proceedings Proceedings Speedup of Adiabatic Multiqubit State-Transfer by Ultrastrong Coupling of Matter and Radiation † Michele Stramacchia 1 , Alessandro Ridolfo 2,3,4 , Giuliano Benenti 1,3 , Elisabetta Paladino 2,3,5 , Francesco Maria Dimitri Pellegrino 2,3 , Daniele Maccarrone 2 and Giuseppe Falci 2,3,5,* 1 Center for Nonlinear and Complex Systems, Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, 22100 Como, Italy 2 Dipartimento di Fisica e Astronomia “Ettore Majorana”, Università di Catania (Italy), Via S. Sofia 64, I-95123 Catania, Italy 3 Istituto Nazionale di Fisica Nucleare, Sez. di Catania, I-95123 Catania, Italy 4 RIKEN, Saitama, Tokyo 351-0198, Japan 5 CNR-IMM, Catania Università, I-95123 Catania, Italy * Correspondence: gfalci@dmfci.unict.it † Presented at the 11th Italian Quantum Information Science conference (IQIS2018), Catania, Italy, 17–20 September 2018. Received: 17 January 2019; Accepted: 6 May 2019; Published: 23 July 2019   Abstract: Ultrastrongly coupled quantum hardware may increase the speed of quantum state processing in distributed architectures, allowing to approach fault-tolerant threshold. We show that circuit QED architectures in the ultrastrong coupling regime, which has been recently demonstrated with superconductors, may show substantial speedup for a class of adiabatic protocols resilient to the main source of errors, namely the interplay of dynamical Casimir effect and cavity losses. Keywords: quantum information; light-matter interaction 1. Introduction Atom-cavity like systems in the strong coupling (SC) regime [1] implemented by circuit-QED solid-state systems [2] are nowadays one of the most promising platforms for quantum hardware [3]. Systems of superconducting artificial atoms coupled in the SC regime to an electromagnetic resonator are at the basis of the IBM, Intel, and Google quantum chips, the latter recently announced up to 72 qubits [4]. Recently entanglement has been demonstrated in systems with ten qubits [5]. Circuit-QED architectures are moreover paradigm models for studying fundamental physics from measurement theory [2] to quantum thermodynamics [6] and quantum communication [7]. We consider the simplest multiqubit architecture, two atoms coupled to a quantized mode, described by the Rabi model [1,8]: H R = ω c a † a − 1 2 2 ∑ i=1 ǫ i σ i 3 + ∑ i g i (t)( a † σ i − + a σ i + )+ ∑ i g i (t)( a σ i − + a † σ i + ) (1) Here ω c is the oscillator frequency and a † (a) are the annihilation (creation) operator acting on the Hilbert space spanned by the Fock states {|n〉}. Pauli matrices σ i α for α = 3, ± refer to the i = 1, 2 qubits whose splittings are ǫ i . The whole Hilbert space is spanned by the factorized basis {|n σ 2 σ 1 〉}, where |σ i 〉 = {| g〉, |e〉} are eigenstates of σ i 3 , with eigenvalue 1 and −1, respectively. Atoms-mode couplings in the dipole approximation [1] are g i , which in the SC limit g i are large enough to overcome the atomic decoherence rates (γ i ) and loss of the oscillator mode (κ). Moreover g ≪ ω c which allows the rotating wave approximations (RWA), where the counterrotating last term of H R is neglected. The resulting Proceedings 2019, 12, 35; doi:10.3390/proceedings2019012035 www.mdpi.com/journal/proceedings