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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?

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)

  • 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. 

  • 18 Mar 2022 11:56 AM | Anonymous

    Thursday 24 March 1pm AEDT

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

    Abstract: Experimental metaphysics is the study of how empirical results can reveal indisputable facts about the fundamental nature of the world, independent of any theory. It is a field born from Bell’s 1964 theorem, and the experiments it inspired, proving the world cannot be both local and deterministic. However, there is an implicit assumption in Bell’s theorem, that the observed result of any measurement is absolutely real (it has some value that is not real only to the observer who made it, or only in the ‘branch’ in which it appears). This assumption is called into question when one thinks of the observer as a quantum system (the “Wigner’s Friend” scenario), which has recently been the subject of renewed interest. In [1], I and co-workers derived a theorem, in experimental metaphysics, for this scenario. It is similar to Bell’s 1964 theorem but dispenses with the assumption of determinism. We show that the remaining assumptions, which we collectively call "local friendliness", are still predicted, by most approaches to quantum mechanics, to be violable. We illustrate this in an experiment in which the “friend” system is a single photonic qubit. In [2], I and other co-workers argue that a truly convincing experiment could be realised if that system were a sufficiently advanced artificial intelligence software running on a very large quantum computer, so that it could be regarded genuinely as a friend. We formulate a new version of the theorem for that situation, using six assumptions, each of which is violated in at least one approach to quantum theory. The popular attitude that “quantum theory needs no interpretation” is untenable because it does not indicate that any of the assumptions are invalid.

    [1] Bong et al., “A strong no-go theorem on the Wigner’s friend paradox”, Nature Physics 16, 1199 (2020).

    [2] Wiseman, Cavalcanti, and Rieffel, “A ‘thoughtful’ Local Friendliness no-go theorem”, in preparation.


  • 23 Feb 2022 10:18 AM | Anonymous

    Thursday 3 March 1pm AEDT

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

    Abstract: A major goal of modern physics is to understand and test the regime where quantum mechanics and general relativity both play a role. Until recently, new effects of this regime were thought to be relevant only at high energies or in strong gravitational fields. I will discuss how and why looking at composite particles — with internal dynamical degrees of freedom — opens new avenues and may finally enable laboratory tests of quantum and general relativistic effects.  I will also show that such particles have a natural interpretation as ideal quantum clocks, detectors, and even thermometers, and will highlight recent insights arising from this approach: e.g. that semi-classical states of free composite particles are not Gaussian but a  new class of states derived from a new uncertainty inequality — for configuration space rather than for phase space variables.

  • 26 Nov 2021 8:59 AM | Anonymous

    Thursday 2 Dec 7pm AEDT

    Click here to watch the recording on YouTube.

    Abstract: Feynman’s original idea of using one quantum system that can be manipulated at will to simulate the behavior of another more complex one has flourished during the last decades in the field of cold atoms. More recently, this concept started to be developed in nanophotonics and in condensed matter. In this talk, I will discuss a few recent experiments, in which 2D electron lattices were engineered on the nanoscale using STM manipulation of adatoms on the surface of copper. First, I will show that it is possible to control the geometry of the lattice and the orbital degrees of freedom by building different Lieb lattices. Then, I will show how to realize topological states of matter using the same procedure. We investigate the robustness of the zero modes in a breathing Kagome lattice, which is the first experimental realization of a designed electronic higher-order topological insulator, and the fate of the edge modes in a Kekule structure, upon varying the type of boundary of the sample. Finally, we will control the effective dimension of the electronic structure by creating a Sierpinski gasket, which has dimension D = 1.58. The realization of this first quantum fractal opens the path to electronics in fractional dimensions. In addition, our recent investigation of quantum transport in fractals by using photonic quantum simulators might shed some light on the issue of consciousness. 

Recorded Talks

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