October 2017
Abstracts of the QSIT Lunch Seminar, Thursday, October 5, 2017
Signatures of a dissipative phase transition in photon correlation measurements
Thomas Fink – Quantum Photonics (Imamoglu group), ETH Zurich
Understanding and characterizing phase transitions in driven-dissipative systems constitutes a new frontier for many-body physics. A generic feature of dissipative phase transitions is a vanishing gap in the Liouvillian spectrum, which leads to long-lived deviations from the steady-state as the system is driven towards the transition. Here, we show that photon correlation measurements can be used to characterize the corresponding critical slowing down of nonequilibrium dynamics.
We focus on the extensively studied phenomenon of optical bistability in GaAs cavity-polaritons, which can be described as a first-order dissipative phase transition. Increasing the excitation strength towards the bistable range results in an increasing photon-bunching signal along with a decay time that is prolonged by more than nine orders of magnitude as compared to that of low density polaritons. In the limit of strong polariton interactions leading to pronounced quantum fluctuations, the mean-field bistability threshold is washed out. Nevertheless, the functional form with which the Liouvillian gap closes as thermodynamic limit is approached provides a signature of the emerging dissipative phase transition. Our results establish photon correlation measurements as an invaluable tool for studying dynamical properties of dissipative phase transitions without requiring phase-sensitive interferometric measurements.
Quantum contextuality tests with a single trapped-ion qutrit
Maciej Malinowski – Trapped Ion Quantum Information (Home group), ETH Zurich
I will discuss recent experiments generating correlations beyond those allowed by purely classical models. Working within a framework known as "quantum contextuality", we reach conclusions similar to Bell tests, but on a local system and without invoking entanglement. For this, we make use of a single qutrit encoded into electronic energy levels of a single ion in a cryogenic surface-electrode trap.
In a first experiment we demonstrate the sustained generation of stronger-than-classical correlations by performing over 50 million consecutive measurements on the same qutrit. We do this with a single, self-correcting sequence, randomising the measurement settings on the go. Our results violate a state-independent inequality by 236 standard deviations.
In the second experiment we tackle the KCBS inequality and its extensions, which are, by some accounts, the most fundamental contextuality structures. We perform the first systematic study of those inequalities, finding violations of classicality while increasing the number of measured observables from 5 all the way to 61.