Entanglement between two superconducting qubits via interaction with nonclassical radiation

Control of the dynamics of a complex quantum system requires a trade-off between tunability and protection against noise. To this end one can be interested in processes where some physical properties of a subsystem are reliably transferred onto the state of a second one (of perhaps different nature) where information can be manipulated. The connection between the two subsystems is effectively realized via a physical interface. An interface is a communication channel used to connect the elements of a quantum register to perform quantum information processing or a physical mechanism that gives full access to the system under investigation and allows to manipulate it. To investigate this problem, in this paper we describe the coupling between a nanoelectronic circuit implementing a pair of quantum bits and a two-mode electromagnetic field. We discuss a mechanism for the transfer of entanglement from a two-mode squeezed state to the pair of qubits. Here, the information sheltered in the electromagnetic medium may be manipulated, using just single-qubit operations, when transferred to the solid-state subsystem. This may offer advantages with respect to integrability and scalability. In particular, we consider the field modes to interact with a pair of (initially independent) superconducting quantum interference devices (SQUIDs) that embody two charge qubits. 1 Direct experimental evidence of the use of these systems as controllable coherent two-level systems has already been provided. 4,5 We find that a nearly maximally entangled state of two qubits can be tailored, with our interaction model, via an effective process of transfer of quantum correlations. The entanglement poured into the joint state of the qubits can be regulated controlling the interaction times between qubits and field modes. At the interaction time corresponding to the maximum of the transferred entanglement, the qubits are in an almost pure state that may be used for efficient quantum information processing. This work is organized as follows. In Sec. II we introduce the system we consider and derive the effective model for the coupling between a superconducting charge qubit and a field mode. Section III is devoted to the study of the process of transfer of quantum correlations from the two-mode field to the qubits. The joint state of these latter, once the field modes are traced out, turns out to be entangled. We quantify the amount of entanglement between the qubits and find the corresponding degree of mixedness of the state. Finally, in Sec. IV, we investigate about the variations in the amount of transferred entanglement as the initial preparation of the qubits is changed. We find that the transfer process is optimized if the qubits are initially in their computational ground state.