The Australian Institute of Physics THEORETICAL PHYSICS (TPG)

Dual-unitary circuits as minimal models for quantum many-body dynamics, Pieter Claeys (MPI, Dresden)

Tue, 25 Mar 2025 07:10:34 GMT

7pm AEST Thursday April 10

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Abstract: In the past years dual-unitary circuits have gained intense attention as minimal models for quantum many-body dynamics. Dual-unitary models are characterized by an underlying space-time duality, which allows for exact calculations of thermalization, scrambling and entanglement dynamics. Next to these exact results on their dynamics, space-time duality allows aspects of quantum chaos and connections to random matrix theory to be explicitly established. In this way dual-unitary circuits present a rare class of models that are both exactly solvable and provably chaotic, as well as being tailor-made for experimental realization in digital quantum computing setups. In this talk I will review dual-unitarity and highlight recent developments extending dual-unitarity.

A strange exchange: paraparticles and where to find them, Kaden Hazzard (Rice)

Fri, 31 Jan 2025 04:50:26 GMT

1pm AEDT Thursday Feb 6

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Abstract: All known particles are fermions or bosons. While exceptions are known in 2D space (anyons), in 3D fermions and bosons were widely believed to be the only possibilities. In this talk, I will describe our results [Z Wang and KRA Hazzard, Nature 637, 314 (2025)] showing it is possible to have particles that are inequivalent to fermions and bosons, known as paraparticles, in arbitrary dimension. I will describe how these emerge in solvable spin models whose excitations are paraparticles. These constructions satisfy all known physical principles required of a quantum theory, including the principle of locality, i.e. that disturbing a system at some point in space does not instantaneously affect far away points. This is crucial, as powerful theorems of algebraic quantum field theory show that locality constrains particle statistics, which presented an obstacle to previous theories of paraparticles. I will describe how our construction evades these theorems' constraints, some implications, and experimental systems in ultracold matter where we may begin to search for paraparticles.

Description of interactions between Kerr black holes in terms of higher spin fields, Mirian Tsulaia (Okinawa)

Tue, 19 Nov 2024 03:49:13 GMT

1pm AEDT Thursday Nov 28

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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.

Frustration-free models and matrix product state solutions, Chisa Hotta (Tokyo)

Tue, 29 Oct 2024 01:48:04 GMT

Thursday Nov 7 1pm AEDT

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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.

Magnetic relaxation times for magnetic nanoparticles, Karen Livesey (Newcastle)

Wed, 16 Oct 2024 21:14:57 GMT

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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).

Topology of vacuum states, quantum gravity and particle physics, Archil Kobakhidze (Sydney)

Tue, 01 Oct 2024 00:09:25 GMT

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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).

Preparing exact eigenstates on a quantum computer, Rafael Nepomechie (Miami)

Tue, 16 Jul 2024 23:32:27 GMT

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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.

Conical intersections and angular momentum, Daniel Leykam (Singapore)

Mon, 17 Jun 2024 05:04:30 GMT

27 June 1pm AEST

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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.

Why quantum correlations are shocking, Michael Hall (ANU)

Mon, 29 Apr 2024 01:04:13 GMT

30 May 1pm AEST

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Abstract: An accessible argument is given for why some correlations between quantum systems boggle our classical intuition. The argument relies on simple properties of joint probabilities, and recovers the standard experimentally-testable Bell inequality in a form that applies equally well to correlations between six-sided dice and between photon polarizations. The observed violation of this inequality implies that the quantities measured on one system cannot have a joint probability distribution that is invariant with respect to the choice of measurement made on a distant system. The possible but extraordinary physical mechanisms underlying this result -- intrinsically incompatible observables, faster-than-light influences and constrained experimental choice -- are briefly discussed. The talk will be at a level suitable for a broad audience.

Quantum hair and the information paradox, Xavier Calmet (Sussex)

Fri, 03 Nov 2023 03:29:50 GMT

9 November 7pm AEDT

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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.

Why Are We Here? Matter-Antimatter Asymmetry Of The Universe, Seyda Ipek (Ottawa)

Thu, 05 Oct 2023 00:28:34 GMT

12 October 11am AEDT

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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.

The scattering of gravitational waves: a geometric perspective, Joerg Frauendiener (Otago)

Wed, 09 Aug 2023 00:28:09 GMT

Thursday 17 Aug 1pm AEST

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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.

Exploring dark matter detection across interdisciplinary frontiers, Jayden Newstead (Melbourne)

Thu, 06 Jul 2023 00:16:19 GMT

Thursday 13 July 1pm AEST

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Abstract: Dark 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.

