Scientific Series

Joint work (in progress) with Ladina Hausmann, Nuriya Nurgalieva and Renato Renner

We can classify contemporary approaches to thermodynamics in roughly four camps:

(1) Top-down microscopic approaches. These are for example resource-theoretical approaches to quantum thermodynamics: they have a microscopic model of states and systems, and which microscopic restrictions implement macroscopic properties. For instance, in the resource theory of quantum thermodynamics, states are represented by density operators, thermal states in particular have a specific micro-canonical form, and constraints like energy preservation are enforced by forcing quantum transformations to commute with a global Hamiltonian. These approaches success at deriving thermodynamic laws in general settings that satisfy the microscopic model (like non-relativistic quantum systems.

(2) Bottom-up microscopic approaches. These also start from a microscopic model, but rather than looking for universal restrictions, they search for explicit thermodynamics protocols: this is the case of recent proposals for quantum work extraction or nano quantum heat engines.

(3) Top-down phenomenological approaches. These try to derive thermodynamic laws from first principles independently of a microscopic model. In principle the results derived in this framework can be applied to a wider variety of explicit systems, and the challenge is then to find the right implementations.  The first derivations of thermodynamics were naturally phenomenological, and some modern information-inspired derivations follow this approach.

(4) Bottom-up phenomenological approaches. These approaches try to find explicit thermodynamic protocols independently of the microscopic model, based only on operational properties of the systems at hand. It was the case for Carnot's original engines and more recently for some approaches to deriving black hole thermodynamics, or thermodynamics of new materials; some experimental results also fit in this camp.

In this work we generalize top-down phenomenological approaches to the case of multiple conserved quantities. Note that multiple conserved quantities have been studied in top-down and bottom-up microscopic approaches to quantum thermodynamics. We argue that our framework is more general, in that it can be applied to systems for which we don't have an explicit microscopic model; in particular we will apply the results of this framework to black hole thermodynamics. Moreover, having a phenomenological axiomatic approach to thermodynamics allows us to identify which properties are specific to a microscopic model like quantum physics, and which hold in any physical theory: our results can be applied to study the thermodynamics of generalized process theories, and other generalizations and foils of quantum mechanics. This generalization makes us reconsider the second law of thermodynamics, adapting for an exchange of different conserved quantities, for example, energy and angular momentum, or energy and spin. Our guiding principle here is to use information as a universal token of exchange to convert between different quantities via Landauer's principle.

Zoom Link: https://pitp.zoom.us/j/96001094153?pwd=YTArTGpPdEJ1NFBMcnFqV1dIRTVyZz09

We analyse models of Matrix Quantum Mechanics in the double scaling limit that contain non-singlet states. The finite temperature partition function of such systems contains non-trivial winding modes (vortices) and is expressed in terms of a group theoretic sum over representations. We then focus on the model of Kazakov-Kostov-Kutasov when the first winding mode is dominant. In the limit of large representations (continuous Young diagrams), and depending on the values of the parameters of the model such as the compactification radius and the string coupling, the dual geometric background corresponds either to that of a long string (winding mode) condensate or a 2d (non-supersymmetric) semi-classical Black Hole competing with the thermal linear dilaton background. In the matrix model we are free to tune these parameters and explore various regimes of this phase diagram. Our construction allows us to identify the origin of the microstates of the long string condensate/2d Black Hole arising from the non trivial representations.

Zoom Link: https://pitp.zoom.us/j/95764320439?pwd=L1E1cEREM29ORjZhK21WdFN0RVgyQT09

Abstract and

Zoom Link: https://pitp.zoom.us/j/91762985902?pwd=djhwYVdsQ25GVVBRVTlwSkQvaDJ4Zz09

Cosmology and astrophysics with the extragalactic light: background and fluctuations

Abstract: The aggregate light emitted by all extragalactic sources can be measured either as an absolute intensity or through its spatial fluctuations; these are known as line-intensity mapping (LIM) when a particular line transition is targeted. I will discuss how these measurements can be used both to learn about galaxy evolution and to investigate the presence of more speculative sources of radiation, such as decaying dark matter. I will then discuss the prospects of LIM to probe the interstellar and intergalactic medium, as well as structure on large scales. In particular, I will focus on the joint information across different line transitions and across different tracers, such as galaxies and extragalactic CMB foregrounds.

Zoom Link: https://pitp.zoom.us/j/94959896129?pwd=QjhzWS9rczdBNGFndUhhNDZ1K1FLQT09

Variational methods have proven to be excellent tools to approximate the ground states of complex many-body Hamiltonians. Generic tools such as neural networks are extremely powerful, but their parameters are not necessarily physically motivated. Thus, an efficient parametrization of the wave function can become challenging. In this talk I will introduce a neural-network-based variational ansatz that retains the flexibility of these generic methods while allowing for a tunability with respect to the relevant correlations governing the physics of the system. I will illustrate the ansatz on a model exhibiting topological phase transitions: The toric code in the presence of magnetic fields. Additionally, I will talk about the use of variational wave functions to gain physical insights beyond lattice models, in particular for the real use-case of two-dimensional materials.

Information is physical, and for a physical theory to be universal, it
should model observers as physical systems, with concrete memories where
they store the information acquired through experiments and reasoning.
Here we address these issues in Spekkens' toy theory, a non-contextual
epistemically restricted model that partially mimics the behaviour of
quantum mechanics. We propose a way to model physical implementations of
agents, memories, measurements, conditional actions and information
processing. We find that the actions of toy agents are severely limited:
although there are non-orthogonal states in the theory, there is no way
for physical agents to consciously prepare them. Their memories are also
constrained: agents cannot forget in which of two arbitrary states a
system is. Finally, we formalize the process of making inferences about
other agents' experiments and model multi-agent experiments like
Wigner's friend. Unlike quantum theory or box world, in the toy theory
there are no inconsistencies when physical agents reason about each
other's knowledge.

