Achieving control over light−matter interaction in custom-tailored nanostructures is at the core of modern quantum electrodynamics. In strongly and ultrastrongly coupled systems, the excitation is repeatedly exchanged between a resonator and an electronic transition at a rate known as the vacuum Rabi frequency Ω R . For Ω R approaching the resonance frequency ω c , novel quantum phenomena including squeezed states, Dicke superradiant phase transitions, the collapse of the Purcell effect, and a population of the ground state with virtual photon pairs are predicted. Yet, the experimental realization of optical systems with Ω R /ω c ≥ 1 has remained elusive. Here, we introduce a paradigm change in the design of light−matter coupling by treating the electronic and the photonic components of the system as an entity instead of optimizing them separately. Using the electronic excitation to not only boost the electronic polarization but furthermore tailor the shape of the vacuum mode, we push Ω R /ω c of cyclotron resonances ultrastrongly coupled to metamaterials far beyond unity. As one prominent illustration of the unfolding possibilities, we calculate a ground state population of 0.37 virtual photons for our best structure with Ω R /ω c = 1.43 and suggest a realistic experimental scenario for measuring vacuum radiation by cutting-edge terahertz quantum detection. KEYWORDS: Quantum electrodynamics, ultrastrong coupling, terahertz, metamaterials I n the strong coupling regime of quantum electrodynamics (QED), where the vacuum Rabi frequency Ω R exceeds the dissipation rates of the electronic excitation and the resonator, new eigenmodes called cavity polaritons emerge. This universal principle is found in a large variety of systems, ranging from atoms 1 to excitons in semiconductors, 2,3 molecules, 4 mid-IR plasmonic structures, 5−9 circuit QED systems at GHz frequencies, 10−13 and structures in the THz spectral range. 14−16 In ultrastrongly coupled structures, Ω R becomes comparable to the resonance frequency ω c itself; the rotating-wave approximation of light−matter interaction falters, and antiresonant coupling terms describing the simultaneous creation of correlated light and matter excitations become relevant. 17−19 Most prominently, the ground state is theorized to be a modified squeezed quantum vacuum with a finite population of correlated virtual photon pairs. 17,19 For sufficiently large values of the relative coupling strength Ω R /ω c ≳1, subcycle switching of Ω R 6,9 may release these photons 17,19,20 in analogy to Unruh− Hawking radiation emerging at the event horizon of black holes. 21 These spectacular perspectives have fuelled the quest of the QED community for ever greater relative coupling strengths, ultimately aiming for Ω R /ω c beyond unity.The key strategy for boosting Ω R /ω c , also referred to as g/ω c , comprises increasing the dipole moment of the electronic transition, decreasing the resonator mode volume and ω c , or enhancing the overlap of the photonic mode and the electroni...
Future information technology demands ultimately fast, low-loss quantum control. Intense light fields have facilitated important milestones, such as inducing novel states of matter 1-3 , accelerating electrons ballistically 4-7 , or coherently flipping the valley pseudospin 8 . These dynamics leave unique signatures, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute -the spin -between two states separated by a potential barrier is to trigger an all-coherent precession. Pioneering experiments and theory with picosecond electric and magnetic fields have suggested this possibility 9-11 , yet observing the actual dynamics has remained out of reach. Here, we show that terahertz (1 THz = 10 12 Hz) electromagnetic pulses allow coherent navigation of spins over a potential barrier and we reveal the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 with the locally enhanced THz electric field of custom-tailored antennas. Within their duration of 1 ps, the intense THz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the magnon resonance, and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with a numerical simulation. The switchable spin states can be selected by an external magnetic bias. The low dissipation and the antenna's sub-wavelength spatial definition could facilitate scalable spin devices operating at THz rates.
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