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arXiv:1809.06878 · Physical review. D/Physical review. D.

Unruh-DeWitt detectors and entanglement: The anti–de Sitter space

Keith K. Ng, Robert B. Mann, Eduardo Martín-Martínez

Published Dec 11, 2018• 63 citations
arXivHTMLPDF

Abstract

We investigate entanglement harvesting in ${\mathrm{AdS}}_{4}$. Applying the general results of [K. K. Ng, R. B. Mann, and E. Mart\'{\i}n-Mart\'{\i}nez, Phys. Rev. D 97, 125011 (2018)], we consider two scenarios: one where two particle detectors are geodesic, with equal redshift; and one where both are static, at unequal redshift. As expected, at large AdS length $L$, our results approximate flat space. However at smaller $L$ we observe nontrivial effects for various field boundary conditions. Furthermore, in the static case we observe a novel feature of the entanglement as a function of switching time delay, which we attribute to different (coordinate) frequencies of the detectors. We also find an island of separability in parameter space, analogous to that observed in ${\mathrm{AdS}}_{3}$, to which we compare and contrast our other results. The variety of features observed in both cases suggest further study in other spacetimes.

DOI
10.1103/physrevd.98.125005

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We investigate entanglement harvesting in AdS4{\mathrm{AdS}}_{4}AdS4​. Applying the general results of [K. K. Ng, R. B. Mann, and E. Mart'{\i}n-Mart'{\i}nez, Phys. Rev. D 97, 125011 (2018)], we consider two scenarios: one where two particle detectors are geodesic, with equal redshift; and one where both are static, at unequal redshift. As expected, at large AdS length LLL, our results approximate flat space. However at smaller LLL we observe nontrivial effects for various field boundary conditions. Furthermore, in the static case we observe a novel feature of the entanglement as a function of switching time delay, which we attribute to different (coordinate) frequencies of the detectors. We also find an island of separability in parameter space, analogous to that observed in AdS3{\mathrm{AdS}}_{3}AdS3​, to which we compare and contrast our other results. The variety of features observed in both cases suggest further study in other spacetimes.
Abstract We examine in (2+1)-dimensional anti-de Sitter (AdS) space the phenomena of entanglement harvesting — the process in which a pair of detectors (two-level atoms) extract entanglement from a quantum field through local interactions with the field. We begin by reviewing the Unruh-DeWitt detector and its interaction with a real scalar field in the vacuum state, as well as the entanglement harvesting protocol in general. We then examine how the entanglement harvested by a pair of such detectors depends on their spacetime trajectory, separation, spacetime curvature, and boundary conditions satisfied by the field. The harvested entanglement is interpreted as an indicator of field entanglement between the localized regions where the detectors interact with the field, and thus this investigation allows us to probe indirectly the entanglement structure of the AdS vacuum. We find an island of separability for specific values of the detectors’ energy gap and separation at intermediate values of the AdS length for which entanglement harvesting is not possible; an analogous phenomena is observed in AdS4, to which we compare and contrast our results. In the process we examine how the transition probability of a single detector, as a proxy for local fluctuations of the field, depends on spacetime curvature, its location in AdS space, and boundary conditions satisfied by the field.
We explore entanglement harvesting using two Unruh-DeWitt (UDW) detectors linearly coupled to the scalar density of a massless spin-1/21/21/2 field in 1+11+11+1 Schwarzschild spacetime. We consider different vacua, including the Boulware, Hartle-Hawking-Israel (HHI), and Unruh vacua, and investigate various configurations of detector trajectories. We find that the transition rate of the static UDW detector exhibits the expected Planckian behavior in the HHI state, while the Unruh state leads to the Helmholtz free energy density of a fermionic thermal bath. We demonstrate that the near-horizon entanglement properties for static detectors in the HHI state have similar behavior to those in Minkowski vacua for uniformly accelerated detectors in Rindler spacetime. We further consider a different interaction Hamiltonian which breaks local Lorentz symmetry and find that the transition rate of the static detector still exhibits Planckian behavior in the HHI state, while in the Unruh state, it leads to the Helmholtz free energy density of a bosonic or fermionic thermal bath corresponding to the static or conformal 2-bein in interaction, respectively. We observe that the anti-Hawking effect enhances the entanglement between the two detectors while the gravitational redshift and Hawking radiation decrease it. In particular, due to the presence of the anti-Hawking effect, the mutual information and concurrence near the event horizon can be non-zero even for static detectors with static 2-bein, which is in contrast with the case of the scalar field. Conclusions are discussed.
