atomic clock

Atomic clock  uses a atomic resonance frequency standard to calculate the real time. It gives the most accurate time and frequency , and are used as primary standards for international time distribution system  , and to control the frequency of television broadcasts and GPS  satellite signals. An extremely accurate reference clock whose operation is based on an atomic process, typically the frequency of electromagnetic radiation associated with a specified energy-level transition in an element such as cesium,Rubidium & Hydrogen etc. It is so precise that it loses only a second every 300 million years .An atomic clock consists of gas atoms captured in a magnetic field where they are held stationary with precise Laser Light and are cooled down to near absolute zero, minus 273 degrees Celsius. In this state the researchers can use the quantum properties of the atoms and get them to function as a clock movement with a pendulum.

As we know “An atom consists of a nucleus and some electrons that spin in clearly defined orbits around the nucleus”. By using the focused laser light one can make the electron swing back and forth in a clearly defined way between these orbits, and it is that which forms the pendulum in the atomic clock.

COMPARISON BETWEEN  ATOMIC CLOCK & MECHANICAL CLOCK

–>An atomic clock timekeeping device that is controlled by atomic or molecular oscillations. A timekeeping device must contain or be connected to some apparatus that oscillates at a uniform rate to control the rate of movement of its hands or the rate of change of its digits where as Mechanical clocks and watches use oscillating balance wheels, pendulums, and tuning forks.

–>In an atomic clock ,oscillation frequency within the atom are determined by the mass of the nucleus ,gravity and electrostatics “spring” between the positive charge on the nucleus and the electron cloud surrounding it.

Much greater accuracy can be attained by using the oscillations of atoms or molecules. Because the frequency of such oscillations is so high, it is not possible to use them as a direct means of controlling a clock. Instead, the clock is controlled by a highly stable crystal oscillator whose output is automatically multiplied and compared with the frequency of the atomic system. Errors in the oscillator frequency are then automatically corrected. Time is usually displayed by an atomic clock with digital or other sophisticated readout devices.

Atomics clocks are not radioactive. They don’t relay an atomic delay. Rather they have an oscillation  mass and spring just like a ordinary clock.

Types of Atomic clocks

Today, though there are different types of atomic clocks, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clocks include:

• Cesium atomic clocks employ a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.
• Hydrogen atomic clocks maintain hydrogen atoms at the required energy level in a container with walls of a special material so that the atoms don’t lose their higher energy state too quickly.
• Rubidium atomic clocks, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.

The most accurate atomic clocks available today use the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beam, reducing small changes in frequency due to the doppler

unknown facts

–>The first atomic clock, invented in 1948, utilized the vibrations of ammonia molecules.

–>In 1955 the first cesium-beam clock (a device that uses as a reference the exact frequency of the microwave spectral line emitted by cesium atoms) was placed in operation at the National Physical Laboratory at Teddington, England.

–>Since 1967, the International System of Units (SI) has defined the second as the duration of 9,192,631,770 cycles of radiation corresponding to the transition between two energy levels of the ground state of the caesium-133 atom.

–>The core of the atomic clock is a TUNABLE MICROWAVE CAVITY  containing the gas. In a hydrogen maser clock the gas emits microwaves (mases) on a hyperfine transition , the field in the cavity oscillates, and the cavity is tuned for maximum microwave amplitude. Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate. For both types the atoms in the gas are prepared in one electronic state prior to filling them into the cavity. For the second type the number of atoms which change electronic state is detected and the cavity is tuned for a maximum of detected state changes.

Applications

–>Without atomic clocks ,GPS (global position system is a actually a constellation of 27 earth orbiting satellite )navigation would be impossible , internet would not synchronize and position of the planet would not be known with enough accuracy for space probes and satellite could be launch with great degree of precision .

–>atomic clocks are used for long-baseline interferometry in radioastronomy.

–>They are also installed at some longwave and mediumwave broadcasting stations to deliver a very precise carrier frequency, which can also function as standard frequency.

Measurement of Atomic Time

The correct frequency for the particular cesium resonance is now defined by international agreement as9,192,631,770 Hz so that when divided by this number the output is exactly 1 Hz, or 1 cycle per second.The long-term accuracy achievable by modern cesium atomic clocks (the most common type) is better than one second per one million years. Hydrogen atomic clocks show a better short-term (one week) accuracy, approximately 10 times the accuracy of cesium atomic clocks. Therefore, the atomic clocks have increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.

WORKING OF CESIUM CLOCK

As we know “Atoms have characteristic oscillation frequencies.. An atom will have many frequencies, some at radio wavelength, some in the visible spectrum , and some in between the two. Cesium 133 is  most suitable element  for atomic clocks.

display using atomic clock

To turn the cesium atomic resonance into an atomic clock, it is necessary to measure one of its transition or resonant frequencies accurately. This is normally done by locking a crystal oscillator to theprincipal microwave resonance of the cesium atom. This signal is in the microwave range of the radio spectrum, and just happens to be at the same sort of frequency as direct broadcast satellite signals. Engineers understand how to build equipment in this area of the spectrum in great detail.

To create a clock, cesium is first heated so that atoms boil off and pass down a tube maintained at a high vacuum. First they pass through a magnetic field that selects atoms of the right energy state; then they pass through an intense microwave field. The frequency of the microwave energy sweeps backward and forward within a narrow range of frequencies, so that at some point in each cycle it crosses the frequency of exactly 9,192,631,770 Hertz (Hz, or cycles per second). The range of the microwave generator is already close to this exact frequency, as it comes from an accurate crystal oscillator. When a cesium atom receives microwave energy at exactly the right frequency, it changes its energy state.

At the far end of the tube, another magnetic field separates out the atoms that have changed their energy state if the microwave field was at exactly the correct frequency. A detector at the end of the tube gives an output proportional to the number of cesium atoms striking it, and therefore peaks in output when the microwave frequency is exactly correct. This peak is then used to make the slight correction necessary to bring the crystal oscillator and hence the microwave field exactly on frequency. This locked frequency is then divided by 9,192,631,770 to give the familiar one pulse per second required by the real world.