Evidence for entangled states of two coupled flux qubits

TL;DR

This study demonstrates the formation of entangled states in two coupled flux qubits by analyzing their magnetic susceptibility and impedance measurements, confirming entanglement through characteristic dips and temperature dependence.

Contribution

It provides experimental evidence for entangled eigenstates in coupled flux qubits using impedance measurement technique and analyzes their properties through susceptibility data.

Findings

Observation of IMT dips indicating flux tunneling

Detection of IMT deficit confirming entanglement

Parameter determination of the qubits' Hamiltonian and density matrix

Abstract

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.