Nonlinear THz dynamics and quantum vacuum radiation from squeezed quantum vacua

• Applicants: Professor Dr. Dominique Bougeard; Professor Dr. Rupert Huber, since 2/2021
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2013 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 231111959

Controlling the interaction of light with electronic excitations defines a key aspect of modern quantum electrodynamics. In strongly light-matter coupled structures, the vacuum Rabi frequency characterizes the continuous emission and re-absorption of a cavity photon by the electronic resonance, which leads to the formation of cavity polaritons representing the new eigenstates of the coupled system. In terahertz (THz) nanoresonators, light-matter interaction can be made so strong that the vacuum Rabi frequency becomes equal to the carrier frequency of light. In such a setting, the quantum ground state contains a finite population of squeezed, correlated photon pairs, which is predicted to be released when the coupling strength is rapidly modulated. The anticipated physics is similar to the yet unobserved dynamical Casimir effect or Unruh-Hawking radiation of black holes. Here, we propose to explore quantum vacuum photonics of custom-cut THz nanostructures ultrastrongly light-matter coupled to cyclotron resonances in semiconductors, few-Landau-electron systems beyond the bosonic approximation, and Landau electrons in graphene. The novel research field will be pioneered in three work streams:(i) Exploiting femtosecond control schemes for light-matter interaction developed in our groups, we will investigate the strongly sub-cycle switching dynamics of squeezed quantum vacua in GaAs-based structures using broadband THz spectroscopy. This work stream will set the stage for quantum vacuum photonics in the more complex structures of work streams (ii) and (iii).(ii) We will target the quantum limit of ultrastrong coupling beyond the bosonic approximation in few-Landau-electron structures fabricated by three-dimensional nanostructuring of custom-designed THz resonators with tiny mode volumes. Highly sensitive THz quantum detection will trace the non-perturbatively nonlinear dynamics and quantum vacuum signatures at extremely low THz amplitudes, holding the prospect of few-photon THz nonlinearities of squeezed quantum vacua. (iii) Going beyond massive electron systems, we will investigate ultrastrong coupling in graphene-based structures, where an instability of the quantum vacuum is predicted to occur. Landau electrons of graphene and its ultrastrongly coupled structures are expected to react distinctly more nonlinear than their massive counterparts due to their non-equidistant energy progression, Rabi flopping, and anharmonic Landau ladder climbing. We will pursue quantum vacuum photonics in these structures using our most sensitive two-dimensional THz spectroscopy and quantum detection.Our work opens a new direction in non-adiabatic quantum electrodynamics, bringing together the most sophisticated ultrastrongly coupled structures and some of the most advanced THz photonics recently developed.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Christoph Lange, until 2/2021