Experimental evidence for entangled states formation in a system of two coupled flux qubits

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  • 12 December 2003
Abstract
The low-frequency response of a system of two inductively coupled flux qubits was monitored using the impedance measurement technique (IMT), which allows to directly read out the magnetic susceptibility of the qubits and the curvature of their average energy as a function of external dc magnetic flux. In a single qubit, an IMT dip marks the level anticrossing due to coherent tunneling and allows to determine the tunneling amplitude. In a two-qubit system, the non-zero difference between the sum of single-qubit IMT dips, and the IMT dip amplitude when the qubits are both at a degeneracy point (IMT deficit) shows that the system is in a mixture of entangled two-qubit states (a necessary condition for two-qubit entanglement). The dependence of the IMT dips on temperature and relative bias between the qubits allows to determine all the parameters of the effective Hamiltonian and equilibrium density matrix and confirm the formation of entangled eigenstates.

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