Ordering of field operators and equations of motion in quantum optics

Jan Peřina, R. Horák

Edwin Albert Power, T. Thirunamachandran

The equations of motion and the class of Hamiltonians for electrodynamics, both classical and quantal, are considered for the interaction of radiation with bound charges within the electric-dipole approximation. It is emphasized that, despite the uniqueness of the equations of motion for physical variables, the form of the Hamiltonian is not unique and that a physical system is associated with an equivalence class of Hamiltonians. Two of the members of this class are the minimal-coupling and multipolar forms extensively used in quantum optics. It is shown that although different choices of Hamiltonians lead to different first-order equations of motion that connect canonical coordinates and momenta, they lead to the same second-order equations of motion for the physical variables and physical electromagnetic fields. In the quantum theory of electrodynamics the role played by the choice of representation for the operators and for the states is analyzed with care. The choice of representation leads to a freedom additional to that provided by canonical transformations in classical electrodyamics, although the two lead to the same operator equations of motion.

Tosizumu Aoki

On the basis of the method of equation of motion, the ground states of the quantum spin systems with a long-range order are studied. The equations of motion for the spin operators are linearized by introduction of the effective field, which is the order parameter itself. The linearized equations of motion and the use of approximate commutation relations for spin operators lead to the boson representation of the systems. It is emphasized that in the ground state with the long-range order along the x -axis, the relationship ( S j y ) 2 +( S j z ) 2 =1/2 should be retained in the average. By use of the above constraint for spin fluctuations, the values of the long-range order and ground-state energy are calculated within the boson approximation. The results agree well with ones obtained by different methods.

Jan Peřina

Kishore Thapliyal, Anirban Pathak, Biswajit Sen +1 more

Higher-order nonclassical properties of fields propagating through a codirectional asymmetric nonlinear optical coupler which is prepared by combining a linear waveguide and a nonlinear (quadratic) waveguide operated by second harmonic generation are studied. A completely quantum mechanical description is used here to describe the system. Closed form analytic solutions of Heisenberg's equations of motion for various modes are used to show the existence of higher-order antibunching, higher-order squeezing, and higher-order two-mode and multimode entanglement in the asymmetric nonlinear optical coupler. It is also shown that nonclassical properties of light can transfer from a nonlinear waveguide to a linear waveguide.

Jan Peřina