Patent GB-881424-A (GB)
Title: Optical absorption monitoring of oriented or aligned quantum systems
IPC: H03L7/26
Filed: 1958-02-13
Abstract: 881,424. Nuclear magnetic resonance apparatus. VARIAN ASSOCIATES. Feb. 13, 1958 [Feb. 13, 1957; March 28, 1957 (2); April 16, 1957], No. 4711/58. Class 37. [Also in Groups XL (b) and XL (c)] The relative number of the atoms occupying the various quantum energy levels in a gas or vapour irradiated by electromagnetic radiation is determined by radiation responsive means detecting absorption charges in the radiation passing through the gas or vapour. Thus in Fig. 1 (not shown), a thermionic diode contains mercury vapour and the bombarding electron beam raises some atoms from the 6<SP>1</SP>S 0  ground state to the 6<SP>3</SP>P 2  metastable state where they are trapped. Upon application of a unidirectional magnetic field Ho, the 6<SP>3</SP>P 2  state is split into five Zeeman sub-levels of which the M=0 and M=Œ1 levels are predominantly excited, where M is the magnetic moment. When 5641A. light (green) from a mercury vapour lamp is focused through a polarizing screen 16 (see Fig. 6) and passed through the excited gas system, only atoms from certain sub-levels absorb the polarized light. Measurement of the absorption by a P.E. cell 17 determines the extent to which the alignment of the atoms in the 6<SP>3</SP>P 2  state has occurred. If paramagnetic realignment of the Hg atoms in the Zeeman levels is obtained by applying a radio frequency magnetic field H, of the appropriate Larmor frequency, the non-absorbing M=Œ2 sublevels are populated at the expense of the absorbing levels, and the absorption of the # 5461 radiation is weakened. By modulating the H o  field or of the radio frequency field H 1 , the point of maximum paramagnetic resonance will be swept through and indicated on an oscilloscope coupled to the P.E. cell. If the value of the Larmor frequency is known, the strength of the magnetic field may be determined as in Fig. 6 where the frequency from the R.F. generator 22 is adjusted until minimum radiation transmission is detected by the P.E. cell 17. A phase-sensitive detector 28 connected both to the photo-cell amplifier 19 and to the audio-sweep generator 25, yields a signal which adds bias, shifting the resonance automatieally to its maximum value. The current meter 29 is calibrated in magnetic field strengths. Atoms may be investigated spectroscopically using precisely determined magnetic fields, radio frequencies and optical transmission frequencies. Transitions between hyperfine states may be detected using micro-wave magnetic fields, e.g. in mercury isotope 199. In a modification (Fig. 7, not shown), a spherical vessel at 130-150‹ C. contains alkali metal vapour, e.g. sodium, and a buffer gas, e.g. argon, and circularly polarized light of # 5896 parallel to H o  causes optical pumping and the occupation of the non-absorbing M=Œ2 sub-levels which is detected in the photo-cell and displayed or recorded. By reversing the duration of the circular polarization or of the magnetic field H 0 , the optical pumping effect may cause variation in absorption with consequential application as a fast light shutter. The operating conditions for potassium, rubidium, caesium and lithium are also described. The device operates as a highly sensitive magnetometer (Fig. 11, not shown), the frequency of the R.F. magnetic field being adjusted for maximum paramagnetic resonance. Initial polarization may be generated by optical pumping using crossed beams, Fig. 14, in which in addition to the absorption of circularly polarizing light being detected by P.E. cell 37, there is a second system comprising sodium lamp 48, circular polarizer 51 from which light passes perpendicularly to H o  and the first light beam and is received by photo-cell 53, the signal being amplified and detected for display on oscilloscope 56. A single beam at 45 degrees to the H o  field may replace the two beams of Fig. 14. In Fig. 17 (not shown), the R.F. amplifier is coupled to a demodulator and a lock-in detector, the error signal automatically adjusting the frequency of the R.F. generator to a value which is a measure of the field strength. Apparatus is described using three beams at right angles, Fig. 18 (not shown). A free-running oscillator, Fig. 19, comprises two optical systems, a portion of the output of R.F. amplifier 54 being fed through a phaseshifter to the radio frequency coil 42. The oscillation is placed on the centre frequency of the resonance for maximum stability and is field dependent. Measurement of the frequency by counter 71 gives the field strength of Ho Rubidium may replace sodium (Fig. 20, not shown). A free precession magnetometer measuring a field He, comprises a sodium system precessing in the field and subjected to an X -beam of circularly polarized light. A pulse of magnetic field H o  is applied by means of coils 73, pulser 74 and source 75 and causes precession round He which is measured by cell 37 and frequency counter 76 (Fig. 21). A single-field at 45 degrees to the beam may be utilized and measured (Fig. 22, not shown); and two light beams at right angles may operate similarly (Fig. 23, not shown). A quantum system for mercury atoms may have the light modulated at twice the precession frequency, detected and displayed on an oscilloscope.