November 2019
Abstracts of the QSIT Lunch Seminar, Thursday, November 7, 2019
Direct field correlation measurement on the electromagnetic ground state
Francesca Fabiana Settembrini - Quantum Optoelectronics Group (Faist group), ETH Zurich
According to quantum mechanics, the ground state of electromagnetic radiation is characterised by the presence of fluctuating electric fields. The experimental proof of their existence came indirectly from the observation of phenomena such as Lamb shift, Casimir effect and spontaneous emission. Only recently, Riek et al. [1,2] succeeded in probing directly the excess in the shot noise affecting their system due to vacuum field fluctuations. A direct method of measuring the spectral properties of these fluctuating fields in situ has been missing so far. Commonly, the spectral composition of a quantum light source is determined through its electric field correlation function. Its measurement is usually performed in an interferometer employing absorption-based detectors, which are not compatible with the detection of vacuum fields.
In this work, we present the first direct electric field correlation measurement on the electromagnetic vacuum state at THz frequencies [3]. The measurement has been implemented by probing the amplitude of the multimode THz vacuum field at two space-time points, through a combination of electro-optic detection and two ultrashort probe pulses mutually delayed in time. The spatial dependence of the field coherence has also been investigated. The experiment has been performed at cryogenic temperatures, where the modes within the detection bandwidth present sub-unity photon occupation number. For comparison, we present the results obtained by exposing the detection crystal to the radiation of a blackbody at 45 K and 300 K. We also analyse how the detection bandwidth depends on the experimental parameters (e.g. dimension of probing beams), which determine the probed space-time volume.
Finally, we will present an alternative measurement scheme, which employs the combination of a metallic cavity with a polymer-based THz detector. The high electro-optic coefficient of the non-linear polymer and the field confinement provided by the cavity would lead to a significant increase in the measurement sensitivity.
[1] C. Riek, D. V. Seletskiy, A. S. Moskalenko, J. F. Schmidt, P. Krauspe, S. Eckart, S. Eggert, G. Burkard, and A. Leitenstorfer, Science 350, 420-423.
[2] C. Riek, P.Sulzer, M. Seeger, A. S. Moskalenko, G. Burkard, D. V. Seletskiy, and A. Leitenstorfer, Nature 541, 376-379.
[3] I.- C. Benea-Chelmus, F. F. Settembrini, G. Scalari, J. Faist, , Nature 568(7751):202-206
Light at a stalemate - Experimental observation of a polarization vortex at a bound state in the continuum
Hugo Doeleman - Hybrid Quantum Systems Group (Chu group), ETH Zurich
Bound states in the continuum (BICs) are modes that, although energy and momentum conservation allow coupling to leaky waves, do not show any radiation loss. Hence, energy can theoretically be stored in the mode for infinite time. Originally discovered by von Neumann for electrons in exotic potentials, such states have been recently shown to exist for e.g. photonic and acoustic waves.
Recently it was proposed theoretically that BICs occur at points where the far-field polarization of the radiated waves shows a vortex, i.e. points where the polarization is undefined . In this work, we verify this claim experimentally. We fabricate a SiN grating and show that it supports an optical BIC around 700 nm wavelength. We then perform polarimetry measurements to map the far-field polarization at every angle and wavelength, demonstrating the existence of a vortex at the BIC.