezing parameters and entanglement entropies are good markers that characterize the different quantum phases, and their corresponding critical points, that take place in these interacting D-level atom models.

Potential Consequences of Wormhole-Mediated Entanglement

There are hints that the connectivity of space-time in quantum gravity could emerge from entanglement, and it has further been proposed that any two entangled particles may be connected by a quantum wormhole. One way to test this proposal is by probing the electric field of an entangled charged particle to determine whether its electric field leaks through the putative wormhole. In addition, if such a wormhole is traversable, then it could be possible for the collapse of the wave function to occur in a causal manner, with information about the collapse travelling through the wormhole at the speed of light, rather than the wave function collapse being a global and instantaneous event.

Target space entanglement in quantum mechanics of fermions and matrices

We consider entanglement of first-quantized identical particles by adopting an algebraic approach. In particular, we investigate fermions whose wave functions are given by the Slater determinants, as for singlet sectors of one-matrix models. We show that the upper bounds of the general Rényi entropies are N log 2 for N particles or an N × N matrix. We compute the target space entanglement entropy and the mutual information in a free one-matrix model. We confirm the area law: the single-interval entropy for the ground state scales as frac{1}{3} log N in the large N model. We obtain an analytical mathcal{O}left({N}^0right) expression of the mutual information for two intervals in the large N expansion.

Locality and entanglement of indistinguishable particles

Entanglement is one of the strongest quantum correlation, and is a key ingredient in fundamental aspects of quantum mechanics and a resource for quantum technologies. While entanglement theory is well settled for distinguishable particles, there are five inequivalent approaches to entanglement of indistinguishable particles. We analyse the different definitions of indistinguishable particle entanglement in the light of the locality notion. This notion is specified by two steps: (i) the identification of subsystems by means of their local operators; (ii) the requirement that entanglement represent correlations between the above subsets of operators. We prove that three of the aforementioned five entanglement definitions are incompatible with any locality notion defined as above.

Diffraction in time of an entangled non-relativistic quantum particle

The diffraction process of an entangled non-relativistic quantum particle in time domain is investigated. The scenario consists of two entangled spin-0 particles and a quantum shutter, which is opened at an initial time. The particles are traveling in two opposite sides and expressed with a single wave function. The integral solution of the Schrödinger equation is considered for two particles case. The initial condition is expressed in terms of an entangled wave function. The time-diffracted matter wave is obtained in terms of the Fresnel integral. The effect of the phenomenon of diffraction in time is examined numerically for the entangled system of two particles.

Is the Past Determined?

In a recent paper, I argued against backward in time effects used by several authors to explain delayed choice experiments. I gave an explanation showing that there is no physical influence propagating from the present to the past and modifying the state of the system at a time previous to the measurement. However, though the solution is straightforward in the case of delayed choice experiments involving only one particle, it is subtler in the case of experiments involving two entangled particles because they give rise to EPR-like situations. Considering that a measurement is not an actual change of the physical state of a system and is relative to the observer allows to understand that there is neither backward in time effects nor instantaneous collapse of the second system when the first one is measured, as is often postulated.This allows also to get rid of any non-locality. In this paper, I want to go further into the consequences of this way of considering the measurement, that I have called Convivial Solipsism, and show that even if, in the usual sense, there is no physical effect of the present or of the future on the past, we must nevertheless consider that the observer's past is sometimes not entirely determined and that it becomes determined only when certain measurements are done latter. This apparent contradiction disappears if one understand that each observer builds, through her own measurements, her own world (that I call the phenomenal world in Convivial Solipsism) which is different from what we are used to consider as the common world shared by everybody.

