Talks

Quantum error correction methods for qubit technologies such as ions, photons, or superconducting qubits can appear very different at first glance. Moreover, as more detailed error models are accounted for, the relationship to the abstract models of fault tolerance can appear to become more distant. In this talk I will discuss two unifying frameworks which connect hardware specific models more closely to the underlying code structures, which can help enable QEC development. First I will introduce a unifying framework for fault tolerance based on the ZX calculus (arXiv:2303.08829) and show how it allows us to view circuit-based, measurement-based, fusion-based quantum computation, and Floquet codes as different flavors of the same underlying stabilizer fault-tolerance structure. Secondly I will introduce fusion complexes (arXiv:2308.07844) which allows a topological interpretation of fault tolerance even under circuit level error models. Both of these frameworks are tools that can aid in the design of quantum error correction methods under hardware-focussed models, and I will give some examples of this applied to the design of photonic architectures.

What is the structure of many-body quantum phases and transitions in the presence of non-unitary elements, such as decoherence or measurements? In this talk we explore two new directions. First, recent works have shown that even if one starts with an ideal preparation of topological order such as the toric code, decoherence can lead to interesting mixed states with subtle phase transitions [e.g., Fan et al, arXiv:2301.05689]. Motivated by a recent experimental realization of non-Abelian topological order [Iqbal et al, Nature 626 (2024)], we generalize this to decohered non-Abelian states, based on work with Pablo Sala and Jason Alicea [to appear]. Second, we study whether and how one can prepare pure states which are already detuned from ideal fixed-point cases---with tunable correlation lengths. This turns out to be possible for large classes of tensor network states which can be deterministically prepared using finite-depth measurement protocols. This is based on two recent works with Rahul Sahay [arXiv:2404.17087; arXiv:2404.16753].

I will review various mechanisms and detection strategies of precursor emission to black holes and neutron stars mergers. I will also discuss other peculiar physical processes at the intersection of electromagnetism, classical General Relativity, and the physics of continuous media.

--

Zoom link 

Quantum many-body scars (QMBS) are atypical eigenstates of chaotic systems that are characterized by sub-volume or area law entanglement as opposed to the volume law present in the bulk of the eigenstates. The term, QMBS, was coined using heuristic correlations with quantum scars - eigenstates with high probability density around unstable classical periodic orbits in quantum systems with a semiclassical description. Through the study of entanglement in a multi-qubit system with a semiclassical description, quantum kicked top (QKT), we show that the properties of QMBS states strongly correlate with the eigenstates corresponding to the very few stable periodic orbits in a chaotic system as opposed to quantum scars in such systems. Specifically, we find that eigenstates associated with stable periodic orbits of small periodicity in chaotic regime exhibit markedly different entanglement scaling compared to chaotic quantum states, while quantum scar eigenstates demonstrate entanglement scaling resembling that of chaotic quantum states. Our findings reveal that quantum many-body scars and quantum scars are distinct. This work is in collaboration with Cheng-Ju Lin and Amirreza Negari.

Some of the most fundamental challenges in quantum gravity involve determining how to take the continuum limit of the underlying regularized theory and how to preserve the causal structure of space-time. Several approaches to quantum gravity attempt to address these questions, but the technical challenges are substantial.

In this talk, we present a novel approach to a lattice-regularized theory of quantum gravity, using techniques from standard lattice quantum field theories to overcome these challenges. Our approach is inspired by quantum geometrodynamics, the earliest attempt at quantizing gravity. While the original approach suffered from the usual shortcomings pertaining to the multiplication of distributions and consequently failed, we propose a novel lattice regularization that is especially well suited for studying the continuum limit. First, we examine the lattice corrections to the theory and quantize these lattice theories in a manner that ensures the manifest causal structure of space-time. Next, we discuss the constructions involved in obtaining a well-defined continuum limit and explain how our approach can overcome some conceptually unappealing aspects.

