# Theoretical Physics

**30 April 2014**

**Time:**15:00 to 16:00

**Location:**EC Stoner SR 8.60

Adil Gangat (Brisbane, Australia)

Topics in open quantum systems: macroscopic quantum effects, many-body quantum simulation, parent Hamiltonians of quantum fields, and dissipation-induced quantum oscillations

I plan to give an overview of my past and current work in the area of open quantum systems.

The theory of environmentally induced decoherence successfully accounts for the emergence of classical physics at macroscopic scales inspite of the manifestly quantum nature of the microscopic world. In a nutshell, the explanation is as follows: larger systems contain more degrees of freedom and are therefore more strongly coupled to their environments, thereby making the time and length scales of quantum phenomena in those systems inaccessibly short. One of the goals in the field of optomechanics is to try to bring macroscopic objects observably within the quantum domain by sufficiently engineering measurement capability and environmental loss. My work in this field (Phys. Rev. A 88, 063846 (2013) and New J. Phys. 13 (2011) 043024) dealt with establishing fundamental constraints for experiments to observe energy quantization and measurement-induced state collapse in a mechanical oscillator.

In the domain of circuit QED, quantum simulation is a hot topic and interest has recently developed in using this platform for simulating many-body physics. The attractive Bose-Hubbard model, which is the Bose-Hubbard model with on-site interactions that are attractive instead of repulsive, is a many-body Hamiltonian whose experimental realization has proved elusive thus far. Separately, entangling large numbers of linear oscillators is of fundamental and applied interest, but feasible protocols to achieve this are lacking. My past work in circuit QED (Phys. Rev. X 3, 031009 (2013)) dealt with proposing a simulation of the attractive Bose-Hubbard model that is feasible with existing circuit QED technology, and further using that simulation to deterministically prepare a large number of microwave resonators in a relatively robust entangled state (more specifically, a W-state).

In quantum information an algorithm exists for finding parent Hamiltonians of a quantum state defined on a lattice, but finding parent Hamiltonians of quantum fields is an open problem. My recent work with Tobias Osborne (in preparation) makes progress on finding parent Hamiltonians of quantum fields by exploiting the link between quantum fields and open quantum systems.

Currently I am working on proposing a way (using circuit QED) to observe dissipation-induced quantum oscillations. Spatial oscillations of quantum probability distributions are conventionally understood as arising from interference between two or more superposed eigenstates of a quantum system, with the frequency of the oscillations being fixed by the eigenspectrum and dissipation to an environment as being detrimental to the oscillations. Here the goal is to observe the counterintuitive scenario where quantum probability oscillations arise only in the presence of dissipation and the frequency of the oscillations is fixed only by the dissipation rate.

Finally, I have some interest concerning the interpretation of quantum theory; I currently find myself sympathetic to the interpretation known as Quantum Bayesianism, which treats the quantum state as subjective, and I like to spend some time trying to understand it better and thinking about objections to and clarifications for it.

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