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Validating quantum computers: are they correct? Peter Drummond (Swinburne)

Tue, 18 Apr 2023 02:48:02 GMT

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

Thursday 27 April 1pm AEST

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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.

From quarks to nuclei: computing the Standard Model, Phiala Shanahan (MIT)

Wed, 29 Mar 2023 00:35:28 GMT

Thursday 6 April 11am AEST

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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.

Taming the Rotating Wave Approximation, Daniel Burgarth (Macquarie)

Thu, 10 Nov 2022 04:25:28 GMT

Thursday 17 Nov 1pm AEDT

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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.

Spin transfer torques and spin-Hall effect due to the bulk states of topological insulators, Dimi Culcer (UNSW)

Wed, 19 Oct 2022 22:25:06 GMT

Thursday 27 Oct 1pm AEDT

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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.

Light-matter coupled quantum systems in flatland, Meera Parish (Monash)

Wed, 28 Sep 2022 23:09:41 GMT

Thursday 6 Oct 1pm AEST

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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.

What’s wrong (and right) with these elements of reality? Margaret Reid (Swinburne)

Tue, 06 Sep 2022 03:52:54 GMT

Thursday 15 Sept 1pm AEST

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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.

Noncommuting charges: Bridging theory to experiment, Shayan Majidy (Waterloo, Canada)

Sun, 31 Jul 2022 07:06:32 GMT

Thursday 18 August 1pm AEST

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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.

Unified framework on conductivities as a response to the time-dependent gauge field, Masaki Oshikawa (ISSP, Tokyo)

Wed, 13 Jul 2022 12:28:41 GMT

Thursday 21 July 1pm AEST

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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.

Searching for Dark Matter Scattering, on Earth and in the Stars, Nicole Bell (Melbourne University)

Wed, 22 Jun 2022 09:22:35 GMT

Thursday 30 June 1pm AEST

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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.

The Life and Death of Turbulence, Nigel Goldenfeld (UCSD)

Mon, 23 May 2022 08:34:27 GMT

Thursday 9 June 1pm AEST

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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.

Realization of a discrete time crystal on IBM's quantum computer, Stephan Rachel (Melbourne University)

Thu, 05 May 2022 22:47:42 GMT

Thursday 19 May 1pm AEST

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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.

Latest developments and challenges in neutrino physics, Srubabati Goswami (PRL, Ahmedabad, India)

Wed, 20 Apr 2022 22:53:16 GMT

Thursday 28 April 2pm AEST

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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.

Can a qubit be your friend? Why experimental metaphysics needs a quantum computer, Howard Wiseman (Griffith University)

Fri, 18 Mar 2022 00:56:01 GMT

Thursday 24 March 1pm AEDT

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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.

photo credit: ANDY AARON/IBM RESEARCH/FLICKR (CC BY-ND 2.0)

Composite quantum particles at the interface with gravity - foundations and new insights, Magdalena Zych (UQ)

Tue, 22 Feb 2022 23:18:06 GMT

Thursday 3 March 1pm AEDT

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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.

Atom-by-atom engineering of novel states of matter, Cristiane Morais Smith (Institute for Theoretical Physics, Utrecht University, The Netherlands)

Thu, 25 Nov 2021 21:59:22 GMT

Thursday 2 Dec 7pm AEDT

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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.

Dynamical control of a non-Hermitian superconducting qubit, Weijian Chen (Washington University in St. Louis)

Mon, 15 Nov 2021 03:00:00 GMT

Thursday 18 November, 11am AEDT

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Abstract: Open systems with loss or gain, described by effective non-Hermitian Hamiltonians, have attracted great attention in recent years. Such systems in general have complex energies and nonorthogonal eigenstates, and their degeneracies are known as exceptional points. The complex energies near an exceptional point form a Riemann manifold, whose topology enables a new control method and has found applications in energy transport and mode switch. In this talk, I will present our recent work on dynamical control of a non-Hermitian superconducting qubit. By varying the Hamiltonian parameters in real time to encircle an exceptional point, we observe that the qubit initialized at one eigenstate is transported to another eigenstate. We further study the chiral geometric phase associated with quantum coherent state transport on the Riemann manifold. In addition, I will discuss non-Hermitian physics based on Liouvillian superoperators, which goes beyond the existing Hamiltonian formalism and allows us to observe decoherence-induced exceptional points.

