The atomic clock is a clock type where the base time is determined by the resonance frequency of an atom.
The first maser atomic clocks were simple supplemented by appropriate detection systems. Today the best clocks to determine the time standards are based on more complex physical principles involving the use of cold atoms and fountains of atoms.
Metrology institutes maintain the standard time with an accuracy of 10-9 seconds per day and an accuracy equal to that of the frequency of the radio transmitter used to "pump" the maser. On this basis, maintains a continuous and stable time scale: the International Atomic Time (International Atomic Time).
For the computation of time civil uses a different scale, Coordinated Universal Time (Coordinated Universal Time - UTC). The second comes from the first but is synchronized with the astronomical time marked by the Earth's rotation.
The first experimental atomic clock was built in 1949 and installed at the National Bureau of Standards in the United States. The first model to be sufficiently accurate, based on transitions of energy levels in the atom of cesium, was built in 1955 by Louis Essen at the National Physical Laboratory in Great Britain. It was installed at the Greenwich Observatory in London.
The use of these watches in 1967 led to the definition of seconds based on atomic time. From 1972 (the date of the introduction of the "atomic time") to 1999 were added to the total of "earth time" 22 seconds.
In August 2004, scientists at the National Institute of Standards and Technology have unveiled a prototype of an experimental atomic clock integrated on a chip. The authors believe that this device has been defined as one hundredth of those of the previous smallest atomic clock. It would also require only 75 milliwatts of electrical power to operate, making it suitable for use in battery-powered portable devices.
For now it is better to use radio-controlled watches, which you can receive the time signal produced by atomic clocks in an economical and practical.
The maser atomic clocks use a resonant cavity containing an ionized gas. It is usually used cesium because this is the basis of the definition of the second as 9,192,631,770 cycles of radiation corresponding to the transition between two specified energy levels of the ground state of this.
This makes the oscillator to the cesium, as it is sometimes called the atomic clock, the primary standard for measuring time and frequency. Other physical quantities such as volt meters and are defined by invoking the latter as a fundamental quantity.
The heart of an atomic clock is in addition to the microwave cavity already hinted, by an oscillator / radio transmitter and a tunable feedback loop (servo system) which regulates the frequency of the oscillator at exactly the frequency at which resonance occurs for the particular type atoms in the cavity.
The transmitter fills the cavity with standing waves, when the frequency coincides with the resonant frequency of the gas, the electrons of the atoms absorb radio waves and jump to higher energy level. Returning to the original level in the form of re-emit light energy previously absorbed.
If the pumping rate differs from the value of resonance, the intensity of light output decreases. A photocell detects the change and then a circuit corrects the frequency in the direction of the light intensity to bring maximum value.
The way this process of feedback work is naturally more complex, since it must also suppress the frequency of side effects such as electronic or other distortion levels in the transitions, temperature changes etc..
For example, the frequency of radio waves can be modulated sinusoidally so that the brightness of the photocell has a similar trend variable. This signal can then be used to monitor the long-term drift rate.
The result is to fluctuate (within a certain margin of error), the microwave generator according to the quantum properties of extremely precise cesium. When the system is turned on you need some time to go to the specific product and system is reliable.
Finally, a counter counts the cycles of the original frequency and communicates them to a computer, which presents them in numerical form, or send them via radio or the Internet.
There are several variations to this configuration. Rubidium clocks have a low cost, small dimensions (business models occupy a volume of 400 cm3) and good thermal stability in the short term. They are used in aerospace and commercial applications. The hydrogen maser (built in Russia in particular) have a short-term stability better than other systems but lower accuracy in the long term.
Often a standard is used to correct another. For example, in some commercial applications has used a rubidium oscillator coupled to a Global Positioning System. This method allows to achieve good accuracy in the short term along with a long-term stability refers to the time standard of the United States (whose government is administered by the GPS).
Of practical importance is also the lifetime of a reference standard. Modern Rubidium maser tubes last over ten years and cost around € 50. Cesium tubes used by national metrology offices have a lifespan of about seven years and cost more than € 30,000. Hydrogen systems are limited by the ratio between the amount of hydrogen stored (typically in cylinders with hydride inside) and energy consumed per unit of time.
Currently, the research aims to make atomic clocks are more compact, economical, accurate and reliable, even if these goals are often in mutual conflict.
Many studies focus on the use of ion traps. Theoretically, a single ion remained suspended in an electromagnetic field can be kept under observation for a long period of time, while obtaining a higher accuracy and lower power consumption and size.
The clock in single ion has a low stability in the short term because the ion is subject to constant vibration due to temperature. For this reason we employ laser cooling of ions combined with optical resonators, in order to suppress the effects due to thermal and mechanical noise. The best technique cools a sapphire resonator at liquid helium temperature. The laser is not widely used instead. It follows that the current ion traps are compact, but the accessories instead occupy much space.
Some researchers have developed ion traps with different geometry, for example elongated cloud of ions give a better accuracy in the short term.
The best system is being developed uses of mercury ions. It was created at NIST that uses a laser pulse in a femtosecond. Has an accuracy of 5 orders of magnitude more than the cesium clock. Its designers say it could miss a second "after 4.5 billion years."
A particular isotope of ytterbium has a precise and specific frequency of a resonance of its levels of a hyperfine transition. Strontium has a hyperfine transition state that is not accurate but can be activated by a solid state laser, allowing the realization of very cheap devices, compact and durevoli.Recentemente was discovered that the atom of aluminum is the most accurate.
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