We have considered [1] the interaction of a qubit with a single mode of the quantized electromagnetic field (Rabi model) and shown that, in the regime of ultrastrong matter-field coupling and when the qubit-field interaction is switched on abruptly, the dynamical Casimir effect (DCE) leads to the generation of a variety of exotic states of the field, which cannot be simply described as squeezed states. Such effect is a consequence of the intrinsic nonlinearity of the qubit and also appears when initially both the qubit and the field are in their ground state. We have demonstrated, always in the regime of ultrastrong matter-field coupling, the strong connection between the DCE and the performance of quantum-information protocols [2]. Our results have been illustrated by means of a realistic quantum communication channel and show that the DCE is a fundamental limit for quantum computation and communication and that novel schemes are required to implement ultrafast and reliable quantum gates. On the other hand, we have also discussed optimal control techniques to amplify the DCE [3]. Moreover, we have considered the DCE in the framework of Nersnst's unattainability principle. We have shown that the DCE sets fundamental, purely quantum limits to cooling and conjectured that it forbids the attainability of the zero-temperature limit in cyclic cooling machines, even in the limits of infinite number of cycles [4].

Exotic states of the field in the ultrastrong coupling regime

[1] G. Benenti, S. Siccardi and G. Strini, Exotic states in the dynamical Casimir effect, Eur. Phys. J. D 68, 139 (2014).
[2] G. Benenti, A. D'Arrigo, S. Siccardi and G. Strini, Dynamical Casimir effect in quantum-information processing, Phys. Rev. A 90, 052313 (2014).
[3] F. Hoeb, F. Angaroni, J. Zoller, T. Calarco, G. Strini, S. Montangero and G. Benenti, Amplification of the parametric dynamical Casimir effect via optimal control, Phys. Rev. A 96, 033851 (2017).
[4] G. Benenti and G. Strini, Dynamical Casimir effect and minimal temperature in quantum thermodynamics, Phys. Rev. A 91, 020502(R) (2015).