We investigate the no-boundary proposal for closed universes with an observer. We argue that the observer’s no-boundary state is the identity operator on the physical Hilbert space, i.e., the maximum entropy state and show this explicitly in Jackiw-Teitelboim gravity. Geometrically, the no-boundary state is a bra-ket wormhole. Expectation values in the no-boundary state provide a trace for the observer’s algebra, which allows one to define von Neumann entropy for observers in different universes as the relative entropy with respect to the no-boundary state. This result is consistent with all previously discussed cases of traces for invariantly defined regions.

United States

Andreas Blommaert

Jonah Kudler-Flam

Erez Urbach

Absolute entropy and the observer’s no-boundary state

<p>We consider an experimentalist, Alice, who creates a quantum superposition of a charged or massive body outside of a black hole (or, more generally, in the presence of a Killing horizon). It was previously shown that emission of soft photons/gravitons into the black hole will result in the decoherence of the components of the superposition if it is held open for a sufficiently long span of time. However, at any finite time, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>t</mi><mi>c</mi></msub></math>, during the process, it is not obvious how much decoherence has irrevocably occurred. Equivalently, it is not obvious how much information an observer inside the black hole can extract about Alice’s superposition prior to time <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>t</mi><mi>c</mi></msub></math>. In this paper, we solve for the optimal experimental protocol to be followed by Alice for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>t</mi><mo>&gt;</mo><msub><mi>t</mi><mi>c</mi></msub></math> so as to minimize the decoherence of the components of her superposition. More precisely, given the entangling radiation that has passed through the horizon prior to the cross-section <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">C</mi></math> corresponding to the time <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>t</mi><mo>=</mo><msub><mi>t</mi><mi>c</mi></msub></math> in Alice’s lab, we determine the “optimal purification” of this radiation beyond <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="script">C</mi></math> such that the global quantum state of the radiation through the horizon has maximal overlap (quantum fidelity) with the Hartle-Hawking or Unruh vacuum. Due to the intricate low frequency entanglement structure of the quantum field theory vacuum state, we find this optimal purification to be nontrivial. In particular, even if Alice has already “closed” her superposition by bringing the components back together, we find that she can decrease the net decoherence of the components of her superposition somewhat by reopening it and performing further manipulations.</p>

Daine L. Danielson

Gautam Satishchandran

Robert M. Wald

How to minimize the decoherence caused by black holes

<p>While von Neumann entropies for subregions in quantum field theory universally contain ultraviolet divergences, differences between von Neumann entropies are finite and well defined in many physically relevant scenarios. We demonstrate that such a notion of entropy differences can be rigorously defined in quantum field theory in a general curved spacetime by introducing a novel, covariant regulator for the entropy based on the modular crossed product. This regulator associates a type II von Neumann algebra to each spacetime subregion, resulting in well-defined renormalized entropies. This prescription reproduces formulas for entropy differences that coincide with heuristic formulas widely used in the literature, and we prove that it satisfies desirable properties such as unitary invariance and concavity. As an application, we provide proofs of the Bekenstein bound and the quantum null energy condition, formulated directly in terms of vacuum-subtracted von Neumann entropies.</p>

Samuel Leutheusser

Adel A. Rahman

Antony J. Speranza

Covariant regulator for entanglement entropy: Proofs of the Bekenstein bound and the quantum null energy condition