Multiparty probabilistic controlled joint remote preparation of an arbitrary <italic>m</italic>-qudit equatorial state with a pure entangled quantum channel

<p indent="0mm">The quantum equatorial state, a special kind of quantum state, has attracted much attention because of its potential for applications in quantum information processing. The remote preparations of arbitrary two-qudit, three-qudit equatorial and <italic>m</italic>-qudit equatorial states have been studied via different entangle quantum channels. In this paper, we propose two protocols for the controlled joint remote preparation of an arbitrary <italic>m</italic>-qudit equatorial state with a pure entangled quantum channel. The first is for the controlled joint remote preparation of the <italic>m</italic>-qudit equatorial state by using <italic>m</italic> multiparticle GHZ-class states as the quantum channel, and the second is for the controlled joint remote preparation of the <italic>m</italic>-qudit equatorial state via a multiparticle entangled state. All senders share information about the prepared state and perform <italic>m</italic>-particle projective measurements on their entangled particles according to their information about the prepared state. All controllers perform generalized X-basis measurements on their entangled particles. If a receiver cooperates with all the senders and controllers, he can prepare the original state probabilistically by performing unitary operations on his entangled particles and an auxiliary qubit. This protocol is more convenient for applications because it only requires nonmaximal entanglement for the controlled joint remote preparation of an arbitrary <italic>m</italic>-qudit equatorial state.

Controlled Cyclic Remote Preparation of an Arbitrary Single-Qudit State by Using a Seven-Qudit Cluster State as the Quantum Channel

We present a protocol for controlled cyclic remote preparation of an arbitrary single-qudit state via a seven-qudit cluster state. In the protocol, Alice can help the remote agent Bob prepare an arbitrary single-qudit state, Bob can help the agent Charlie prepare an arbitrary single-qudit state and at the same time Charlie can help Alice prepare an arbitrary single-qudit state under the controller David’s control. Alice, Bob and Charlie first perform positive operator-valued measurement (POVM) on their entangled particles according to the information of the prepared state, then perform generalized X-basis measurement. The controller performs generalized X-basis measurement on his entangled particle. The arbitrary single-qudit states can be cyclic remote prepared under the controller’s control. The protocol is more convenient in application since it only requires single-particle measurement and single-particle unitary operations for controlled cyclic remote preparation of the single-qudit states.

Nonlocal single particle steering generated through single particle entanglement

In 1927, at the Solvay conference, Einstein posed a thought experiment with the primary intention of showing the incompleteness of quantum mechanics; to prove it, he employed the instantaneous nonlocal effects caused by the collapse of the wavefunction of a single particle—the spooky action at a distance–, when a measurement is done. This historical event preceded the well-know Einstein–Podolsk–Rosen criticism over the incompleteness of quantum mechanics. Here, by using the Stern–Gerlach experiment, we demonstrate how the instantaneous nonlocal feature of the collapse of the wavefunction together with the single-particle entanglement can be used to produce the nonlocal effect of steering, i.e. the single-particle steering. In the steering process Bob gets a quantum state depending on which observable Alice decides to measure. To accomplish this, we fully exploit the spreading (over large distances) of the entangled wavefunction of the single-particle. In particular, we demonstrate that the nonlocality of the single-particle entangled state allows the particle to “know” about the kind of detector Alice is using to steer Bob’s state. Therefore, notwithstanding strong counterarguments, we prove that the single-particle entanglement gives rise to truly nonlocal effects at two faraway places. This opens the possibility of using the single-particle entanglement for implementing truly nonlocal task.

Efficient entanglement distillation for quantum channels with polarization mode dispersion

Quantum entanglement shared by remote network nodes serves as a valuable resource for promising applications in distributed computing, cryptography, and sensing. However, distributing high-quality entanglement via fiber-optic routes could be challenging due to the various decoherence mechanisms in fibers. In particular, one of the primary polarization decoherence mechanisms in optical fibers is polarization mode dispersion (PMD), which is the distortion of optical pulses by randomly varying birefringences. To mitigate the effect of decoherence in entangled particles, quantum entanglement distillation (QED) algorithms have been proposed. One particular class, the recurrence QED algorithms, stands out because it has relatively relaxed requirements both on the size of the quantum circuits involved and on the initial quality of entanglement between particles. However, because the number of required particles grows exponentially with the number of distillation rounds, an efficient recurrence algorithm needs to converge quickly. We present a recurrence QED algorithm designed for photonic qubit pairs affected by PMD-degraded channels. Our proposed algorithm achieves the optimal fidelity as well as the optimal success probability (conditioned on the fact that optimal fidelity is achieved) in every round of distillation. The attainment of the maximal fidelity improves the convergence speed of fidelity with respect to the number of distillation rounds from linear to quadratic and, hence, significantly reduces the number of rounds. Combined with the fact that the optimal success probability is achieved, the proposed algorithm provides an efficient method to distribute entangled states with a high fidelity via optical fibers.

