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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 2023 Committee 

  • Chair: Archil Kobakhidze (Sydney)
  • Vice-chair: Jacinda Ginges (UQ)
  • Secretary: Murray Batchelor (ANU)

Program Committee:

Murray Batchelor (ANU), Nicole Bell (Melbourne), Krzysztof Bolejko (Tasmania), Gavin Brennan (Macquarie), Eric Cavalcanti (Griffith), Susan Coppersmith (UNSW), Jacinda Ginges (UQ), Archil Kobakhidze (Sydney), Sergei Kuzenko (UWA), Karen Livesey (Newcastle), Meera Parish (Monash), Margaret Reid (Swinburne), David Tilbrook (ANU), James Zanotti (Adelaide)

News and Upcoming Events

Asia-Pacific Center for Theoretical Physics (APCTP) 

Who Are APCTP?

Link to APCTP Colloquium Series

AIP TPG Seminar Series

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

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  • 9 Aug 2023 10:28 AM | Anonymous

    Thursday 17 Aug 1pm AEST 

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

    AbstractScattering 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.

  • 6 Jul 2023 10:16 AM | Anonymous

    Thursday 13 July 1pm AEST 

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

    AbstractDark matter is an elusive substance which, despite considerable effort, continues to evade detection. In this talk we will tour through the realms of particle, nuclear, atomic, and astro-physics, surveying the rich interdisciplinary research that propels our search for dark matter. The efforts of astrophysics and cosmology provide us with the necessary cosmic context, while the fields of nuclear and atomic physics ground us, informing and guiding the search for dark matter in the laboratory. We will explore the compelling evidence for the existence of dark matter and how we can best determine its true nature. 

  • 18 Apr 2023 12:48 PM | Anonymous

    Peter Drummond, Margaret Reid, Alex Dellios, Bogdan Opanchuk, Simon Kiesewetter, and Run-Yan Teh

    Thursday 27 April 1pm AEST

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

    Abstract:   There are experimental claims of computational advantage on quantum computers. This raises  theoretical questions of validation for the random-number generation tasks that are solved. How does one verify the output? Are the answers obtained even correct, and how can one test this in practise?

    Brute-force computational verification is not possible. No classical computer is large or fast enough for this, without taking billions of years. Even computing the distributions is exponentially hard, not just from time and memory, but also due to precision constraints, as there is insufficient precision.

    For Gaussian boson sampling tasks, we show that simulations in quantum phase-space can solve this, by generating any diagnostic that is measurable. This uses an FFT binning algorithm to obtain computable statistics, with up to 16,000 qubits in large test cases, far larger than in any experiment.

    The result is that recent experimental data from China and USA is significantly different from theory, with over 100 standard deviations of discrepancy for some measured output statistics. Possible explanations are explored, but this is a nontrivial physics problem, and we do not have a complete explanation.

    This does not disprove the computational advantage claims. These are very hard tasks, and we do not directly generate the required numbers. However, the outputs do not survive the chi-squared tests one would normally use to test validity of random numbers, as used in numerous cryptography applications.

    Finally, we point out how similar techniques may in future be useful in testing other quantum network and computer designs. The principle is to use scalable methods that generate probabilities, rather than trying to use naive algorithms on classical machines, which are now totally impractical.

  • 29 Mar 2023 11:35 AM | Anonymous

    Thursday 6 April 11am AEST

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

    Abstract:   Our understanding of the structure of matter, encapsulated in the Standard Model of particle physics, is that protons, neutrons, and nuclei emerge dynamically from the interactions of underlying quark and gluon degrees of freedom. I will describe how first-principles theory calculations have given us new insights into this structure, including recent predictions of the contributions of gluons to the pressure and shear distributions in the proton, which will be measurable at the planned Electron-Ion Collider. I will also discuss studies of light nuclei which provide insights relevant to dark matter direct detection experiments and to searches for evidence of the Majorana nature of neutrinos through neutrinoless double beta decay. Finally, I will explain how provably exact machine learning algorithms are providing new possibilities in this field.

  • 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.

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Recorded Talks

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