Ph.D. Thesis - Physics

Figure 3-1: A schematic of a generic NMR system.
the quantization ˆz axis for the nuclear spins contained therein. Coils of wire surrounding the
sample carry carefully-controlled ac currents to provide the rotations of the magnetizations
of the atomic nuclei. Fig. 3-1 is a diagram of a basic NMR system. Here we briefly discuss
each part, providing some specific details about our experimental system. The information
in this section is a somewhat abbreviated treatment of the extensive discussion presented
in the Ph.D. theses of Vandersypen [Van01] and Steffen [Ste03].
3.4.1 Sample
The system under study consists of a set of spin-1/2 nuclei that are contained within
molecules. Because the signal from a single molecule is difficult to detect, a large ensemble
of molecules is used. These are dissolved in a liquid solution. Although “liquid-state”
NMR is commonly used for such a system, we prefer the somewhat more accurate term
“solution-state.” If the sample molecule is dilute enough, this effectively eliminates inter-
molecular couplings. What one has then is a sample of identical quantum systems. For
these experiments, on the order of 10 18 molecules are used.
The sample is contained in a glass tube with a 5 mm outer diameter and 4.2 mm
inner diameter. These sample tubes have very straight sides and a uniform thickness;
this minimizes the effects of a varying magnetic susceptibility on the sample. Our tubes
were supplied mostly by Wilmad. The solution must be purged of impurities such as
paramagnetic oxygen through a freeze-thaw process before being flame-sealed. This process
involves freezing the sample, subjecting it to vacuum, then thawing it. With each step,
oxygen is removed. The sample is finally placed in a device called a “spinner,” which,
true to its name, enables the tube to be rotated around the magnetic field axis (ˆz) at 20
revolutions per second. This helps to cancel inhomogeneities in the ˆx-ˆy magnetic field.
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