Entanglement and decoherence of massive particles due to gravity

We analyze the dynamics of a gravity-induced entanglement for N massive particles. Considering the linear configuration of these particles, we investigate the entanglement between a specific pair of particles under the influence of the gravitational interaction between the massive particles. As the particle number increases, the specific particle pair decoheres more easily due to the gravitational interaction with other particles. The time scale of the gravity-induced decoherence is analytically determined. We also discuss the entanglement dynamics of initially entangled particles, which exemplify the monogamy of the gravity-induced entanglement.

Aggregation of a Ferruginous Nodular Gleysol in a pasture area in Cuba, under the influence of Arbuscular mycorrhizal fungi associated with hybrid Urochloa

The objective of this study was to evaluate in the field the influence of different arbuscular mycorrhizal fungi (AMF) inoculated in a pasture area of hybrid Urochloa (cv. CIAT BR 02/1752 “Caymam Pasture”) regarding the stability of aggregates of a Ferruginous Nodular Gleysol in Cuba. The experiment was conducted at the Pasture and Forage Experimental Station in Cuba (22°39′58.77″ N and 80°31′06.35″ W), using Urochloa hybrid, inoculated with three strains of AMF EcoMic® (Glomus cubense, Funneliformis mosseae and Rhizoglomus irregularis). The chosen design was in plots subdivided over time, with four treatments: three combined inoculations + non-inoculated treatment and two evaluation times (at 140 and 876 days after sowing). The number of spores, glomalin-related soil protein production, the quantification of total extraradicular mycelia, the structural stability coefficient of the soil (dry and wet) and the Soil Structural Stability Index were all evaluated. Greater stability of the aggregates was verified in dry sieving at 140 and 876 days of cultivation, however, influence of inoculation was only observed at 876 days. On the other hand, less stability was found for aggregates immersed in water. The effect of inoculation on soil aggregates can be associated with glomalin increments at 140 days, which has a cementing and cumulative character in the soil, and the length of mycelia at 876 days, which acts in the entanglement of mineral and organic soil particles, favoring the aggregation. Inoculation with AMF associated with the roots of hybrid Urochloa contributed to the best aggregate stability index at 876 days of cultivation, with an emphasis on the species G. cubense, which also showed greater competitiveness with a greater number of spores over time. Inoculation and/or soil management aimed at increasing the infectious potential of AMF is of great relevance for the formation and stability of soil aggregates in these grazing areas.

Дискретность пространства и ее следствия

Based on the general principle of the unity of the nature of interacting entities and the principle of the relativity of motion, as well as following the requirement of an indissoluble and conditioning connection of space and time, the model of a discrete space-time consisting of identical interacting particles is proposed as the most acceptable one. We consider the consequences of the discreteness of space, such as: the occurrence of time quanta, the limiting speed of signal propa­gation, and the constancy of this speed, regardless of the motion of the reference frame. Regularly performed acts of particles of space-time (PST) interaction en­sure the connectivity of space, set the quantum of time and the maximum speed – the speed of light. In the process of PST communication, their mixing occurs, which ensures the relativity of inertial motion, and can also underlie quantum uncertainty. In this case, elementary particles are spatial configurations of an excited “lattice” of PST, and particles with mass must contain loop struc­tures in their configuration. A new interpretation of quantum mechanics is pro­posed, according to which the wave function determines the probability of de­struction of a spatial configuration (representing a quantum object) in its corresponding region, which leads to the contraction of the entire structure to a given, detectable component. Particle entanglement is explained by the appear­ance of additional links between the PST – the appearance of a local coordinate along which the distance between entangled objects does not increase. It is shown that the movement of a body should lead to an asymmetry of the tension of the bonds between the PST – to the asymmetry of its effective gravity, the es­tablishment of which is one of the possibilities for experimental verification of the proposed model. It is shown that the constancy of the speed of light in a vac­uum and the appearance of relativistic effects are based on ensuring the connec­tivity of space-time, i.e. striving to prevent its rupture.

