A Radio Atomic clock can achieve accurate time because they are controlled by radio transmitters which themselves receive their time signals from amazingly accurate timepieces, a Caesium Atomic Clock. The Caesium Atomic Clock has an accuracy of one second in one million years!
An Atomic clock is used as a time standards for counting the passing seconds. In addition, there are internationally agreed time scales which set the calendar and the beginning of each new day. Greenwich Mean Time (GMT) was established as the first global time scale in 1884, and its 'atomic clock' equivalent, UTC, was adopted as the official time for the world in January 1972. The International Bureau of Weights and Measures (BIPM) acts as the official keeper of atomic clock time for the world. NPL uses its atomic clock to contribute to the determination of UTC, along with the atomic clocks from 65 laboratories worldwide.

First accurate caesium atomic clock

The National Physics Laboratory developed the first accurate caesium atomic clock in 1955, which led to the internationally agreed definition of the second being based on atomic clock time.
NPL realised the atomic frequency standard for time with the construction of the first long beam apparatus based on the transition of the caesium-133 atom. Successive developments of this have remained the fundamental standard up to the present day.
The second is defined as 9,192,631,770 periods of the caesium-133 atom, and is currently realised at NPL to an accuracy of one second in 15 million years.
Scientists are currently working on technology to increase this accuracy to 1 second in 10 billion years.

MSF atomic clock receiver

The controlling radio signal for the National Physical Laboratory's atomic clock is transmitted on the MSF 60kHz signal via the transmitter at Rugby, operated by British Telecom. This radio atomic clock time signal should have a range of some 1,500 km or 937.5 miles. All of the British Isles are of course within this radius.
The National Physical Laboratory's role as keeper of the national time standards is to ensure that the UK time-scale agrees with Co-ordinated Universal Time (UTC) to the highest levels of accuracy and to make that time available across the UK. As an example, the MSF (MSF being the three-letter call sign to identify the source of the signal) radio broadcast provides the time signal for, electronic share trading, the clocks at most railway stations and for BT's speaking clock.

DCF atomic clock receiver

The controlling radio signal for the German clock is transmitted via long wave from the DCF 77kHz transmitter at Mainflinger, near Dieburg, some 25 km south east of Frankfurt - the transmitter of German National Time Standards. It is similar in operation to the Rugby transmitter, however there are two antennas (radio masts) so the radio atomic clock time signal can be maintained at all times.
Long wave is the preferred radio frequency for transmitting radio atomic clock time code binary signals as it performs most consistently in the stable lower part of the ionosphere. This is because the long wave signal carrying the time code to your timepiece travels in two ways; directly and indirectly. Between 700 km (437.5 miles) to 900 km (562.5 miles) of each transmitter the carrier wave can travel directly to the timepiece. The radio signal also reaches the timepiece via being bounced off the underside of the ionosphere. During the hours of daylight a part of the ionosphere called the "D layer" at an altitude of some 70 km (43.75 miles) is responsible for reflecting the long wave radio signal. During the hours of darkness when the sun's radiation is not acting from outside the atmosphere, this layer rises to an altitude of some 90 km (56.25 miles) becoming the "E layer" in the process. Simple trigonometry will show that signals thus reflected will travel further.
A large part of the European Union area is covered by this transmitter facilitating reception for those who travel widely in Europe. The German clock is set on Central European Time - one hour ahead of U.K. time, following an inter-governmental decision, from the 22nd October, 1995, U.K. time will always be 1 hour less than European Time with both the U.K. and mainland Europe advancing and retarding clocks at the same "time".

