Here we demonstrate how para-hydrogen can be used to prepare a two-spin system in an almost pure state which is suitable for implementing nuclear magnetic resonance (NMR) quantum computation. A 12 ns laser pulse is used to initiate a chemical reaction involving pure para-hydrogen (the nuclear spin singlet of H2). The product, formed on the µs timescale, contains a hydrogen-derived two-spin system with an effective spin-state purity of 0.916. To achieve a comparable result by direct cooling would require an unmanageable (in the liquid state) temperature of 6.4 mK or an impractical magnetic field of 0.45 MT at room temperature. The resulting spin state has an entanglement of formation of 0.822 and cannot be described by local hidden variable models.PACS numbers: 03.67. Lx, 03.67.Mn, Introduction. While quantum computing [1] offers the potential of using new quantum algorithms to tackle problems that are intractable for classical processors, its implementation requires the development of quantum devices, which are as yet unavailable. The most complex implementations of quantum algorithms to date have used techniques adapted from nuclear magnetic resonance (NMR) spectroscopy [2,3,4,5], but current liquid state NMR approaches cannot be extended to systems with many quantum bits, as it is not possible to prepare pure initial states by directly cooling the spin system into its ground state [6]. Furthermore, it has been shown that current NMR experiments involve only separable states [7], and thus could in principle be described by local hidden variable models.The conventional approach in NMR quantum computing [4] is to use an ensemble of spins, and to prepare a pseudo-pure ground state [2,4] of the form
Electric quadrupole coupling constants and linewidths of the Li NMR spectrum of a single crystal of LiNbO3 were measured at temperatures between 24 and 680°C and are interpreted in terms of the motion of Li. The linear increase of the quadrupole coupling constant is unusual and is assumed to be due to the anisotropic vibration of Li. A simple expression for the temperature variation is derived which depends only on 〈z2〉 − (x2〉 and ∂4V / ∂4z, where 〈z2〉 is a mean square amplitude of vibration of Li, and V is the crystal electric potential at a Li site. ∂4V / ∂z4 was computed by an ionic lattice summation. It is suggested that a linear increase of the quadrupole coupling constant is characteristic of a light atom that vibrates anisotropically. Analysis of the linewidth data, assuming that Li atoms diffuse, gives D0 = 1.8 × 10−5m2sec−1 and ε = 1.62 eV.
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