Quantum Measurement and Control with Massive Mechanical Oscillators, Matt Woolley, UNSW Canberra

Fri, 17 Sep 2021 06:00:37 GMT

Abstract: Massive mechanical oscillators have recently been measured and controlled in the quantum regime, providing a testbed for investigating the limits of quantum mechanics and its possible interplay with gravity. The stabilized entanglement of massive mechanical oscillators has been measured both indirectly and directly. Further, sensing of the motion of a mechanical oscillator beyond conventional quantum limits has been demonstrated. There exist further proposals for the realization of enhanced force sensing and many-body quantum state control in optomechanics, and problems in optomechanics have spurred the development of novel theoretical techniques.

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Quantum Nature of Gravity in the Lab, Sougato Bose (University College London)

Mon, 13 Sep 2021 23:01:06 GMT

Quantum Nature of Gravity in the Lab: Assumptions, Implementation and Applications on the Way

Abstract: There is no empirical evidence yet as to “whether” gravity has a quantum mechanical origin. Motivated by this, Sougato Bose presents a feasible idea for testing the quantum origin of the Newtonian interaction based on the simple fact that two objects cannot be entangled without a quantum mediator. He shows that despite its weakness, gravity can detectably entangle two adjacent micron sized test masses held in quantum superpositions even when they are placed far apart enough to keep Casimir-Polder forces at bay. A prescription for witnessing this entanglement through spin correlations is also provided. Further, he clarifies the assumptions underpinning the above proposal such as our reasonable definition of “classicality”, as well as relativistic causality. He notes a few ways to address principal practical challenges: Decoherence, Screening EM forces and Inertial noise reduction. He also describes how unprecedented compact sensors for classical gravity (including meter scale sensors for low frequency gravitational waves) will arise on the way to the above grand goal.

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PT Symmetry, Carl Bender (Washington University in St. Louis)

Thu, 02 Sep 2021 01:13:21 GMT

Abstract: By using complex-variable methods one can extend conventional Hermitian quantum theories into the complex domain. The result is a huge and exciting new class of non-Hermitian parity-time-symmetric (PT-symmetric) theories that still obey the fundamental laws of quantum mechanics. These new theories have remarkable physical properties, which are currently under intense study by theorists and experimentalists. Many theoretical predictions have been verified in recent beautiful laboratory experiments.

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The Thermodynamics of Clocks, Gerard Milburn (University of Queensland)

Fri, 20 Aug 2021 08:00:00 GMT

Abstract: All clocks, periodic and non-periodic, are open dissipative systems driven from thermal equilibrium so that the Helmholtz free energy is increased. In this talk Gerard Milburn discusses the thermodynamic constraints for classical and quantum clocks. He also discusses clocks driven not by work but by information extraction and makes a connection to Rovelli's thermal time hypothesis as a proposed solution to the problem of time in quantum gravity.

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Quantum stochastic resonance of individual Fe atoms, Susan Coppersmith (UNSW)

Mon, 19 Jul 2021 03:13:39 GMT

Abstract: Stochastic resonance, where noise synchronizes a system’s response to an external drive, is a phenomenon that occurs in a wide variety of noisy systems ranging from the dynamics of neurons to the periodicity of ice ages. In this webinar Susan Coppersmith will present theory and experiments on a quantum system that exhibits stochastic resonance — the quantum tunneling of the magnetization of a single Fe atom measured using spin-polarized scanning tunneling microscopy. Stochastic resonance is shown deep in the quantum regime, where fluctuations are driven by tunneling of the magnetization, as well as in a semi-classical crossover region where thermal excitations set in. Combining theory and experiment enables one to probe the dynamics on time scales shorter than can be resolved experimentally.

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Are misbehaving muons taking us beyond the Standard Model of particle physics? Raymond Volkas (Melbourne)

Thu, 17 Jun 2021 03:00:00 GMT

Abstract: After reviewing the basics of our current understanding of fundamental particles and their interactions, as enshrined in the Standard Model, Raymond Volkas briefly surveys the well-established evidence that this is an incomplete theory. The main part of the talk is then about recent measurements of the muon anomalous magnetic moment, and the rates of some B-meson decays, which point to possible further inadequacies of the Standard Model. Interestingly, the strongest current hints for this all involve muons.

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Dark Matter Puzzles from Indirect Searches, Tracy Slatyer (MIT)

Fri, 28 May 2021 01:00:00 GMT

Abstract: The nature and origin of dark matter is one of the key unresolved questions of fundamental physics. Astrophysical and cosmological data provide powerful probes of dark matter properties, although to date no signal has been confirmed. In this webinar Tracy Slatyer discusses a number of claimed possible signals of novel dark matter physics in astrophysical datasets, alternative explanations, and open questions, with a focus on the Galactic Center Excess in GeV-scale gamma rays.

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