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

A. Izmalkov, M. Grajcar, E. V. Il’ichev, Th. Wagner, H.‐G. Meyer, Anatoly Yu. Smirnov, M. H. S. Amin, Alec Maassen van den Brink, Alexandre M. Zagoskin

Published Dec 12, 2003• 2 citations
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We have studied the low-frequency magnetic susceptibility of two inductively coupled flux qubits using the impedance measurement technique (IMT), through their influence on the resonant properties of a weakly coupled high-quality tank circuit. In a single qubit, an IMT dip in the tank's current-voltage phase angle at the level anticrossing yields the amplitude of coherent flux tunneling. For two qubits, the difference (IMT deficit) between the sum of single-qubit dips and the dip amplitude when both qubits are at degeneracy shows that the system is in a mixture of entangled states (a necessary condition for entanglement). The dependence on temperature and relative bias between the qubits allows one to determine all the parameters of the effective Hamiltonian and equilibrium density matrix, and confirms the formation of entangled eigenstates.

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Abstract We investigate hybrid flux-nitrogen-vacancy (NV) entanglement in the system consisting of two flux qubits and an NV-center ensemble (NVE). The influence of the initial entanglement degree of the flux qubit C, the coupling strength between two flux qubits, the coupling strength between the flux qubit M and the NVE on the bipartite entanglement and the tripartite entanglement is discussed. The maximally bipartite entangled state and the maximally tripartite entangled state can be obtained under certain conditions. In addition, it is shown that the time of disentanglement of the hybrid quantum system can be controlled by adjusting the coupling strength between two flux qubits and the coupling strength between the flux qubit M and the NVE.

Laser Physics•Jun 27, 2018•0 citations
We have studied the low-frequency magnetic susceptibility of two inductively coupled flux qubits using the impedance measurement technique (IMT), through their influence on the resonant properties of a weakly coupled high-quality tank circuit. In a single qubit, an IMT dip in the tank's current-voltage phase angle at the level anticrossing yields the amplitude of coherent flux tunneling. For two qubits, the difference (IMT deficit) between the sum of single-qubit dips and the dip amplitude when both qubits are at degeneracy shows that the system is in a mixture of entangled states (a necessary condition for entanglement). The dependence on temperature and relative bias between the qubits allows one to determine all the parameters of the effective Hamiltonian and equilibrium density matrix, and confirms the formation of entangled eigenstates.
We theoretically study the macroscopic quantum entanglement in superconducting flux qubits. A phase-coupling scheme is proposed to offer enough strength of interactions between qubits. It is shown that due to the two-qubit tunneling processes both the ground state and excited states of coupled two flux qubits can be Bell type states, maximally entangled, in experimentally accessible regimes. The parameter regimes for the Bell states are discussed in terms of external magnetic flux and Josephson coupling energies. We also investigate two types of genuine three-qubit entanglement, known as the Greenberger-Horne- Zeilinger(GHZ) and W states. While an excited state can be the W state, the GHZ state is formed at the ground state of the coupled three flux qubits. The GHZ and W states are shown to be robust against external flux fluctuations for feasible experimental realizations.
Abstract We investigate hybrid flux-nitrogen-vacancy (NV) entanglement in the system consisting of two flux qubits and an NV-center ensemble (NVE). The influence of the initial entanglement degree of the flux qubit C, the coupling strength between two flux qubits, the coupling strength between the flux qubit M and the NVE on the bipartite entanglement and the tripartite entanglement is discussed. The maximally bipartite entangled state and the maximally tripartite entangled state can be obtained under certain conditions. In addition, it is shown that the time of disentanglement of the hybrid quantum system can be controlled by adjusting the coupling strength between two flux qubits and the coupling strength between the flux qubit M and the NVE.