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1. Gravity from thermodynamics: Optimal transport and negative effective dimensions

   https://doi.org/10.21468/SciPostPhys.15.2.039  

   De Luca, Giuseppe Bruno; De Ponti, Nicolò; Mondino, Andrea; Tomasiello, Alessandro (January 2023, SciPost Physics)

   We prove an equivalence between the classical equations of motion governing vacuum gravity compactifications (and more general warped-product spacetimes) and a concavity property of entropy under time evolution. This is obtained by linking the theory of optimal transport to the Raychaudhuri equation in the internal space, where the warp factor introduces effective notions of curvature and (negative) internal dimension. When the Reduced Energy Condition is satisfied, concavity can be characterized in terms of the cosmological constant\Lambda $\Lambda$; as a consequence, the masses of the spin-two Kaluza-Klein fields obey bounds in terms of\Lambda $\Lambda$alone. We show that some Cheeger bounds on the KK spectrum hold even without assuming synthetic Ricci lower bounds, in the large class of infinitesimally Hilbertian metric measure spaces, which includes D-brane and O-plane singularities. As an application, we show how some approximate string theory solutions in the literature achieve scale separation, and we construct a new explicit parametrically scale-separated AdS solution of M-theory supported by Casimir energy. 

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2. Wilson loop in general representation and RG flow in 1D defect QFT

   https://doi.org/10.1088/1751-8121/ac7018  

   Beccaria, M.; Giombi, S.; Tseytlin, A. A. (June 2022, Journal of Physics A: Mathematical and Theoretical)

   Abstract The generalized Wilson loop operator interpolating between the supersymmetric and the ordinary Wilson loop in $\mathcal{N}=4$SYM theory provides an interesting example of renormalization group flow on a line defect: the scalar coupling parameterζhas a non-trivial beta function and may be viewed as a running coupling constant in a 1D defect QFT. In this paper we continue the study of this operator, generalizing previous results for the beta function and Wilson loop expectation value to the case of an arbitrary representation of the gauge group and beyond the planar limit. Focusing on the scalar ladder limit where the generalized Wilson loop reduces to a purely scalar line operator in a free adjoint theory, and specializing to the case of the rankksymmetric representation ofSU(N), we also consider a certain ‘semiclassical’ limit wherekis taken to infinity with the productkζ²fixed. This limit can be conveniently studied using a 1D defect QFT representation in terms ofNcommuting bosons. Using this representation, we compute the beta function and the circular loop expectation value in the largeklimit, and use it to derive constraints on the structure of the beta function for general representation. We discuss the corresponding 1D RG flow and comment on the consistency of the results with the 1D defect version of the F-theorem. 

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3. Null states and time evolution in a toy model of black hole dynamics

   https://doi.org/10.1007/JHEP08(2024)199  

   Dong, Xi; Kolanowski, Maciej; Liu, Xiaoyi; Marolf, Donald; Wang, Zhencheng (August 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> Spacetime wormholes can provide non-perturbative contributions to the gravitational path integral that make the actual number of statese^Sin a gravitational system much smaller than the number of states$$ {e}^{S_{\textrm{p}}} $$ $e^{S_p}$predicted by perturbative semiclassical effective field theory. The effects on the physics of the system are naturally profound in contexts in which the perturbative description actively involvesN=O(e^S) of the possible$$ {e}^{S_{\textrm{p}}} $$ $e^{S_p}$perturbative states; e.g., in late stages of black hole evaporation. Such contexts are typically associated with the existence of non-trivial quantum extremal surfaces. However, by forcing a simple topological gravity model to evolve in time, we find that such effects can also have large impact forN≪e^S(in which case no quantum extremal surfaces can arise). In particular, even for smallN, the insertion of generic operators into the path integral can cause the non-perturbative time evolution to differ dramatically from perturbative expectations. On the other hand, this discrepancy is small for the special case where the inserted operators are non-trivial only in a subspace of dimensionD≪e^S. We thus study this latter case in detail. We also discuss potential implications for more realistic gravitational systems. 

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4. Building Multiple Access Channels with a Single Particle

   https://doi.org/10.22331/q-2022-02-16-653  

   Zhang, Yujie; Chen, Xinan; Chitambar, Eric (February 2022, Quantum)

   A multiple access channel describes a situation in which multiple senders are trying to forward messages to a single receiver using some physical medium. In this paper we consider scenarios in which this medium consists of just a single classical or quantum particle. In the quantum case, the particle can be prepared in a superposition state thereby allowing for a richer family of encoding strategies. To make the comparison between quantum and classical channels precise, we introduce an operational framework in which all possible encoding strategies consume no more than a single particle. We apply this framework to an $N$-port interferometer experiment in which each party controls a path the particle can traverse. When used for the purpose of communication, this setup embodies a multiple access channel (MAC) built with a single particle.We provide a full characterization of the $N$-party classical MACs that can be built from a single particle, and we show that every non-classical particle can generate a MAC outside the classical set. To further distinguish the capabilities of a single classical and quantum particle, we relax the locality constraint and allow for joint encodings by subsets of $1<K\le N$parties. This generates a richer family of classical MACs whose polytope dimension we compute. We identify a generalized fingerprinting inequality'' as a valid facet for this polytope, and we verify that a quantum particle distributed among $N$separated parties can violate this inequality even when $K=N-1$. Connections are drawn between the single-particle framework and multi-level coherence theory. We show that every pure state with $K$-level coherence can be detected in a semi-device independent manner, with the only assumption being conservation of particle number. 

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5. Deconfined quantum criticality of nodal $d$ -wave superconductivity, Néel order, and charge order on the square lattice at half-filling

   https://doi.org/10.1103/PhysRevResearch.6.033018  

   Christos, Maine; Shackleton, Henry; Sachdev, Subir; Luo, Zhu-Xi (July 2024, Physical Review Research)

   We consider a SU(2) lattice gauge theory on the square lattice, with a single fundamental complex fermion and a single fundamental complex boson on each lattice site. Projective symmetries of the gauge-charged fermions are chosen so that they match with those of the spinons of the $\pi$-flux spin liquid. Global symmetries of all gauge-invariant observables are chosen to match with those of the particle-hole symmetric electronic Hubbard model at half-filling. Consequently, both the fundamental fermion and fundamental boson move in an average background $\pi$-flux, their gauge-invariant composite is the physical electron, and eliminating gauge fields in a strong gauge-coupling expansion yields an effective extended Hubbard model for the electrons. The SU(2) gauge theory displays several confining/Higgs phases: a nodal $d$-wave superconductor, and states with Néel, valence-bond solid, charge, or staggered current orders. There are also a number of quantum phase transitions between these phases that are very likely described by $(2+1)$-dimensional deconfined conformal gauge theories, and we present large flavor expansions for such theories. These include the phenomenologically attractive case of a transition between a conventional insulator with a charge gap and Néel order, and a conventional $d$-wave superconductor with gapless Bogoliubov quasiparticles at four nodal points in the Brillouin zone. We also apply our approach to the honeycomb lattice, where we find a bicritical point at the junction of Néel, valence bond solid (Kekulé), and Dirac semimetal phases. Published by the American Physical Society2024 

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