量子频率标准Quantum Frequency Standards Chapter8

8.1.1 Principle of the Hydrogen Maser
The H maser utilises a transition between two ground state levels |F=1 ,mF=0> and |F=0 ,mF=0> of atomic hydrogen, with a frequency separation of 1.42 GHz (Fig. 5.22). Hydrogen masers used today do not differ much from the first realisation in the group of Norman Ramseser
• The first microwave amplification by stimulated emission of radiation (maser) was proposed in 1954.
• Nobel prize in physics 1964.
Charles H. Townes "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle"
• H-maser with a short and medium-term stability which surpasses the best Cs clocks.
• Compact and cheaper Rb cell clock, used for applications with less demanding accuracy
8.2 Rubidium-cell Frequency Standards 8.2.1 Principle and Set-up 8.2.2 Performance of Lamp-pumped Rubidium Standards 8.2.3 Applications of Rubidium Standards
Figure 8.1: Schematic of an active hydrogen maser.
Hydrogen atoms from the source (Fig. 8.1) effuse in a vacuum of about 10−4 Pa maintained with an ion pump. Low-field seeking atoms are focussed into a storage bulb by means of a state selecting magnet which, at the same time, deflects the high-field seekers in the lowest states such that they do not enter the bulb. Hence, the storage cell, which is placed in a microwave resonator, contains more atoms in the upper state which can emit radiation by stimulated emission. The radiation is detected by an antenna and used to tune the frequency of a voltage-controlled crystal oscillator (VCXO) to the transition of the hydrogen atoms. The maser design allows the detection of the atomic transition with a small linewidth as a result of the long possible interaction time of about a second which is the time the atoms typically spend in the storage bulb. During this time the atoms are kept in a volume whose extension of about 15 cm is smaller than the wavelength of the transition. This confinement to the Lamb– Dicke regime leads to a suppression of the first-order Doppler effect of the atoms interacting with the standing-wave field allowed in the microwave resonator.
8.1 Maser
• Masers have been built using various atomic or molecular species in order to perform high-resolution microwave spectroscopic investigations.
Quantum Frequency Standards
Chapter 8: Microwave Frequency Standards
H-maser, Rb-cell clock
Microwave Frequency Standards
8.1 Masers 8.1.1 Principle of the Hydrogen Maser 8.1.2 Theoretical Description of the Hydrogen Maser 8.1.3 Design of the Hydrogen Maser 8.1.4 Passive ydrogen Maser 8.1.5 Cryogenic Masers 8.1.6 Applications
Nikolai Gennadievich Basov ,
Alexander Mikhailovich Prokhorov
"for basic researches in the field of experimental physics, which led to the discovery of the maser and the laser"
8.3 Alternative Microwave Standards 8.3.1 Laser-based Rubidium Cell Standards 8.3.2 All-optical Interrogation of Hyperfine Transitions
Introduction
• In addition to Cs clock, there are other microwave standards based on neutral atoms, which are used for different purposes.
• For frequency standard: Ammonia, hydrogen, rubidium, cesium
From the various kinds of masers developed for frequency standards and based on, e.g., ammonia, hydrogen, rubidium, or caesium, the hydrogen maser (H maser) has achieved the greatest widespread use