Conservation of Angular Momentum in a Flux Qubit

TL;DR

This paper investigates how flux qubits conserve angular momentum through entanglement with a macroscopic solid, analyzing decoherence effects and implications for quantum state stability in large systems.

Contribution

It presents an exactly solvable quantum model demonstrating angular momentum conservation and discusses the conditions for slow decoherence in macroscopic systems.

Findings

Slow decoherence is possible in large macroscopic systems.

Flux qubit states can be entangled with mechanical rotations.

Angular momentum conservation requires entanglement with the solid's states.

Abstract

Oscillations of superconducting current between clockwise and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. To 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 macroscopic body. The question then arises whether slow decoherence of quantum oscillations of the current is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is analyzed within an exactly solvable quantum model of a qubit embedded in an absolutely rigid solid 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 states, slow decoherence is permitted if the system is macroscopically large. Practical implications of entanglement of qubit states with mechanical rotations are discussed.