NMR quantum computing

Abstract:

We describe the development and application of a novel rapid sample-mixing technique for real-time NMR (nuclear magnetic resonance) spectroscopy. The apparatus consists of an insert inside a conventional NMR tube coupled to a rapid injection syringe outside the NMR magnet. Efficient and homogeneous mixing of solutions in the NMR tube is achieved with a dead time of tens of milliseconds, without modification of the NMR probe or additional hardware inside the magnet. Provision is made for the inclusion of an optical fiber to allow in situ laser irradiation of samples, for example to generate photo-CIDNP (chemically induced dynamic nuclear polarization). An NMR water suppression method has been implemented to allow experiments in H(2)O as well as in deuterated solvents. The performance of the device has been tested and optimized by a variety of methods, including sensitive detection of residual pH gradients and the use of NMR imaging to monitor the extent of mixing in real time. The potential utility of this device, in conjunction with the sensitivity and selectivity of photo-CIDNP, is demonstrated by experiments on the protein hen lysozyme. These measurements involve the direct detection of spectra during real-time refolding, and the use of CIDNP pulse labeling to study a partially unfolded state of the protein under equilibrium conditions. Magnetization transfer from this disordered state to the well-characterized native state provides evidence for the remarkable persistence of nativelike elements of structure under conditions in which the protein is partially denatured and aggregation prone.

Abstract:

While nuclear-magnetic-resonance (NAIR) techniques are unlikely to lead to a large-scale quantum computer, they are well suited to investigating basic phenomena and developing new techniques. Indeed, it is likely that many existing NMR techniques will find uses in quantum information processing. Here I describe how the composite-rotation (composite-pulse) method can be used to develop quantum logic gates which are robust against systematic errors.

Abstract:

The oxidative folding of the Amaranthus alpha-amylase inhibitor, a 32-residue cystine-knot protein with three disulfide bridges, was studied in vitro in terms of the disulfide content of the intermediate species. A nonnative vicinal disulfide bridge between cysteine residues 17 and 18 was found in three of five fully oxidized intermediates. One of these, the most abundant folding intermediate (MFI), was further analyzed by (1)H NMR spectroscopy and photochemically induced dynamic nuclear polarization, which revealed that it has a compact structure comprising slowly interconverting conformations in which some of the amino acid side chains are ordered. NMR pulsed-field gradient diffusion experiments confirmed that its hydrodynamic radius is indistinguishable from that of the native protein. Molecular modeling suggested that the eight-membered ring of the vicinal disulfide bridge in MFI may be located in a loop region very similar to those found in experimentally determined 3D structures of other proteins. We suggest that the structural constraints imposed on the folding intermediates by the nonnative disulfides, including the vicinal bridge, may play a role in directing the folding process by creating a compact fold and bringing the cysteine residues into close proximity, thus facilitating reshuffling to native disulfide bridges.

Abstract:

The use of composite rotations to combat systemetic errors in single-qubit quantum logic gates was described. Three families of composite rotations which could be used to correct off-resonance and pulse length errors was also discussed. Results showed that any implementation of a quantum computer must ultimately be concerned with rotations on the Bloch sphere, and so composite pulse techniques may find a broader application in quantum computing.