Dephasing-assisted entanglement in a system of strongly coupled qubits

Abstract

Creation of entangled states of quantum systems with low decoherence rates is a cornerstone in practical implementation of quantum computations. Processes of separate dephasing in each qubit in experimentally feasible systems is commonly accepted to destroy entanglement. In this work, we consider a system of two strongly coupled qubits that interact with dephasing reservoirs. We demonstrate that interaction with dephasing reservoirs can contribute to the formation of a long-lived mixed entangled state with nonzero concurrence. The weight of the subradiant state in this mixed state tends toward unity if the dephasing rate is much larger than the radiative rate and less than the coupling constant between qubits. The lifetime of this state is proportional to the exponent of the ratio of the coupling constant to environmental temperature and can be, by orders of magnitude, larger than the system's characteristic dephasing and dissipation times. Therefore, high dephasing, along with strong coupling, contributes to the creation of an entangled state with a long lifetime. This result paves the way for creation of long-lived entangled states. I. INTRODUCTIONEntangled states of two or more qubits are the building blocks of quantum computers and elements of quantum communication systems [1]. The concept of entanglement was first proposed for the pure state, which can be described in terms of wave function [2][3][4][5]. However, all real systems inevitably interact with the environment. This leads to a state of the system ceases to be pure and becomes a mixed state, which can be described only in terms of a density matrix. The concept of entanglement can be extended to mixed states, as well [6]. The system-environment interaction leads to processes of dissipation and dephasing or decoherence. The first leads to change in both energy and coherence, while the second process does not change the energy and results in destruction of coherence. This manifests as dissipation of only the non-diagonal density matrix elements. In real systems, the dephasing rate is up to five orders of magnitude higher than the dissipation rate [7,8] and it is responsible for the fast destruction of the entanglement.One basic element for entanglement creation is an ensemble of two two-level systems (TLSs) interacting with the environment and, possibly, each other [9][10][11][12][13]. The problem of entanglement creation in such a system has been investigated in many works [14][15][16][17][18][19][20], and several opportunities to create and conserve entanglement exist. One can enhance the interaction between TLSs in such a way that the ground state of the system becomes entangled [17,20] or use coherent external drive to move the system into the entangled state [21,22]. The use of common reservoirs for two qubits is another way to create entanglement [14,16,18,23].Free space modes of electromagnetic field are an example of common reservoirs for dipole moments of atoms or molecules. When they occupy subwavelength volume, an entangled subradiant state,...