---

Zoom link 

Quantum-chaotic systems are known to exhibit eigenstate thermalization and to generically thermalize under unitary dynamics. In contrast, quantum-integrable systems exhibit a generalized form of eigenstate thermalization and need to be described using generalized Gibbs ensembles after equilibration. I will discuss evidence that the entanglement properties of highly excited eigenstates of quantum-chaotic and quantum-integrable systems are fundamentally different. They both exhibit a typical bipartite entanglement entropy whose leading term scales with the volume of the subsystem. However, while the coefficient is constant and maximal in quantum- chaotic models, in integrable models it depends on the fraction of the system that is traced out. The latter is typical in random Gaussian pure states. I will also discuss the nature of the subleading corrections that emerge as a consequence of the presence of abelian and nonabelian symmetries in such models.

We discuss families of approximate quantum error correcting codes which arise as the nearly-degenerate ground states of certain quantum many-body Hamiltonians composed of non-commuting terms. For exact codes, the conditions for error correction can be formulated in terms of the vanishing of a two-sided mutual information in a low-temperature thermofield double state. We consider a notion of distance for approximate codes obtained by demanding that this mutual information instead be small, and we evaluate this mutual information for the Sachdev-Ye-Kitaev (SYK) model and for a family of low-rank SYK models. After an extrapolation to nearly zero temperature, we find that both kinds of models produce fermionic codes with constant rate as the number, N, of fermions goes to infinity. For SYK, the distance scales as N^1/2, and for low-rank SYK, the distance can be arbitrarily close to linear scaling, e.g. N^.99, while maintaining a constant rate. We also consider an analog of the no low-energy trivial states property and show that these models do have trivial low-energy states in the sense of adiabatic continuity. We discuss a holographic model of these codes in which the large code distance is a consequence of the emergence of a long wormhole geometry in a simple model of quantum gravity

Atmospheric Cold Pools (ACP) or regions of large-scale masses of cold air are often observed beneath precipitating deep convective clouds as a result of rain evaporation. An ACP is typically identified as a drop in air temperature that is greater than 10C within a period of 30 minutes to an hour at a given location. The dense air pockets relative to warmer surroundings sink and lead to low-level outflows that may propagate as gravity currents. It has been postulated that propagating ACPs could trigger secondary convection when ensuing gravity currents undercut and mechanically lifts warm air to the level of free convection. Detailed understanding of ACP dynamics has been stymied by the lack of high-resolution field data or numerical simulations, in particular in cases where a cold-pool induced gravity current is propagating in an ambience with a mean flow. As such, and motivated by observations of ACPs in the Bay of Bengal during recent MISO-BOB field studies, laboratory experiments we...

The Indian Ocean has been warming rapidly over the last few decades. However, this warming is not uniform, with the South Indian Ocean (SIO, south of 5S) exhibiting the strongest warming after 2000, an abrupt reversal from the cooling trend observed until the late 20th Century. Increased Indonesian throughflow (ITF) into the Indian Ocean during the recent climate hiatus was considered to be the primary reason for this SIO warming. Here, we show that the role of ITF on the IO decadal variability has reduced considerably after 2010. We find that the warming of the SIO during the climate hiatus (1998-2010) resulted in a weaker Mascarene High and decoupled it from the Southern Ocean atmospheric variabilities. Subsequently, while the Pacific Ocean subtropical gyre continued to migrate poleward in response to the anthropogenic warming in the Southern Ocean, it stalled in the Indian Ocean. This caused a three-fold increase in the Tasman inflows into the Indian Ocean, compensating for the weak...

 It is well known that heatwaves are influenced by both atmospheric and land-surface forcings. As the climate warms, both these forcings are likely to change. To clarify the role of each these forcings on the intensity-duration-frequency (IDF) characteristics of heatwaves, we use an idealised to study the dry static energy (DSE) budget of heatwaves, and how the sources and sinks of DSE are affected when atmospheric opacity and Bowen ratio are separately changed. Furthermore, we will look at how the changing energetics impacts the IDF characteristics and return times of heatwaves. Since the heatwaves in this model are primarily driven by the circulation, this configuration also provides insight into the character oF atmospheric macroturbulence near the tail of the distribution.