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)

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.

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. 

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.

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: 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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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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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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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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