Rotational decoherence

Another practical tool is dynamical decoupling, a technique that uses sequences of fast qubit rotations to mitigate the effects of decoherence. The pulse sequences are designed such that the interactions of each qubit with its environment tend to average out [34, 35]. While still a major concern, decoherence may thus not be the strong impediment it originally seemed to represent for the advent of QC. 

In this section, we describe the motion of vibronic WPs created in a diatomic molecule that has only one vibrational mode. The influence of other degrees of motions such as rotation and nuclear spins are omitted for simplicity. Since our studies deal with the quantum property of the system in which relaxations can be neglected, the decoherence process is not taken into account in the following formulations. Assuming that the molecule occupies a single vibrational level v = 0 as an initial state, the WP generated by the absorption of a pump laser pulse is given as... 

Needless to say, the system S (the conformation ) is (likewise in the (semi-)classical approach) a characteristic of a (single) molecule as a whole. That is, as usual, we do not take into account the local details of the conformational rotations themselves, which essentially take into account the electron-state transitions. As much as we can see, these are of the secondary importance to our model, which abandons the concept of the transitions in T-space. Abandoning the T-space is key to the possible success of our model. It is a decoherence-Iike process that breaks stability of conformations (in the nonstationary state), eventually giving rise to the possibility of rather fast conformational transitions. 

Choice of qubits. We need long-lived states to store the information. These should interact as little as possible with the environment to minimize decoherence. Good candidates are hyperfine and rotational states of a ground electronic molecular state. 

Spin relaxation phenomena are usually described by the semiclassical theory developed by Wangsness, Bloch and Redfield and known as the WBR theory or Redfield theory. The semiclassical nature of the theory implies that the spin system is treated quantum mechanically, while the remaining degrees of freedom (such as molecular rotations) are treated classically. Few years ago, Segnorile and Zamar studied the issue of quantum decoherence (loss of system phase memory) in proton NMR of nematic liquid crystals. The spin dynamics - and the decay of the free induction decay - was found to be governed by several different processes, partly of purely quantum nature. During the period under the present review, the same group reported a related work concerned with the Jeener-Broekaert experiment on liquid crystals. ... 

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