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Theoretical Physics  Group (TPG)

The TPG in the AIP is focused on all areas of theoretical physics, from elementary particles in the quantum realm to the universe, and everything in between. Many, if not all, of these areas have an overlap with the other AIP topical groups. Purely theoretical studies in physics have lead to amazing technological changes in society, including computers and satellite communication.

Who Can Join the TPG?

Any members of the AIP who are interested in theoretical physics can join the TP Group as part of their AIP membership at no extra charge. To sign up to the TP Group, login to the Membership portal, then click on Theoretical Physics (TPG) under Topical Groups in your Membership Profile. Please take the time to do this as it gives the AIP a gauge of how much interest there is in TPG across Australia and beyond.

TPG 2022 Committee 

News and Upcoming Events

Asia-Pacific Center for Theoretical Physics (APCTP) 

Who Are APCTP?

Call for APEC Topical Research Program 2023

AIP TPG Seminar Series

Organisers: Murray Batchelor (ANU), Nicole Bell (Melbourne), Gavin Brennan (Macquarie), Eric Cavalcanti (Griffith), Susan Coppersmith (UNSW), Archil Kobakhidze (Sydney), Sergei Kuzenko (UWA), Karen Livesey (Newcastle), Meera Parish (Monash), Margaret Reid (Swinburne), James Zanotti (Adelaide), Magdalena Zych (UQ)

Host presenter: David Tilbrook (ANU)

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  • 10 Nov 2022 3:25 PM | Anonymous

    Thursday 17 Nov 1pm AEDT

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

    Abstract:   The Rotating Wave Approximation (RWA) is one of the oldest and most successful approximations in quantum mechanics. It is often used for describing weak interactions between matter and electromagnetic radiation. In the semi-classical case, where the radiation is treated classically, it was introduced by Rabi in 1938. For the full quantum description of light-matter interactions it was introduced by Jaynes and Cummings in 1963. Despite its success, its presentation in the literature is often somewhat handwavy, which makes it hard to handle both for teaching purposes and for controlling the actual error that one gets by performing the RWA. Bounding the error is becoming increasingly important. Recent experimental advances in achieving strong light matter couplings and high photon numbers often reach regimes where the RWA is not great. At the same time, quantum technology creates growing demand for high-fidelity quantum devices, where even errors of a single percent might render a technology useless for error-corrected scalable quantum computation.

    I will give a gentle introduction to the history of the RWA and then report a conceptually simple way of explaining it. Finally, I will show how to tame it by providing non-perturbative error bounds, both for the semi-classical case and the full quantum case.

  • 20 Oct 2022 9:25 AM | Anonymous

    Thursday 27 Oct 1pm AEDT

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

    Abstract:  Spin torques at topological insulator (TI)/ferromagnet interfaces have received considerable attention in recent years with a view towards achieving full electrical manipulation of the spin degree of freedom. The most important question in this field concerns the relative contributions of bulk and surface states to the spin torque, a matter that remains incompletely understood. Whereas the surface state contribution has been extensively studied, the contribution due to the bulk states has received comparatively little attention. I will first discuss spin torques due to TI bulk states and show that they give rise to a spin transfer torque (STT) due to the inhomogeneity of the magnetisation in the vicinity of the interface. This spin transfer torque is somewhat unconventional since it arises from the interplay of the bulk TI spin-orbit coupling and the gradient of the monotonically decaying magnetisation inside the TI. We find, likewise, that there is no spin-orbit torque due to the bulk states on a homogeneous magnetisation, in contrast to the surface states, which give rise to a spin-orbit torque via the Edelstein effect. Whereas we consider an idealised model in which the magnetisation gradient is small and the spin transfer torque is correspondingly small, I will argue that in real samples the spin transfer torque should be sizable and may provide the dominant contribution due to the bulk states. I will show that an experimental smoking gun for identifying the bulk states is the fact that the spin transfer torque has a comparable size for in-plane and out-of-plane magnetisations when the bulk states dominate, distinguishing them from the surface states, which are expected to give a much stronger torque on an out-of-plane magnetisation than on an in-plane magnetisation. I will also discuss our latest insights into the spin-Hall effect arising from TI bulk states. I will show that, contrary to popular belief, we do not expect any intrinsic spin-Hall effect due to the bulk. There is the possibility of an extrinsic spin-Hall effect, but we expect this to be destroyed near the interface, while the possibility also exists for an intrinsic spin-Hall effect to be generated near the interface. In the last part of my talk I will attempt to put together all the pieces of this rather complex puzzle.

  • 29 Sep 2022 9:09 AM | Anonymous

    Thursday 6 Oct 1pm AEST

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

    Abstract:  A strong coupling between light and matter can be achieved by embedding two-dimensional layers of semiconductor in an optical microcavity. This results in the formation of exciton-polaritons, which are hybrid part-light, part-matter particles that are capable of realising a range of quantum many-body phenomena. However, the interactions between such polaritons are still not well understood despite their fundamental role. In this talk, I will discuss recent theoretical progress in understanding the microscopic properties of polaritons. In particular, I will show how the two-dimensional geometry plays an important role and leads to highly counterintuitive results, such as light-enhanced polariton-polariton interactions.

