zum Hauptkapitel Emerging Quantum Mechanics:


The problem: a single qubit loses coherence within nanoseconds, whereas the qubits in the D-wave quantum computer behave coherently over microseconds. The experimental data of Chiorescu et al. \cite{chiorescu} on the dynamics of a single superconducting flux qubit shows decay of the probability for Rabi switching between the two states of a micron-size superconducting ring to the value of 0.6 within 80 ns. On the other hand, the experiment of Dickson et al. \cite{dickson} using 16 qubits of a superconducting quantum processor and quantum annealing with the help of applied transverse magnetic fields shows that, even with annealing time eight orders of magnitude longer than the time predicted for decoherence of a single qubit, the system behaves coherently.

Eight orders of magnitude discrepancy is a strong challenge for quantum field theory. It is our aim to reproduce the observation of apparent insensitivity of the system state to the effects of the environment and to provide an understanding of this phenomenon. We suggest a quantum field theoretical approach to a local system of 4 qubits entangled to different environmental excitations. The solution of Schrödinger's time dependent equation for the 4 qubit system, isolated from the environment, shows a coherent time development on the global ground state potential if the transverse magnetic fields, acting on the qubits, vary slowly with time. However fast changes, especially in the time domain of avoided crossing between the lowest and first excited states, lead to Landau-Zener transitions and drive the system diabatically into an excited state. Entanglement to phonons in 3+1 spacetime does lead to chaotic spin flip, which is a result of the time dependent Schrödinger equation. Entanglement of the local qubits to soft environmental modes has yet to be studied.