Quantum Dynamical Control

Quantum computation and quantum information processing provides a powerfultechnique in the design of new computers based on quantum mechanics. Since quantumstates possess coherence and entanglement, there are advantages than the classical infor-mation. However, it is often very fragile due to environmental perturbations. The focusof our research is fault tolerant quantum computation which needed to be designed tosuppress or use system-environment interactions.Firstly, we propose to nest3M layers of explicitly constructed Uhrig’s dynamicaldecoupling sequences of local control operators to achieve universal high-order pro-tection of arbitrary and unknown M-qubit states on a2M-dimensional computationalsubspace. This is the case even if the mechanism responsible for population leakagefrom the computational subspace is unclear and the actual form of system–environmentcoupling is not given.Secondly, inspired by a method of purification through Zeno-like measurements,we have designed a new strategy to obtain entangled states through only two measure-ments with a control field. A determinated purification method is also provided.Thirdly, we propose a scheme to create entanglement of two qubits that interactwith a common heat bath. Dynamical decoupling is applied in the scheme so that itworks even in the regime of strong interaction between qubits and environment. There-fore the scheme can create entanglement rather fast.Finally, we study the usefulness of dynamical decoupling (DD) in protectingsqueezing processes. Concatenated dynamical decoupling (CDD) and Uhrig dynami-cal decoupling (UDD) methods are applied to continuous variable systems by aperiodicmodulations of the harmonic oscillator frequency. In contrast to squeezing process-es with periodic parity kicks where the error is suppressed to the first order, errors insqueezing processes with CDD and UDD are suppressed to the second order. We show,using CDD or UDD, one can create quantum entanglement via two-mode squeezingprocess even at room temperature.