Quantum Entanglement Results from Quantum State Transition at Fast-Than-Light Speed with Matter Wave’s Phase Velocity

The quantum entanglement, that violates the local realism and other classical physics theories, leads to various counterintuitive phenomena, is a primary feature of quantum mechanics and probably results from the quantum state’s conservation and the quantum state’s transition with the matter wave’s phase velocity at the fast-than-light speed. The quantum state transition of entangled particles proceeds with the phase velocity, while the observer measures the process with the electromagnetic or the light speed. This speed difference makes the causality law no longer fully valid everywhere except in certain areas.

Realism and causality. I. Pilot wave and retrocausal models as possible facilitators

Of all basic principles of classical physics, realism should arguably be the last to be given up when seeking a better interpretation of quantum mechanics. We examine the de Broglie-Bohm pilot wave theory as a well developed example of a realistic theory. We present three challenges to a naive reading of pilot-wave theory, each based on a system of several entangled particles. With the help of a coarse graining of pilot wave theory into a discrete system, we show how these challenges can be answered. However this comes with a cost. In the description of individual systems, particles appear to scatter off empty branches of the wave function as if they were particles, and conversely travel through particles as if they were waves. More generally, the "particles" of pilot wave theory are led by the guidance equation to move in ways no classical particle would, involving apparent violations of the principles of inertia and momentum conservation.We next argue that the aforementioned cost can be avoided within a retrocausal model. In the proposed version of the pilot wave theory, the particle is guided by a combination of advanced and retarded waves. The resulting account for quantum physics seems to have greater heuristic power, it demands less damage to intuition, and moreover provides some general hints regarding spacetime and causality. This is the first of two papers. In the second [1] we show that, in the context of an explicit model, retrocausality, with respect to an effective, emergent spacetime metric, can coexist with a strict irreversibility of causal processes.

Entangling bosons through particle indistinguishability and spatial overlap.

Particle identity and entanglement are two fundamental quantum properties that work as major resources for various quantum information tasks. However, it is still a challenging problem to understand the correlation of the two properties in the same system. While recent theoretical studies have shown that the spatial overlap between identical particles is necessary for nontrivial entanglement, the exact role of particle indistinguishability in the entanglement of identical particles has never been analyzed quantitatively before. Here, we theoretically and experimentally investigate the behavior of entanglement between two bosons as spatial overlap and indistinguishability simultaneously vary. The theoretical computation of entanglement for generic two bosons with pseudospins is verified experimentally in a photonic system. Our results show that the amount of entanglement is a monotonically increasing function of both quantities. We expect that our work provides an insight into deciphering the role of the entanglement in quantum networks that consist of identical particles.

On-chip multiphoton Greenberger—Horne—Zeilinger state based on integrated frequency combs

One of the most important multipartite entangled states, Greenberger-Horne-Zeilinger state (GHZ), serves as a fundamental resource for quantum foundation test, quantum communication and quantum computation. To increase the number of entangled particles, significant experimental efforts should been invested due to the complexity of optical setup and the difficulty in maintaining the coherence condition for high-fidelity GHZ state. Here, we propose an ultra-integrated scalable on-chip GHZ state generation scheme based on frequency combs. By designing several microrings pumped by different lasers, multiple partially overlapped quantum frequency combs are generated to supply as the basis for on-chip polarization-encoded GHZ state with each qubit occupying a certain spectral mode. Both even and odd numbers of GHZ states can be engineered with constant small number of integrated components and easily scaled up on the same chip by only adjusting one of the pump wavelengths. In addition, we give the on-chip design of projection measurement for characterizing GHZ states and show the reconfigurability of the state. Our proposal is rather simple and feasible within the existing fabrication technologies and we believe it will boost the development of multiphoton technologies.