Ph.D. Thesis - Physics

Figure 3-2: A typical sample tube filled with molecules in solution that are used for NMR
quantum computation and simulation.
3.4.2 Magnet
The static magnetic field is provided by a superconducting magnet manufactured by Oxford
Instruments. A superconducting coil cryogenically cooled with liquid helium to 4.2 K creates
a magnetic field of B0 = 11.7 T using about 100 A of current. It features an active vibration
isolation system, and is actively shielded so that the 5 gauss line is about 3 m away from
the outer shell of the magnet. The cryogenic environment is maintained by surrounding the
liquid helium reservoir with a vacuum shell, which in turn is surrounded by a 77 K liquid
nitrogen reservoir, which itself is shielded by another vacuum shell. The hold time for liquid
nitrogen is about 10 days, while that for liquid helium is about three months.
The magnetic field due to this coil is not perfectly homogeneous. In practice, this leads
to a loss of coherence as variations in the field effect a “spread” in the chemical shifts of the
nuclei in the sample, and thus errors in the frequency at which the nuclei are being addressed.
Homogeneity is fine-tuned by a set of “shim” coils that surround the sample. One set of
coils is also superconducting, and is adjusted during magnet installation, not to be changed
by the user. The other set is not superconducting, and produces smaller spatially-varying
fields that are to be adjusted by the experimenter. The field homogeneity is maximized by
observing the linewidth of one nuclear resonance as a function of the currents in each of
these coils. This is an iterative process that can take some time. Automated routines do
exist, but we find that the best results generally come from manually tweaking the shim
currents. The shim settings depend strongly on the sample and probe being used, but for
each pair of these, there is a good “baseline” set of shim currents that is saved and re-used.
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