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Coupled flux qubits

We have performed spectroscopic measurements on two coupled flux qubits consisting of small super-conducting loops interrupted by three Josephson junctions with a small junction capacitance. The qubit eigenstates correspond to super-positions of clockwise and anti-clockwise circulating currents. Because the basic states of these qubis are flux states they are very insensitive to charge noise.

The qubits are coupled inductively, which results in an Ising type of interaction. By applying microwave radiation, one observes resonances due to transitions from the ground state to the first two excited states. The position of these resonances as a function of the magnetic field applied reveals the coupling of the qubits. The coupling strength agrees well with calculations of the mutual inductance.

The analysis makes clear that the spectroscopic data are fully consistent with the two-qubit Hamiltonian. This Hamiltonian, in turn, opens the possibility of well-chosen one and two-qubit operations that lead to controlled entanglement. The new results support the notion that super-conducting flux qubits can be used to study entanglement in macroscopic quantum systems and for the development of non-trivial two-qubit gates such as the controlled-not.

Reported by

Kavli Institute of Nanoscience
Lorentzweg 1
2612HD Delft
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