Physics – Quantum Physics
Scientific paper
2010-06-13
J. Phys. A: Math. Theor. 43, 465302 (2010)
Physics
Quantum Physics
22 pages, 5 figures Somewhat condensed version with minor modifications to improve readability, misprints corrected. Accepted
Scientific paper
We derive the general formula giving the Berry phase for an arbitrary spin, having both magnetic-dipole and electric-quadrupole couplings with external time-dependent fields. We assume that the effective E and B fields remain orthogonal during the quantum cycles. This mild restriction has many advantages. It provides simple symmetries leading to selection rules and the Hamiltonian-parameter and density-matrix spaces coincide for S=1. This implies the identity of the Berry and Aharonov-Anandan phases, which is lost for S>1. We have found that new features of Berry phases emerge for integer spins>2. We provide explicit numerical results of Berry phases for S=2,3,4. We give a precise analysis of the non-adiabatic corrections. The accuracy for satisfying adiabaticity is greatly improved if one chooses for the time derivatives of the parameters a time-dependence having a Blackman pulse shape. This has the effect of taming the non-adiabatic oscillation corrections which could be generated by a linear ramping. For realistic experimental conditions, the non-adibatic corrections can be kept < 0.1%. For quantum cycles,involving as sole periodic parameter the precession angle of E around B, the corrections odd upon the reversal of the angular velocity can be cancelled exactly if the quadrupole to dipole coupling ratio takes a "magic" value. The even ones are cancelled by subtraction of the phases relative to opposite velocities. As a possible application of the results of this paper we suggest a route to holonomic entanglement of N non-correlated 1/2-spins by performing adiabatic cycles governed by a Hamiltonian which is a non-linear function of the total spin operator S defined as the sum of the N spin operators. The case N=4 and Sz=1 is treated explicitly and maximum entanglement is achieved.
Bouchiat Claude
Bouchiat Marie-Anne
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