Oscillations of superconducting current between clockwis e and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. T o compensate for this effect the solid containing the qubit must oscillate in unison with the current. This requires entanglement of quantum states of the qubit with quantum states of a macrosco pic body. The question then arises whether slow decoherence of quantum oscillations of the cur rent is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is ana lyzed within an exactly solvable quantum model of a qubit embedded in an absolutely rigid soli d and for the elastic model that conserves the total angular momentum. We show that while the quantum state of a flux qubit is, in general, a mixture of a large number of rotational stat es, slow decoherence is permitted if the system is macroscopically large. Practical implicatio ns of entanglement of qubit states with mechanical rotations are discussed.
Chudnovsky, E. M., Garanin, D. A., & O'Keeffe, M. F. (2012). Conservation of angular momentum in a flux qubit. Journal of Superconductivity and Novel Magnetism, 25(4), 1007-1016. DOI: 10.1007/s10948-012-1410-y