We derive analytic response functions for Unruh--DeWitt detectors coupled to real, massless, conformally coupled scalar fields, to distinguish the effects of the field spectrum and global geometry from those of the interaction protocol. For Gaussian-switched static pairs in Minkowski, anti-de Sitter, and de Sitter spacetimes, we obtain the leading-order excitation probabilities and two-detector coherences in spacetime dimension D=d+1≥3\mathcal D=d+1\ge3D=d+1≥3. Time-ordered coherence requires distinct spatial worldlines; the local response and single-excitation coherence also admit coincidence. For equal-redshift pairs, Gaussian switching separates the gap dependence of the time-ordered coherence. In de Sitter space, KMS detailed balance fixes the gap that maximizes the ratio of nonlocal coherence to local excitation, giving a single-gap test for the existence of leading-order entanglement and restricting any entangling gaps to one bounded interval. When entanglement is present, the concurrence itself peaks at a smaller positive gap. In the three-dimensional BTZ black hole, an integrated Legendre series gives the sharply switched response of a radially infalling detector. A uniform image-sum estimate establishes logarithmic growth near the singularity and determines its coefficient from the covering-AdS response. A sufficiently smooth monotone onset removes the finite-time onset glitches but leaves a positive logarithmic coefficient; the accumulated leading-order response remains finite. Independent numerical calculations test the analytic expressions, their convergence properties, and these physical distinctions.
This thesis is focused on using a pair of Unruh-DeWitt detectors, which interact locally with a quantum field, to gain information about properties of the field and the underlying spacetime. \n \nFirst, we consider a massless scalar field with a bandlimit, a hard ultraviolet cutoff. We show that that when UDW detectors interact with a bandlimited field, they are able to become entangled over larger distances then would be expected in the case of no cutoff. The bandlimit introduces some non locality into the interaction, which boosts the nonlocal correlations between the detectors. This boost in entanglement could, in principle, be used to put a lower bound . \n \nThen we study the affect of a quantum controlled switch to the entanglement harvesting protocol. This switch will control when the detectors interact with the quantum scalar field and can allow for them to each couple to the field in a superposition of two times. We show that when a detector interacts with a field at a superposition of times, its transition probability is reduced compared to what it interacted at a mixture of times. This in turn increases the entanglement harvested by a pair of detectors, which can even violate no-go theorems. \n \nNext, we turn our attention to (2+1)-dimensional AdS spacetime. We map out a large amount of the parameter space, tuning both detector and spacetime properties. Most interestingly, we find an "island of seprability" in the parameter space, which occurs at a relatively small Ads length. When the detectors' properties fall in the "island of seprablity", they cannot become entangled through local interactions with the field; however for a good range of parameters around the island, they can. \n \nWe then apply the entanglement harvesting protocol to a black hole spacetime, specifically the BTZ black hole. We find an entanglement "death zone" which marks the point where entanglement harvesting is no longer possible as two detectors with fixed proper separation approach the horizon. \n \nFinally, we make a change to the detector model itself. We consider the the center of mass degree of freedom of the detectors to be quantum, and so will it coherently delocalize over the course of the interaction with the quantum field. We find that this delocalization inhibits entanglement harvesting, and it is only in the limit of very heavy and very sharply localized detectors that we recover the case of a pair of UDW detectors classical center center of mass.
What is the boundary dual of an Unruh-DeWitt detector in anti-de Sitter space? We argue that this question is naturally answered by the HKLL reconstruction program. A local detector in the bulk can be associated with a smeared detector in the boundary, where the choice of smearing depends on the choice of HKLL kernel. We use this formalism to analyze entanglement harvesting in the Poincaré patch of AdS3_{3}3​ for a bulk Unruh-DeWitt detector paired either with a boundary detector or with a second bulk detector. We compute the amount of harvesting in the standard way, namely by evaluating the two-detector density matrix to quadratic order in the coupling λλλ. Additionally, we address an apparent tension between microcausality for spacelike-separated bulk detectors and the fact that the dual HKLL detectors may directly overlap in the boundary theory.
The aim of this work is present the phenomenon denoted entanglement harvesting.We begin by introducing entanglement historically.Following, we go beyond the one particle theory in flat spacetime and introduce Quantum Field Theory in Curved Spacetime, showing two famous consequences: the Unruh effect and the Hawking radiation.Finally, we analyze entanglement harvesting for two Unruh-deWitt detectors.In the fisrt example, we see that there is a "sudden death" point of entanglement harvesting when the detectors are near the BTZ black hole event horizon, due to redshift effect and Hawking radiation.Then, we compare the phenomenon for different scenarios, and find out that it is sensitive to the structure of spacetime.Finally, we see how detectors' parameters affect it and find out that the smoothness of the switching of the detectors' coupling to the field is extremely relevant.We also see how the parameters can be used to optimize entanglement harvested.