  • 6 Sep 2022 1:52 PM | Anonymous

    Thursday 15 Sept 1pm AEST

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

    Abstract:  Einstein, Podolsky and Rosen (EPR) presented an argument that quantum mechanics is an incomplete theory. However, the argument assumes local realism which is falsifiable by Bell’s theorem. Here, we re-examine the argument, by presenting a mapping between microscopic and macroscopic Bell tests.  The macroscopic tests involve qubits based on two macroscopically-distinct coherent states and suitable unitary interactions. This compels us to address how the macro-Bell tests can be compatible with the important concept of macroscopic realism. We show that deterministic macroscopic realism is falsified by the macro-Bell tests, and therefore define the weaker assumption that we call “weak macroscopic realism”, which takes into account the dynamics associated with the choice of measurement setting. We show consistency of weak macroscopic realism with the Bell violations, as well as macroscopic versions of Greenberger-Horne-Zeilinger, Wigner’s friend and delayed-choice experiments. This brings us to deduce a macroscopic version of the EPR paradox based on weak macroscopic realism, thereby re-opening the question of the incompleteness of quantum mechanics. We then examine the measurement problem by proposing a model for measurement using simultaneous forward- and backward-propagating equations in time, derived from Q function dynamics. We demonstrate a causal consistency, and distinguish measurable from unobservable variables, which leads to models of realism and causal relations involving loops. We show that the new model supports weak macroscopic realism and explain how consistency with EPR-Bell correlations can be achieved. 

  • 31 Jul 2022 5:06 PM | Anonymous

    Thursday 18 August 1pm AEST

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

    Abstract:  Noncommuting conserved quantities have recently launched a subfield of quantum thermodynamics. In conventional thermodynamics, a system of interest and an environment exchange quantities– energy, particles, electric charge, etc.– that are globally conserved and are represented by Hermitian operators. These operators were implicitly assumed to commute with each other, until a few years ago. Freeing the operators to fail to commute has enabled many theoretical discoveries – about reference frames, entropy production, resource-theory models, etc. Little work has bridged these results from abstract theory to experimental reality. This work provides a methodology for building this bridge systematically: we present a prescription for constructing Hamiltonians that conserve noncommuting quantities globally while transporting the quantities locally. The Hamiltonians can couple arbitrarily many subsystems together and can be integrable or nonintegrable. Our Hamiltonians may be realized physically with superconducting qudits, with ultracold atoms, and with trapped ions.

  • 13 Jul 2022 10:28 PM | Anonymous

    Thursday 21 July 1pm AEST

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

    Abstract: (Optical) conductivity is one of the most important characteristics of materials. Recently, nonlinearconductivities are of increasing experimental and theoretical interest, but the subject is largely open. I will discuss our recent approach to the fundamentals of the nonlinear optical conductivities. A system under the periodic boundary condition in one direction can be regarded as a "ring". A static magnetic flux through the ring induces the quantum mechanical Aharonov-Bohm effect. Furthermore, increasing the flux over time induces a uniform electric field in the system. The uniform electric conductivity can be regarded as the response of the system to the increase of the magnetic flux. This not only allows a unified formulation of the "frequency sum rule" (f-sum rule) and "Kohn formula", which have been important in many applications of linear response theory, but their natural generalization to nonlinear conductivities at all orders.

  • 22 Jun 2022 7:22 PM | Anonymous

    Thursday 30 June 1pm AEST

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

    Abstract: The quest to identify the cosmological dark matter is one of the foremost goals of science today. Yet the very nature of dark matter makes this a formidable task. l outline the status of dark matter direct detection searches, and describe new strategies to probe dark matter scattering using existing detectors, such as the Migdal effect, with particular application to light or inelastic dark matter. Complementary information about dark matter scattering can be obtained by considering the capture of dark matter in stars. For a wide range of parameters, collisions between ambient dark matter and the constituents of a star would result in sufficient energy loss for the dark matter to become gravitationally bound to the star, with important observational consequences. I describe applications of dark matter capture in the Sun, white dwarfs, and neutron stars. 

  • 23 May 2022 6:34 PM | Anonymous

    Thursday 9 June 1pm AEST

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

    Abstract:  Turbulence is the last great unsolved problem of classical physics. But there is no consensus on what it would mean to actually solve this problem. In this colloquium, I propose that turbulence is most fruitfully regarded as a problem in non-equilibrium statistical mechanics, and will show that this perspective explains turbulent drag behavior measured over 80 years, and makes predictions that have been experimentally tested in 2D turbulent soap films. I will also explain how this perspective is useful in understanding the laminar-turbulence transition, establishing it as a non-equilibrium phase transition whose critical behavior has been predicted and tested experimentally.  This work connects transitional turbulence with statistical mechanics and renormalization group theory, high energy hadron scattering, the statistics of extreme events, and even population biology.

  • 6 May 2022 8:47 AM | Anonymous

    Thursday 19 May 1pm AEST

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

    Abstract:  Novel dynamical phases that violate ergodicity have been a subject of extensive research in recent years. A periodically driven system is naively expected to lose all memory of its initial state due to thermalisation, yet this can be avoided in the presence of many-body localization. A discrete time crystal represents a driven system whose local observables spontaneously break time translation symmetry and retain memory of the initial state indefinitely. Here, we report the observation of a discrete time crystal on a chain consisting of 57 superconducting qubits on a state-of-the art quantum computer.

  • 21 Apr 2022 8:53 AM | Anonymous

    Thursday 28 April 2pm AEST

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

    Abstract:  Results from oscillation experiments have established that neutrinos have small but non-zero mass and there is mixing between different neutrino flavours. This signals that there is Physics Beyond the Standard Model. The remaining neutrino oscillation parameters to be determined by the current and future experiments are the neutrino mass ordering, octant of the atmospheric mixing angle and the CP phase of the neutrinos.  In my talk, I will discuss the current status of the neutrino oscillation parameters, the challenges in the precise determination of the parameters and the prospects of determining these in the future experiments. I will also discuss the possibilities of probing other physics scenarios beyond the standard model in neutrino oscillation experiments. 

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Recorded Talks

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