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 computing, we propose new NMR pulse sequences that exchange information between two sites I and S in a weakly coupled IS spin system for any arbitrary preparation 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 heteronuclear IS systems without the need for frequency-selective pulses. Experiments on the IS protons in uracil at 600 MHz demonstrate several examples of the universal SWAP operation and are matched by simulations based on the Liouville–von Neumann equation.