It has been argued that, while the individual terms in the generalized entropy, <math><msub><mi>S</mi><mrow><mi>gen</mi></mrow></msub><mo>=</mo><mi>A</mi><mo>/</mo><mn>4</mn><msub><mi>G</mi><mi>N</mi></msub><mo>+</mo><msub><mi>S</mi><mrow><mi>ext</mi></mrow></msub></math>, are ill-defined in the semiclassical limit, their sum is well-defined if one takes into account perturbative quantum gravitational effects. The first term diverges as <math><msub><mi>G</mi><mi>N</mi></msub><mo>→</mo><mn>0</mn></math>, and the second diverges due to the infinite entanglement across the horizon which is characteristic of type III von Neumann algebras. It was recently shown that the von Neumann algebra of observables “gravitationally dressed” to the mass of a Schwarzschild-AdS black hole or the energy of an observer in de Sitter spacetime admit a well-defined trace. The algebras are type <math><msub><mrow><mi>II</mi></mrow><mi>∞</mi></msub></math> (which does not admit a maximum entropy state) and type <math><mrow><msub><mrow><mi>II</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math> (which admits a maximum entropy state) respectively, and the von Neumann entropy of “semiclassical” states was found to be (up to an additive constant) the generalized entropy. However, these arguments rely on the existence of a stationary “equilibrium (KMS) state” and do not apply to, for example, black holes formed from gravitational collapse, Kerr black holes, or black holes in asymptotically de Sitter spacetime. These spacetimes are stationary but not in thermal equilibrium. In this paper, we present a general framework for obtaining the algebra of “gravitationally dressed” observables for a linear, Klein-Gordon field on any spacetime with a (bifurcate) Killing horizon. We prove, assuming the existence of a stationary state—which is not necessarily KMS—and suitable asymptotic decay of solutions, a “structure theorem” that the algebra of “gravitationally dressed” observables always contains a type II factor of observables “localized” on the horizon. These assumptions have been rigorously proven in most cases of interest in this paper. Applying our general framework to the algebra of observables in the exterior of an asymptotically flat Kerr black hole where the fields are dressed to the black hole mass and angular momentum we find that the algebra is the product of a type <math><msub><mrow><mi>II</mi></mrow><mi>∞</mi></msub></math> algebra on the horizon and a type <math><msub><mrow><mi>I</mi></mrow><mi>∞</mi></msub></math> algebra at past null infinity. The full algebra is type <math><msub><mrow><mi>II</mi></mrow><mi>∞</mi></msub></math>, and the von Neumann entropy of semiclassical states is the generalized entropy. In the case of Schwarzschild-de Sitter, despite the fact that we must introduce an observer, the algebra of observables dressed to the perturbed areas of the black hole and cosmological horizons is the product of type <math><msub><mrow><mi>II</mi></mrow><mi>∞</mi></msub></math> algebras on each horizon. The entropy of semiclassical states is given by the sum of the areas of the two horizons as well as the entropy of quantum fields in between the horizons. Our results suggest that in all cases where there exists another “boundary structure” (e.g., an asymptotic boundary or another Killing horizon) the algebra of observables is type <math><msub><mrow><mi>II</mi></mrow><mi>∞</mi></msub></math> and in the absence of such structures (e.g. de Sitter spacetime) the algebra is type <math><mrow><msub><mrow><mi>II</mi></mrow><mrow><mn>1</mn></mrow></msub></mrow></math>.

Generalized black hole entropy is von Neumann entropy

The entanglement negativity $$ \mathcal{E} $$ ($\textit{A}$ : $\textit{B}$) is a useful measure of quantum entanglement in bipartite mixed states. In random tensor networks (RTNs), which are related to fixed-area states, it was found in ref. [1] that the dominant saddles computing the even Rényi negativity $$ {\mathcal{E}}^{(2k)} $$ generically break the $\textit{ℤ}$ replica symmetry. This calls into question previous calculations of holographic negativity using 2D CFT techniques that assumed $\textit{ℤ}$ replica symmetry and proposed that the negativity was related to the entanglement wedge cross section. In this paper, we resolve this issue by showing that in general holographic states, the saddles computing $$ {\mathcal{E}}^{(2k)} $$ indeed break the $\textit{ℤ}$ replica symmetry.

Xi Dong

Pratik Rath

Entanglement negativity and replica symmetry breaking in general holographic states

We establish an equivalence between two different quantum quench problems, the joining local quantum quench and the Möbius quench, in the context of (1 + 1)-dimensional conformal field theory (CFT). Here, in the former, two initially decoupled systems (CFTs) on finite intervals are joined at t = 0. In the latter, we consider the system that is initially prepared in the ground state of the regular homogeneous Hamiltonian on a finite interval and, after t = 0, let it time-evolve by the so-called Möbius Hamiltonian that is spatially inhomogeneous. The equivalence allows us to relate the time-dependent physical observables in one of these problems to those in the other. As an application of the equivalence, we construct a holographic dual of the Möbius quench from that of the local quantum quench. The holographic geometry involves an end-of-the-world brane whose profile exhibits non-trivial dynamics.

