Uniqueness of the phase transition in many-dipole cavity QED systems

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• Ahsan Nazir (The University of Manchester)
• Wednesday 27 January 2021, 16:00-17:00
• https://bham-ac-uk.zoom.us/j/87933879601?pwd=V0ttWlJWQVIwVldrNVdJU2gwR1BBZz09.

If you have a question about this talk, please contact Dr Miguel Navarro-Cia.

Zoom Meeting ID: 879 3387 9601. Passcode: 9a976i

The possibility of a superradiant phase transition is one of the most surprising collective phenomena to have been predicted in light-matter physics. It is however, still debated, because of numerous apparently conflicting no-go and counter no-go theorems which are proven in different gauges. It has been suggested for example, that artificial systems are required to circumvent the no-go theorem. We show that a unique phase transition does occur in archetypal many-dipole cavity QED systems, and that it manifests unambiguously via a macroscopic gauge-invariant polarisation. We show further that the gauge choice controls the extent to which this polarisation is included as part of the radiative quantum subsystem and thereby determines the degree to which the abnormal phase is classed as superradiant. This resolves the long-standing paradox of no-go and counter no-go theorems for superradiance, which refer to different definitions of radiation and are therefore not contradictory, but actually equivalent. A separate issue is that each gauge provides a different two-level approximation of the material dipoles. Our approach enables characterisation of physical behaviour in terms of any chosen material and radiation subsystems, using any of the non-equivalent approximate models. We provide exact numerical results for a finite number of dipoles, exhibiting clear precursors to the phase transition and demonstrating how accurate approximate predictions can be identified. Arbitrary-gauge QED therefore eliminates all apparent inconsistencies in the description of many-dipole cavity QED systems and their thermodynamic phases.

This talk is part of the Metamaterials and Nanophotonics Group Seminars series.

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