Theory Seminar

Random unitaries in extremely low depth

Random unitaries serve as indispensable toy models for complex processes in quantum many-body physics and form the backbone of numerous components of quantum technologies. This idea raises a fundamental question: In what time-scale (i.e. circuit depth) can a quantum circuit behave like a random unitary? I will present recent work in which we show that local quantum circuits can realize random unitaries in exponentially shorter time-scales than previously thought. We prove that random quantum circuits on any geometry, including a 1D line, can form so-called approximate unitary designs over n qubits in log n depth. 
 
Similarly, we show that 1D circuits can form pseudorandom unitaries, a recent object of interest in quantum cryptography, in poly(log n) circuit depth. Both depths can be further exponentially reduced using long-range connectivity. These shallow quantum circuits have low complexity and create only short-range entanglement, yet are indistinguishable from unitaries with exponential complexity. Applications of our results include:
  • Demonstrating fast thermalization of the magic and output distributions of random quantum circuits. 
  • Efficient implementations of classical shadow tomography.
  • New superpolynomial quantum learning advantages. 
  • Establishing computational hardness for recognizing phases of matter in quantum experiments. 
     
Time pending, I will also discuss recent extensions of our results to symmetric quantum systems and time-reversal experiments.

 
 
Date:
Location:
Zoom
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Universal Signatures of First-Order Phase Transitions in Cosmology

The universe may still be hiding traces of dramatic events long after its earliest moments. In this talk, I will explore how first-order phase transitions — processes where bubbles of a new vacuum spontaneously nucleate and expand — can imprint both scalar and tensor perturbations on the cosmic microwave background (CMB) and large-scale structure (LSS) of the universe. 
 
As I will show, precise CMB and LSS measurements already have placed strong limits on such phase transitions, even if they happen in a completely hidden dark sector that interacts only through gravity. Future CMB temperature and B-mode measurements, however, can still have a chance to observe these phase transition signals. Moreover, when vacuum energy starts to dominate the energy budget of the universe, it may naturally trigger a new phase transition as thermal effects fade away. In that regime, CMB anisotropies again offer a uniquely sensitive probe, providing constraints far stronger than those from Hubble expansion measurements.
Date:
Location:
CP 303
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Simulations of Gauge Theories on Quantum Computers

Gauge theories describe the fundamental interactions, but their complexity makes questions involving real-time dynamics beyond the reach of classical computation. Quantum computers open a new path by naturally representing quantum fields and evolving them in real time thus circumventing for example the sign problem that limits classical Monte Carlo methods. In this talk, we will discuss the challenges and recent progress in encoding and simulating Gauge theories on fault-tolerant quantum computers.

Date:
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Location:
CP 303
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Uncertainty-aware generative models for autonomous discovery in particle theory

Uncertainty quantification (UQ) plays a crucial role in the predictive power of nonperturbative quantum correlation functions at high precision. My research explores novel approaches to UQ in the context of parton distribution functions (PDFs), using uncertainty-aware machine learning techniques to map observables to underlying theoretical models and to navigate the complex parametric landscape of phenomenological scenarios, including the vast ecosystem of beyond-the-Standard-Model (BSM) configurations. By leveraging modern generative AI methods, I investigate how the inherent uncertainties in phenomenological extractions of collinear PDFs impact the landscape of potential New Physics models. My approach integrates explainability methods to trace underlying theory assumptions back to the input feature space - specifically the x-dependence of PDFs - thereby identifying the salient features that shape constraints and model interpretations. Additionally, this work aims to enhance the incorporation of lattice inputs in phenomenological fits and refine the consistency between lattice QCD and collider phenomenology. Together, these components point towards uncertainty-aware autonomous workflows, pushing the frontier of particle physics discovery.
 
Date:
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Location:
CP 303
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The Fermi Function, Factorization and the Neutron’s Lifetime

The neutron lifetime is a precision observable of the Standard Model probing the CKM matrix element |Vud| and beyond the Standard Model physics. For nuclear beta decay, in the region of small electron velocity or the limit of large nuclear charge Z, a Fermi function is used to account for enhanced perturbative effects. In this talk, I will present the derivation of the quantum field theoretic analog of the Fermi function valid for neutron beta decay in which neither of the aforementioned limits apply. This QFT analog is related to renormalization group effects of objects occurring in the context of a factorization formula valid in the limit of small electron mass. I will introduce this factorization formula and present results through two-loop order. The main phenomenological results are two-loop input to the long-distance corrections to neutron beta decay and an accompanying calculation of |Vud|.

Date:
Location:
Zoom
Event Series:

Temperature-Resistant Order in 2+1 Dimensions

High temperatures are typically thought to increase disorder. Here we examine this idea in Quantum Field Theory in 2+1 dimensions. For this sake we explore a novel class of tractable models, consisting of nearly-mean-field scalars interacting with critical scalars. We identify UV-complete, local, unitary models in this class and show that symmetry breaking $\mathbb{Z}_2 \to \emptyset$ occurs at any temperature in some regions of the phase diagram. This phenomenon, previously observed in models with fractional dimensions, or in the strict planar limits, or with non-local interactions, is now exhibited in a local, unitary 2+1 dimensional model with a finite number of fields.

Date:
Location:
CP 179
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Fractionally Charged Particles at the Energy Frontier

The pattern in the Standard Model (SM) charges can be traced to an ambiguity in the gauge group of the SM. The easiest way to resolve this ambiguity would be to discover a particle with electric charge that is an integer multiple of e/6 (beyond +/- e, 0). The discovery of such fractionally charged particles would also challenge and potentially rule out many minimal unification models. In this talk I will review the connection between the `global structure' of the SM and the charges particles can have, then explore the phenomenology of fractionally charged particles, focusing on the constraints provided by current searches at the Large Hadron Collider (LHC). By reinterpreting existing results, we assess the bounds on various fractionally charged representations, uncovering scenarios where collider limits are unexpectedly weak or entirely absent.

 

Date:
Location:
CP 303
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Prospects of supermassive millicharged particles as dark matter

We explore the prospects of very heavy millicharged particles as candidates for some or all of dark matter (DM). Constraints on their properties like charge, dipole moment and mass are obtained by translating the current limits set by state-of-the-art nuclear recoil experiments like XENONnT and LZ. We contrast the extracted limits with those coming from astrophysical considerations, such as the decoupling of DM from regular matter during the recombination epoch and the suitability of the dynamical environments in the galactic halo for these heavy and charged beyond the Standard Model particles to remain lodged-in and available for future detections. 


 


 

Thanks! 

Date:
Location:
CP 303
Event Series:

Higher moments of parton distribution functions from Lattice QCD

The parton distribution functions (PDFs) are crucial to the understanding of the internal structure of hadrons, and their precise determination is necessary for searches of BSM physics in collider experiments. Directly accessing PDFs in Lattice QCD calculations is impossible due to the Euclidean space-time geometry. The moments of PDFs are defined in terms of local operators that can be computed in Lattice QCD, but suffer from power divergent mixing beyond a certain order due to the hypercubic symmetry of the lattice. Recently, a method was proposed [1] to use gradient flow in order to circumvent this mixing and access moments of PDFs of any order. In this talk, I will summarize this method, and present preliminary results on moments of the unpolarized isovector PDF of the pion using four stabilized Wilson fermion ensembles generated by the OpenLat [2] initiative.

[1] A. Shindler 2311.18704

[2] https://openlat1.gitlab.io

Date:
Location:
Zoom
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