Dynamical suppression of unwanted transitions in multistate quantum systems

Summary form only given. Experiments in many fields of quantum physics require well-defined quantum states and well-defined interactions. For example, the basic ingredient of the quantum computer is a well-defined qubit - a two-state quantum system. Real quantum systems, however, possess many states and special care is needed to isolate just two of them. In real and artificial atoms this is usually done with polarized laser light, carefully aligned with the quantization axis. However, unwanted transition channels may still be present, which reduce the fidelity of the operations. For example, when an ultracold atomic ensemble held in an optical dipole trap is addressed by right circularly polarized light, many atoms often “see” an admixture of σ+ and σ- light (i.e., elliptical polarization [1]) since not all of them are exactly in the focus of the laser fields. Unwanted transitions may be present also due to imperfect polarization or alignment, off-resonant couplings, perturbing excitations in molecules and solids, etc.We propose a simple and efficient technique for automatic compensation of such errors, even without knowing their magnitudes, which uses composite pulse sequences [2-5] to dynamically suppress unwanted transitions, while simultaneously controlling the qubit in a robust way. We illustrate the technique in threeand four-state quantum systems forming linkages reminiscent of the letters V, Ξ and Y, but it can readily be adapted to more complex systems. By suitably choosing the phases of the constituent pulses, the unwanted transitions can be suppressed with very high fidelity, while compensation of deviations in laser polarizations, intensities and detunings can be done simultaneously. The accuracy, the flexibility and the simplicity of the proposed technique make it a potentially important tool in applications requiring high control fidelity, such as quantum information processing and quantum optics.transitions