Three-qubit nuclear magnetic resonance quantum information processing with a single-crystal solid

We report the first implementation of NMR quantum information processing in a static single-crystal solid, illustrating pseudopure state preparation and gate manipulations on a three-qubit system, U–13C2, N15-glycine (H215N13CH213COOH). We consider issues important to single-crystal NMR implementations of quantum information processing and introduce several innovations specific to such implementations. In particular, concurrent cross-polarization of both the C13 and N15 spins from proton magnetization enhances and equalizes populations on these nuclei in a single step. This multiple cross-polarization step simplifies preparation of pseudopure spin states relative to previously described multiple-pulse sequences. Also, proton decoupling, which prolongs coherence during spin-evolution and detection periods, is switched off to dephase unwanted coherences during preparation of pseudopure spin states. Such “relaxation averaging” (or T2 averaging) is an alternative to spatial averaging, which requires special hardware and large gradient strengths for small crystals.