How to exchange information between two coupled nuclear spins: the universal SWAP operation

Starting from the concept of the SWAP logic gate used in NMR quantum comput ing, we propose new NMR pulse sequences that exchange information between t wo sites I and S in a weakly coupled IS spin system for any arbitrary prepa ration of the I and S spins. The pulse sequences are constructed from three carefully chosen controlled-not operations. They offer the possibility of monitoring the state of a spin of low inherent sensitivity by interchanging product operator terms with a second spin of high sensitivity. One of the proposed pulse sequence modules is applicable to both homonuclear and heter onuclear IS systems without the need for frequency-selective pulses. Experi ments on the IS protons in uracil at 600 MHz demonstrate several examples o f the universal SWAP operation and are matched by simulations based on the Liouville-von Neumann equation. (C) 1999 Elsevier Science B.V. All rights r eserved.