Japan

China

Masahiro Nozaki

Tokiro Numasawa

Shinsei Ryu

South Korea

Mao Tan

Bridging two quantum quench problems — local joining quantum quench and Möbius quench — and their holographic dual descriptions

We explore a large class of correlation measures called the α − z Rényi mutual informations (RMIs). Unlike the commonly used notion of RMI involving linear combinations of Rényi entropies, the α − z RMIs are positive semi-definite and monotonically decreasing under local quantum operations, making them sensible measures of total (quantum and classical) correlations. This follows from their descendance from Rényi relative entropies. In addition to upper bounding connected correlation functions between subsystems, we prove the much stronger statement that for certain values of α and z, the α − z RMIs also lower bound certain connected correlation functions. We develop an easily implementable replica trick which enables us to compute the α − z RMIs in a variety of many-body systems including conformal field theories, free fermions, random tensor networks, and holography.

Laimei Nie

Akash Vijay

Rényi mutual information in quantum field theory, tensor networks, and gravity

We propose a new formula for computing holographic Renyi entropies in the presence of multiple extremal surfaces. Our proposal is based on computing the wave function in the basis of fixed-area states and assuming a diagonal approximation for the Renyi entropy. For Renyi index n ≥ 1, our proposal agrees with the existing cosmic brane proposal for holographic Renyi entropy. For n &lt; 1, however, our proposal predicts a new phase with leading order (in Newton’s constant G) corrections to the cosmic brane proposal, even far from entanglement phase transitions and when bulk quantum corrections are unimportant. Recast in terms of optimization over fixed-area states, the difference between the two proposals can be understood to come from the order of optimization: for n &lt; 1, the cosmic brane proposal is a minimax prescription whereas our proposal is a maximin prescription. We demonstrate the presence of such leading order corrections using illustrative examples. In particular, our proposal reproduces existing results in the literature for the PSSY model and high-energy eigenstates, providing a universal explanation for previously found leading order corrections to the n &lt; 1 Renyi entropies.

A modified cosmic brane proposal for holographic Renyi entropy

While recent progress in the black hole information problem has shown that the entropy of Hawking radiation follows a unitary Page curve, the quantum state of Hawking radiation prior the Page time is still treated as purely thermal, containing no information about the microstructure of the black hole. We demonstrate that there is significant quantum information regarding the quantum state of the black hole in the Hawking radiation prior to the Page time. By computing of the quantum fidelity in a 2D boundary conformal field theory (BCFT) model of black hole evaporation, we demonstrate that an observer outside of an evaporating black hole may distinguish different black holes via measurements of the Hawking radiation at any time during the evaporation process, albeit with an exponentially large number of measurements. Furthermore, our results are universal, applicable to general BCFTs including those with large central charge and rational BCFTs. The techniques we develop for computing the fidelity are more generally applicable to excited states in CFT. As such, we are able to characterize more general aspects of thermalization in 2D conformal field theory.

Yuya Kusuki

On quantum information before the Page time

We study the universal behavior of quantum information-theoretic quantities in thermalized isolated quantum many-body systems and evaporating black holes. In particular, we study a genuine mixed-state entanglement measure called the logarithmic negativity, other correlation measures including the Renyi negativities and the mutual information, and a signature of multipartite entanglement called the reflected entropy. We also probe the feasibility of recovering quantum information from subsystems of a thermalized quantum many-body system or from the radiation of an evaporating black hole, using quantities such as relative entropy and Petz map fidelity. A recently developed technique called the equilibrium approximation allows us to probe these quantities at finite temperature. We find striking qualitative differences from the infinite temperature case, which has been the topic of previous studies using Haar-random states. In particular, we find regimes where the logarithmic negativity is extensive but the mutual information is sub-extensive, indicating a large amount of undistillable, bound entanglement in thermalized states. For evaporating black holes at finite temperature, both the logarithmic negativity and the Petz map fidelity reveal an important new time scale t  b , which is earlier than the Page time t  p  by a finite fraction of the total evaporation time. We find that t  b , as opposed to t  p , is the time scale at which quantum entanglement between different parts of the radiation becomes extensive, and the fidelity of information recovery for a large diary thrown into the black hole starts to grow.