Nonlinear THz dynamics and quantum vacuum radiation from squeezed quantum vacua

控制光与电子激发的相互作用定义了现代量子电动力学的一个关键方面。在强光-物质耦合结构中，真空拉比频率表征了腔光子通过电子共振的连续发射和再吸收，从而形成了代表耦合系统新本征态的腔极化子。在太赫兹(THz)纳米谐振器中，光与物质的相互作用非常强，以至于真空拉比频率与光的载波频率相等。在这种情况下，量子基态包含有限的压缩的、关联的光子对，当耦合强度被快速调制时，这些光子对被预测将被释放。预期的物理学类似于尚未观测到的动态卡西米尔效应或黑洞的Unruh-Hawking辐射。在这里，我们打算探索定制切割的太赫兹纳米结构的量子真空光子学，半导体中耦合到回旋共振的超强光物质，超出玻色近似的少数朗道电子系统，以及石墨烯中的朗道电子。这一新的研究领域将在三个方面开拓：(I)利用我们团队开发的用于光-物质相互作用的飞秒控制方案，我们将利用宽带太赫兹光谱研究GaAs基结构中压缩量子真空的强亚周期开关动力学。这一工作流将为量子真空光子学在更复杂的工作流结构(II)和(III)中奠定基础。(II)我们将把超强耦合的量子极限作为目标，超越玻色子近似，通过三维纳米结构定制设计的具有微模体积的THz谐振器来制造少数朗道电子结构。高灵敏度的太赫兹量子探测将在极低的太赫兹振幅下追踪非微扰的非线性动力学和量子真空信号，有望实现压缩量子真空的少光子太赫兹非线性。(Iii)超越大质量电子系统，我们将研究基于石墨烯的结构中的超强耦合，其中预计会发生量子真空的不稳定性。石墨烯及其超强耦合结构的朗道电子由于其非等距能量级数、拉比翻转和非简谐朗道阶梯攀升，预计将比其质量对应的朗道电子反应更明显的非线性。我们将利用我们最灵敏的二维太赫兹光谱学和量子探测器在这些结构中探索量子真空光子学。我们的工作开辟了非绝热量子电动力学的新方向，将最复杂的超强耦合结构和最近发展起来的一些最先进的太赫兹光子学结合在一起。

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.