In this letter we study qubits coupled to the bath formed by their environment. Although entanglement of the qubits is a well-known topic, much less effort has gone into the description of the correlations between the qubits and the bath. Here, we investigate these correlations, and study their effect on the qubits in equilibrium and their dynamics following the interaction with one or several external pulses. We find that a correct description of the correlations at the moment of these interactions is essential for a correct understanding of the dynamics.KEYWORDS: qubit, system-bath, nonlinear response, initial correlations, non-MarkovianThe entanglement of two qubits (two-level systems) measures the extent to which they are not separable, and is a purely quantum effect. It is formed and destroyed by the interaction of the qubit system with its environment. Formation of entanglement is often the result of a deliberate preparation procedure, while the destruction occurs because of uncontrollable interactions. After the preparation of an entangled state, the entanglement typically decays with time to an equilibrium value. In the case of Markovian dynamics, the entanglement smoothly decays with time, while temporary increases are possible if the evolution is nonMarkovian. The dynamics of entanglement shows some surprising characteristics. Even if the single qubit coherence decays smoothly, the entanglement can go to zero in a finite time, a phenomenon called ''sudden death'' of entanglement.1) Furthermore, the entanglement can be revived from death, i.e., the system can become entangled after a period of zero entanglement.All these time dependent phenomena are caused by the interaction of the qubits with a bath. Entanglement which is lost from the system during the time evolution is stored in the bath.2) Crucially, correlations are created between the qubit system and the bath. Their presence influences the dynamics in the system. In particular, correlations present at the moment of the interaction with an external force are not well studied. Their detailed understanding is the topic of this letter. Of course, correlations are formed continuously when a system is placed in contact with a heat bath. This is closely related to the reorganization energy known in chemical physics. 3,4) In the case of non-Markovian dynamics, the correlations acts as a memory for the system, and influences the system entanglement at a later point in time.5) Besides this, there is another phenomenon which is important for a complete description of the dynamics: the entanglement of system and bath states at time zero (where zero can be chosen arbitrarily, but in this paper will represent the moment of interaction with an external pulse). In general, this quantity, which reports on initial correlations, 6-9) does not vanish and can influence the system during its time evolution.
10)Theoretically, the dynamics of a system in contact with its environment is often studied using master equations, which have been extended to include initial cor...