June 2018

Abstracts of the QSIT Lunch Seminar, Thursday, June 7, 2018

Quantum clocks: applications and advantages over classical ones

Mischa Woods – Quantum Information Theory (Renner group), ETH Zurich

Clocks have proven to be extremely useful for our everyday life and devices. But what can we use quantum clocks for, what are their fundamental limitations, and finally, are they more efficient than classical clocks? In this talk, after introducing the notion of a finite dimensional quantum clock, we will study two related but different tasks: how well they can autonomously control another quantum system (e.g. perform the gates in a quantum computation) and how well they can tell the time by producing a periodic stream of “ticks”. For the former, we will prove they can do so while only enduring an exponentially small error in clock dimension and clock energy; while for the latter, we will derive the optimal quantum clock as a function of its dimension and compare it to the optimal classical clock of the same dimension; thus proving a separation in space resources between the optimal quantum and classical clocks. We will briefly comment on how this is related to to other fundamental limitations, such as the inescapable entropy gain of a clock, as a function of its accuracy.

Transport of neutral optical excitations using electric fields

Ovidiu Cotlet – Quantum Photonics (Imamoglu group), ETH Zurich

Polaritons are composite bosonic particles formed by hybridization of propagating photons and quanta of polarization waves in a solid. Remarkably, these polariton excitations combine an ultra-light effective mass dictated by their photonic content with a sizable interparticle interaction strength stemming from their excitonic character. This unique combination allows for the realization of a driven dissipative interacting boson system that has been shown to exhibit a myriad of many-body phenomena such as nonequilibrium condensation, superfluidity and the Josephson effect.

Despite this progress, the realization of topological states of polaritons remains elusive and finding means to implement the required effective gauge fields is a challenge for theory and experiment. Exciton polaritons are charge-neutral particles and, hence, it is not possible to couple them directly to dc electric or magnetic fields.

In this talk I will show that it is possible to exploit the polariton interaction with charge carriers to mediate such a coupling[1]. In a simple picture, excitons in the presence of degenerate electrons can be considered as mobile impurities interacting with a Fermi sea. To lower its energy, the exciton can create a polarization cloud in its environment forming a new quasiparticle known as attractive exciton-polaron. Remarkably, although polarons are charge-neutral optical excitations, their interaction with the Fermi sea forces them to follow the motion of charge carriers in an electric or magnetic field. Indeed, we find that the drag conductivity of polarons can ideally be of the same order as the electron conductivity, promising the realization of sizable photonic gauge fields. From a more general perspective, we describe a novel transport mechanism that is mediated by interactions beyond the perturbative regime. This paves the way to probing quantum many-body effects using transport measurements. Our results apply generically to polaron systems at low density in cold atom or condensed matter settings.


[1] Cotlet, O., Pientka, F., Schmidt, R., Zarand, G., Demler, E., & Imamoglu, A. (2018). Transport of neutral optical excitations using electric fields. arXiv preprint arXiv:1803.08509