Environmentally induced Quantum Dynamical Phase Transition in the spin swapping operation

الأصل · EN

Quantum Information Processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states |+,-> and |-,+>. In NMR, this is achieved turning on and off the spin-spin interaction b=ΔE that splits the energy levels and induces an oscillation with a natural frequency ΔE/ℏ. Interaction of strength ℏ/τSE, with an environment of neighboring spins, degrades this oscillation within a decoherence time scale τϕ. While the experimental frequency ωand decoherence time τϕ were expected to be roughly proportional to b/ℏ and τSE respectively, we present here experiments that show drastic deviations in both ωand τϕ. By solving the many spin dynamics, we prove that the swapping regime is restricted to ΔE τSE > ℏ. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1/τϕ ∝ (b/ℏ)² τSE. The transition between quantum dynamical phases occurs when ω∝ √(b/ℏ)²-(k/τSE)² becomes imaginary, resembling an overdamped classical oscillator. Here, 0<k²<1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the Quantum Zeno effect opens up new opportunities for controlling quantum dynamics.