Quantum Computing and Entanglement Generation using Intramolecular Degrees of Freedom

Kenji Mishima

Research Center for Advanced Science and Technology, The University of Tokyo

	e-mail: erdao@tcl.t.u-tokyo.ac.jp

In this talk, I will give an outline of my calculation results of quantum computing using molecular internal degrees of freedom and the development of new optimal control theories for quantum systems in connection with molecular quantum computing and entanglement generation.  
First, I have used the conventional multi-target optimal control theory (MTOCT) to numerically construct two-qubit general-purpose phase-correct global quantum gates by using several combinations of molecular internal degrees of freedom (electronic-vibrational, vibrational-rotational, rotational-rotational).  It turns out that the fidelities of each elementary quantum gate and measurement results are the best compared with any mode-combinations reported so far when intermolecular rotational modes of two polar molecules coupled by dipole-dipole interaction are used to encode two qubits and implement the Deutsch-Jozsa algorithm.  Second, in connection with quantum computing and entanglement manipulation by laser fields, I have constructed free-time and fixed end-point optimal control theories (FRFP-OCTs) for the quantum systems without and with dissipation for the first time.  Using the theories, I have performed simulations of entanglement generation and maintenance.  From the numerical results, I have found that FRFP-OCT is more efficient than the conventional fixed-time and fixed end-point optimal control theory (FIFP-OCT) because the optimal time duration of the external laser fields can also be determined exactly using FRFP-OCT.  
Finally, I would like to make two comments on ongoing researches: the first one is the implementation of molecular quantum computing using cold polar molecules because the actual molecules are affected by finite temperature in experiments.  The second one is the entanglement generation of rotational degrees of freedom by the dissociation of two water dimers.