Page 180 - Principles and Applications of NanoMEMS Physics

168 Chapter 4

spin orients itself in the direction of the field. If, in addition, an RF field is
applied in a pulsed fashion, the spins are made to tip off-axis, while
precessing about the direction of the constant field. The precession
frequency is the so-called Larmor frequency and is given by Ȧ = µB . For
the hydrogen atom (proton), the magnetic moment is 42.7MHz/T and, at a
typical field of B=11.7T, its precession frequency is 500MHz. Sample
examination is accomplished placing a coil around it, tuned to the precession
frequency, which picks up the oscillating currents induced as a consequence
of the magnetic field produced by the precessing protons. The device that
applies the static magnetic field and the RF control pulses, and then detects
the magnetic induction is called an NMR spectrometer, see Fig. 4-15.

Sa m p le T u b
Sa m p le T u b e e

Mix er r
Mix e
C C
RF
RF
Co m p u ter r
D irectio n a l a l
D irectio n Co m p u te
C o uple Osc illa to
C o upler r
Osc illa to r r
RF Co il il
RF Co
Am p lifier r
Am p lifie
B B
0 0
Sta tic F ie ld C o
Sta tic F ie ld C o il il
Figure 4-15. Sketch of experimental NMR spectrometer. (After [195].)
NMR phenomena, which were first observed in 1946 [196], [197],
became the basis for a multitude of analytical studies of materials, in
particular, the determination of molecular structures [198], and magnetic
resonance imaging [199]. In these contexts, the technology of NMR
spectroscopy is rather mature.
The application of NMR to QC was advanced Cory, Fahmy and Havel
[200], and Gershenfeld and Chuang [201] in 1997. To overcome the
difficulty in detecting the spin of individual, adoption was made of qubits
implemented in the liquid state, where additive effects could be exploited to
yield a reasonably large signal amplitude. Also adopted were methods to
discern the fraction of nuclear spins pointing in the external field direction,
despite the effects of temperature-induced random spin orientation. The two-
state quantum system was realized by choosing molecules possessing spin-
1/2 nuclei, which in the presence of the external magnetic field adopts two