We discuss the properties of bound states in finite-bandwidth waveguide QED beyond the Rotating Wave Approximation or excitation number conserving light-matter coupling models. Therefore, we extend the calculations to a broader range of light-matter strengths, in particular, in the so-called ultrastrong coupling regime. We do this using the Polaron technique. Our main results are as follows. We compute the spontaneous emission rate, which is renormalized as compared to the Fermi Golden Rule formula. We generalise the existence criteria for bound states, their properties and their role in the qubits thermalization. We discuss effective spin-spin interactions through both vacuum fluctuations and bound states. Finally, we sketch a perfect state-transfer protocol among distant emitters mediated by bound states.

中文翻译：

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超强波导QED中的束缚态

我们讨论了有限带宽波导QED中束缚态的性质，这些性质超出了旋转波近似或激发数守恒的光质耦合模型。因此，我们将计算扩展到更广泛的光强度范围，尤其是在所谓的超强耦合方案中。我们使用Polaron技术进行此操作。我们的主要结果如下。我们计算自发发射率，与费米黄金定律公式相比，将其重新归一化。我们概括了束缚态的存在准则，它们的性质及其在量子位热化中的作用。我们讨论了通过真空波动和束缚态进行的有效自旋-自旋相互作用。最后，我们在绑定状态介导的远距离发射器之间绘制了一个完美的状态转移协议。