空间变化电磁场中的光子凝聚理论
Theory of Photon Condensation in a Spatially-Varying Electromagnetic Field
摘要：
在空间均匀的量子腔场中实现平衡超辐射量子相（光子凝聚）被源于规范不变性的“不通过”定理所禁止。我们在这里表明，不去定理不适用于空间变化的量子腔场。我们发现其发生的标准，完全取决于静态的，非本地的轨道磁化率$χ_{\rm orb}(q)$，在腔光子动量$\hbar q$的电子系统（ES）的评价。只有满足Condon不等式$χ_{\rm orb}(q)>1/(4π)$的3DES才能产生光子凝聚。对于实验相关的情况下，二维（2D）ES嵌入准2D腔的标准再次涉及$χ_{\rm orb}(q)$，但也垂直尺寸的腔。我们使用这些考虑来确定理想的光子凝聚的电子特性。我们的理论是非微扰的电子-电子相互作用的强度，因此适用于强关联ES。
英文摘要：
The realization of equilibrium superradiant quantum phases (photon condensates) in a spatially-uniform quantum cavity field is forbidden by a "no-go" theorem stemming from gauge invariance. We here show that the no-go theorem does not apply to spatially-varying quantum cavity fields. We find a criterion for its occurrence that depends solely on the static, non-local orbital magnetic susceptibility $χ_{\rm orb}(q)$, of the electronic system (ES) evaluated at a cavity photon momentum $\hbar q$. Only 3DESs satisfying the Condon inequality $χ_{\rm orb}(q)>1/(4π)$ can harbor photon condensation. For the experimentally relevant case of two-dimensional (2D) ESs embedded in quasi-2D cavities the criterion again involves $χ_{\rm orb}(q)$ but also the vertical size of the cavity. We use these considerations to identify electronic properties that are ideal for photon condensation. Our theory is non-perturbative in the strength of electron-electron interaction and therefore applicable to strongly correlated ESs.
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