Laser phase modulation approaches towards ensemble quantum computing

Selective control of decoherence is demonstrated for a multilevel system by generalizing the instantaneous phase of any chirped pulse as individual terms of a Taylor series expansion. In the case of a simple two-level system, all odd terms in the series lead to population inversion while the even terms lead to self-induced transparency. These results also hold for multiphoton transitions that do not have any lower-order photon resonance or any intermediate virtual state dynamics within the laser pulse-width. Such results form the basis of a robustly implementable CNOT gate.Comment: 10 pages, 4 figures, PRL (accepted

A Complete Formulation of Baum-Conens' Conjecture for the Action of Discrete Quantum Groups

We formulate a version of Baum-Connes' conjecture for a discrete quantum group, building on our earlier work (\cite{GK}). Given such a quantum group \cla, we construct a directed family \{\cle_F \} of $C^*$-algebras ($F$ varying over some suitable index set), borrowing the ideas of \cite{cuntz}, such that there is a natural action of \cla on each \cle_F satisfying the assumptions of \cite{GK}, which makes it possible to define the "analytical assembly map", say $\mu^{r,F}_i$, $i=0,1,$ as in \cite{GK}, from the \cla-equivariant $K$-homolgy groups of \cle_F to the $K$-theory groups of the "reduced" dual \hat{\cla_r} (c.f. \cite{GK} and the references therein for more details). As a result, we can define the Baum-Connes' maps \mu^r_i : \stackrel{\rm lim}{\longrightarrow} KK_i^\cla(\cle_F,\IC) \raro K_i(\hat{\cla_r}), and in the classical case, i.e. when \cla is $C_0(G)$ for a discrete group, the isomorphism of the above maps for $i=0,1$ is equivalent to the Baum-Connes' conjecture. Furthermore, we verify its truth for an arbitrary finite dimensional quantum group and obtain partial results for the dual of $SU_q(2).$Comment: to appear in "K Theory" (special volume for H. Bass). A preliminary version was available as ICTP preprint since the early this yea