A two-level system resonantly interacting with an a.c. magnetic or electric field constitutes the physical basis of diverse phenomena and technologies. However, Schrödinger’s equation for this seemingly simple system can be solved exactly only under the rotating-wave approximation, which neglects the counter-rotating field component. When the a.c. field is sufficiently strong, this approximation fails, leading to a resonance-frequency shift known as the Bloch–Siegert shift. Here, we report the vacuum Bloch–Siegert shift, which is induced by the ultra-strong coupling of matter with the counter-rotating component of the vacuum fluctuation field in a cavity. Specifically, an ultra-high-mobility two-dimensional electron gas inside a high-Q terahertz cavity in a quantizing magnetic field revealed ultra-narrow Landau polaritons, which exhibited a vacuum Bloch–Siegert shift up to 40 GHz. This shift, clearly distinguishable from the photon-field self-interaction effect, represents a unique manifestation of a strong-field phenomenon without a strong field.

与交流磁场或电场共振相互作用的两级系统构成了各种现象和技术的物理基础。但是，对于这个看似简单的系统，薛定ding方程只能在旋转波近似下精确求解，而忽略了反向旋转场分量。当交流场足够强时，这种近似将失败，从而导致共振频率偏移，称为布洛赫-西格特偏移。在这里，我们报告真空Bloch–Siegert位移，它是由物质与腔中真空波动场的反向旋转分量的超强耦合引起的。具体而言，高Q值内的超高迁移率二维电子气太赫兹腔在量化磁场中显示出超窄的Landau极化子，该极化子在高达40 GHz的频率下具有Bloch-Siegert真空位移。这种变化与光子场的自相互作用效应是明显不同的，它代表了没有强场的强场现象的独特表现。