  We investigate the thermal entanglement in two superconducting qubits for
arbitrary interaction strength and ground state frequencies. We calculate the
concurrence of the system to quantify the thermal entanglement. We suggest a
scheme, where an external tunable coupler qubit sandwich between two
superconducting qubits generates entanglement. The behavior of concurrence is
analyzed for three different cases, in which we consider the effects of the
temperature, the qubit-qubit effective coupling strength, and the qubit
frequencies on the thermal entanglement. What deserves mentioning here is that
to achieve maximally entangled states, it is better to use two superconducting
qubits with the same frequencies. We also note that for a given temperature,
the thermal entanglement can be tuned by qubit internal capacitance and
inductance.
