1pm AEDT  Thursday Nov 28 

Zoom link         Meeting ID: 846 3409 5447        Passcode: theophys

Abstract: The detection of gravitational waves from merging black hole binaries triggered a search for an efficient theoretical description. One such approach is a field theoretical description of interacting Kerr black holes in terms of massive higher spin fields. In this talk, we shall give a brief introduction to some necessary ingredients of this approach, such as worldline description of Black Holes, spinor helicity formalism for scattering amplitudes and higher spin gauge theories. Then we shall show how to construct an action that reproduces the cubic interactions between Kerr Black Holes and Gravitational Waves.

Thursday Nov 7 1pm AEDT 

Click here  to watch the recording on the AIP YouTube channel.

Abstract:  Frustration-free quantum models represent a class of models where the Hamiltonian is a sum of local projectors, and the ground state minimizes the energy of all these local projectors simultaneously. However, "to know whether a given Hamiltonian is frustration-free or not" is a question that is widely believed to be intractable, as it belongs to a k-QSAT (quantum satisfaction problem) known to be QMA_1-complete -- a quantum analogue of the NP-complete class for classical problems. Beyond a simple "yes or no," condensed matter physics also seeks to understand the nature of ground states that may arise from these models. In this work, we introduce an algorithm that not only determines frustration-free ground states but also constructs them as "cluster-projected matrix product states" (MPS). Our method progressively builds the MPS by starting with a single-site tensor and incrementally adding tensors for each site. Each tensor element is chosen by applying projectors that impose local constraints, ensuring the wave function aligns with the desired configuration of local basis states. This algorithm is applicable to models such as the Motzkin and Fredkin chains, zigzag frustrated magnets, and lattices like diamond, triangular, kagome, and square structures. Our approach captures gapless and long-range entangled ground states for systems up to a hundred sites, depending on the model, and works for both one- and two-dimensional systems. Notably, it can even describe the long-range entangled spin liquid state in the two-dimensional toric code model, distinguishing all the topological sectors very easily.
[1]  H. Saito and C.Hotta,  Phys. Rev. Lett, 132 , 166701 (2024)
[2]  H. Saito and C. Hotta, arXiv:2406.12357.

Click here  to watch the recording on the AIP YouTube channel.  

Abstract: The magnetization of an ensemble of magnetic nanoparticles changes over time in response to external applied fields. This is important as the time it takes for the magnetization to relax impacts the particles’ use in biomedical applications, such as killing cancerous tumors when an AC field is applied. Although the calculation of magnetic relaxation times is a problem in non-equilibrium thermal physics dating back to the 1920s, most calculations are for special cases only. Debye calculated the relaxation time when it is due to physical rotation (Brownian motion) of nanoparticles. However, the relaxation can also be due to internal motion of magnetic dipoles within a stationary particle (Néel relaxation). In this talk I will discuss our efforts to extend the calculation of Brownian and Néel relaxation times to more general situations. A combination of our new, analytic expressions [1,2] and numerical simulations will be presented.

[1] Chalifour, A. R., Davidson, J. C., Anderson, N. R., Crawford, T. M., & Livesey, K. L., Phys. Rev. B 104, 094433 (2021).

[2] Davidson, J. C., Anderson, N. R., & Livesey, K. L., Phys. Rev. B, accepted (2024).

Click here  to watch the recording on the AIP YouTube channel.   

Abstract: Quantum mechanics and relativity are two foundational pillars of modern physics that are believed to provide an underlying description of all natural phenomena. Several quantum effects in relativistic systems, especially in the context of the relativistic theory of gravitation, are not fully understood theoretically, as well as lack empirical verification. From the observational point of view, the major obstacle lies in the fact that any detectable gravitational effect manifests through local interactions of macroscopic objects for which quantum effects are notoriously difficult to detect. On the other hand, for microscopic systems where quantum effects are prominent, the gravitational interactions are miniscule and are undetectable with current and feasible future technologies.

 In this talk, I will discuss how global (topological) features of the lowest energy states (the vacuum states) of relativistic quantum systems, rather than their local interactions, predict new phenomena potentially detectable with present experimental technologies. Namely, I will argue for the existence of new particle states and charge and parity (CP) violating interactions within the well-established theories of particle physics (the Standard Model) and gravity (Einstein's theory of General Relativity). 

Click here  to watch the recording on the AIP YouTube channel.

Abstract: The spin-1/2 Heisenberg quantum spin chain is a paradigmatic model of theoretical physics. We consider the problem of preparing exact eigenstates of this model on a quantum computer. We begin by briefly reviewing the basics of coordinate Bethe ansatz and quantum computing. We then describe an efficient construction of Dicke states, and finally its generalization to Bethe states. The algorithm is explicit, deterministic, and does not use ancillary qubits. 

27 June 1pm AEST

Click here  to watch the recording on the AIP YouTube channel.

Abstract: Conical intersections occur between energy bands in certain two-dimensional periodic lattices. Wavepacket dynamics in the vicinity of a conical intersection mimics that of relativistic spinor particles, where the role of the particle spin is played by an internal spin-like "pseudospin" degree of freedom within the lattice. I will discuss intruiging relations between this pseudospin and other forms of angular momentum, focusing on the pseudospin-1/2 "Dirac cone" between two bands, which occurs in the electronic band structure of graphene. I will then show how Dirac cones can be generalised to pseudospin-1 and pseudospin-2 conical intersections using relatively simple and experimentally-feasible periodic lattice potentials. These findings are applicable to a variety of systems admitting mean field dynamics governed by Schrödinger-type equations, including photonic crystals, Bose-Einstein condensates in optical lattices, and exciton-polariton condensates in structured microcavities.

9 November 7pm AEDT 

Click here  to watch the recording on the AIP YouTube channel.

Abstract: In this talk I present our solution to the information paradox published in Phys. Rev. Lett. 128 (2022) 11, 111301 and Phys. Lett. B 827 (2022) 136995 (see EPL 139 (2022) 4, 49001 for a review). Long wavelength quantum gravitational effects allow the interior state of the black hole to influence Hawking radiation, leading to unitary evaporation. I explain why the Mathur theorem is evaded due to the complex nature of the Hawking radiation superposition state. 

12 October 11am AEDT 

Click here  to watch the recording on the AIP YouTube channel.

Abstract: Everything in our Universe is virtually only made up of matter and not antimatter. This baryon asymmetry of the Universe cannot be explained within the Standard Model of particle physics. This asymmetry drives a lot of new physics models. I will explain how this asymmetry can be generated in a few different new physics models. I will then focus on particle-antiparticle oscillations in the early Universe as a source of the asymmetry. 

Thursday 17 Aug 1pm AEST 

Click here  to watch the recording on the AIP YouTube channel.

Abstract: Scattering is described in physics by the relations between asymptotically ingoing and outgoing states. The corresponding notions in Einstein’s theory of gravity are the past and future light-like infinities as introduced by Roger Penrose. They rely on the conformal structure of the Lorentz manifold describing the system. In this talk, we will discuss how conformal methods can be used to describe the scattering of gravitational waves geometrically and numerically.