Zoom Link: TBD

In this talk we combine integrability and conformal bootstrap to learn about correlation functions of planar maximally supersymmetric Yang- Mills theory. Focusing on correlators of four stress-tensor multiplets, we first introduce a set of dispersive sum rules that are only sensitive to single-traces in the OPE expansion (this is advantageous because this data is available from integrability). We then construct combinations of the sum rules which determine one-loop correlators. Further, we discuss how to employ the sum rules in numerical bootstrap to nonperturbatively bound planar OPE coefficients. As an example, we show a nontrivial upper bound on the OPE coefficient of the Konishi operator outside the perturbative regime. 

In this talk, I will show that supergravity on asymptotically flat spaces possesses a (nonlinear) asymptotic symmetry algebra, containing an infinite number of fermionic generators. Starting from the Hamiltonian action for supergravity with suitable boundary conditions on the graviton and gravitino fields, I will derive a graded extension of the BMS_4 algebra at spatial infinity, denoted by SBMS_4. These boundary conditions are not only invariant under the SBMS_4 algebra, but lead to a fully consistent canonical description of the supersymmetries, which have well-defined Hamiltonian generators. One finds, in particular, that the graded brackets between the fermionic generators yield BMS supertranslations, of which they provide therefore “square roots”.  I will comment on some key aspects of extending the asymptotic analysis at spatial infinity to fermions and on the structure of the SBMS_4 algebra in terms of Lorentz representations.

Zoom link: https://pitp.zoom.us/j/95951230095?pwd=eHIwUXB5SUkvd0IvZnVUN3JJMFE1QT09

As a recent combination of two strong global leaders in quantum computing, Honeywell Quantum Solutions and Cambridge Quantum, Quantinuum integrates quantum hardware and software, including solutions for drug discovery, materials science, finance, and other applications. Quantinuum aims to be a center of gravity for quantum computing, supporting collaboration across the ecosystem. For this we have also developed “TKET”, an open-source architecture-agnostic quantum software stack and ‘best in class’ compiler. This enables our partners, collaborators and clients to effortlessly work across multiple platforms and tackle some of the most intriguing and important problems in quantum computing. 

Bio: Dr. Mark Jackson is the Senior Quantum Evangelist at Quantinuum. He received his B.S. in Physics and Mathematics from Duke University and Ph.D. in Theoretical Physics from Columbia University. He then spent 10 years researching superstring theory and cosmology, co-authoring almost 40 technical articles. To promote the public understanding of science, he founded the science crowdfunding platform Fiat Physica and non-profit Science Partnership Fund. He is Adjunct Faculty at Singularity University and a Director of Astronomers Without Borders.

Zoom link: https://pitp.zoom.us/j/98433088425?pwd=UzgwcGpUYnBKNzJmMnQ1ZVNOdGVXZz09

The Entanglement Bootstrap is a program to derive the universal properties of a phase of matter from a single representative wavefunction on a topologically-trivial region.  Much (perhaps all) of the structure of topological quantum field theory can be extracted starting from a state satisfying two axioms that implement the area law for entanglement.  This talk will focus on recent progress (with Bowen Shi and Jin-Long Huang) using this approach to prove remote detectability of topological excitations in various dimensions.  This is an axiom of topological field theory.  Two key ideas are a quantum avatar of Kirby's torus trick to construct states on closed manifolds, and the new concept of pairing manifold, which is a closed manifold associated with a pair of conjugate excitation types that encodes their braiding matrix.  The pairing manifold also produces Verlinde formulae relating the S-matrix to the structure constants of a generalized symmetry algebra of flexible operators.  

Zoom link:  https://pitp.zoom.us/j/92633473610?pwd=eEhqR3BaQXljQm5ScHZvZm81N2FyZz09

The elementary CuO2 plane sustaining cuprate high temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO5 pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap E, generate ‘superexchange’ spin-spin interactions of energy J≈4t4/E3 in an antiferromagnetic correlated-insulator state. However, hole doping this CuO2 plane converts this into a very high temperature superconducting state whose electron-pairing is exceptional. A leading proposal for the mechanism of this intense electron-pairing is that, while hole doping destroys magnetic order it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale E.

To explore this hypothesis directly at atomic-scale, we developed high-voltage single-electron and electron-pair (Josephson) scanning tunneling microscopy, to visualize the interplay of E and the electron-pair density nP in Bi2Sr2CaCu2O8+x. Changing the distance δ between each pyramid’s apical O atom and the CuO2 plane below, should alter the energy levels of the planar Cu and O orbitals and thus vary E.  Hence, the responses of both E and nP to alterations in δ that occur naturally in Bi2Sr2CaCu2O8+x were visualized. These data revealed, directly at atomic scale, the crux of strongly correlated superconductivity in CuO2: the response of the electron-pair condensate to varying the charge transfer energy. Strong concurrence between these observations and recent three-band Hubbard model DMFT predictions for superconductivity in hole-doped Bi2Sr2CaCu2O8+x (PNAS 118, e2106476118 (2021)) indicate that charge-transfer superexchange is the electron-pairing mechanism (PNAS 119, 2207449119 (2022)).

Zoom link:  https://pitp.zoom.us/j/95592484157?pwd=YU56Wno3WnBIUTlyaC9VSHJ3cGxZUT09