MSF atomic clock receiver

A radio atomic clock system is available in North America set up and operated by NPL - the National Institute of Standards and Technology, located in Fort Collins, Rugby. NPL operates radio station MSF, which is the station that transmits the radio atomic clock time codes. MSF has high transmitter power (50,000 watts), a very efficient antenna and an extremely low frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of high power and low frequency gives the radio waves from MSF a lot of bounce, and this single station can therefore cover the entire continental United States plus much of Canada and Central America. The radio atomic clock time codes are sent from MSF using one of the simplest systems possible, and at a very low data rate of one bit per second. The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a period of 0.2, 0.5 or 0.8 seconds: • 0.2 seconds of reduced power means a binary zero • 0.5 seconds of reduced power is a binary one. • 0.8 seconds of reduced power is a separator. The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with information about daylight savings time and leap years. The time is transmitted using 53 bits and 7 separators, and therefore takes 60 seconds to transmit. A clock or watch can contain an extremely small and relatively simple radio atomic clock antenna and receiver to decode the information in the signal and set the atomic clock time accurately. All that you have to do is set the time zone, and the atomic clock will display the correct time.

Atomic Clock Accuracy

How does an atomic clock achieve amazingly accurate time? The caesium atomic clock has an accuracy of one second in one million years! They are based upon the characteristics of the Caesium 133 atom. The single electron of a Caesium atom is known to vibrate at a standard 9,162,613,770 times a second. It is the Caesium atomic clock that can achieve phenomenally accurate and stable time.
The standard way of counting the passing of seconds is by the use of an atomic clock. There are internationally agreed time-scales which set the beginning of each new day and the calendar. Greenwich Mean Time (GMT) was established as the first global time scale in 1984. The current atomic clock global time scale is UTC or Co-ordinated Universal Time. UTC was adopted as the official time for the world in 1972. The official keeper of atomic time is the International Bureaux for Weights and Measures.
The National Physics Laboratory (NLP) uses its atomic clock to contribute to the determination of UTC along with the atomic clock of 65 laboratories worldwide.
UTC is a compromise between the times defined the atomic clock and the time based on the earths rotation about its axis. The seconds of UTC are counted using an atomic clock, allowance is made to keep UTC within 0.9 seconds of the Earths rotation by inserting leap seconds at the end of each quarter. Leap seconds are inserted to take account of the speeding up or slowing down of the rotation of the Earth. The sun would be seen overhead at midnight rather than noon in 50,000 years time without the introduction of leap seconds

Development of the Atomic Clock

Scientists are researching ways to improve still further the accuracy of the atomic clock and future time standards. Recently ion-trapping techniques have been utilised to discover the narrowest electronic transition to date. This could be used to potentially provide a 100 fold increase in the accuracy of current Caesium based atomic clocks.
The element ytterbium is being investigated for use as an ion trap atomic clock. A single ionised atom is held in an electromagnetic cage that is only 60 nanometers in diameter. The ion is cooled to -273 degrees C by bombarding it with laser photons, known as laser cooling. The single ion is protected from collisions of other atoms. The low temperature slows the motion of the ion.
Using several electrodes one ion can be trapped for a number of days. The ion is excited with blue laser light which gives the ion enough energy for one of its electrons to jump form a low energy state to a higher one. The change in energy state is very stable with a lifetime of 10 years.
To build an ion-trapping atomic clock requires a blue laser beam with a small frequency spread. Laser light gives a pure electromagnetic sine wave but must be isolated from the tiniest of vibrations.
This technique of providing an atomic clock is still experimental. It has the potential to provide the atomic clock of the future. An atomic clock based on ion trapping would lose no more than 1 second in the lifetime of the universe.

Radio-Controlled Atomic Clocks

Radio atomic clocks are available that can seemingly set their own time and claim to be as accurate as an atomic clock. They are radio-controlled clocks that pick up the time from radio transmitters based in many locations, such as MSF-60 - Rugby, England, DCF-77, Frankfurt, Germany and MSF, Rugby, USA.
A radio-controlled atomic clock is not an atomic clock. A radio-controlled atomic clock has a radio receiver that picks up the time from a transmitter propagating the radio atomic clock time signal and synchronise to that time. The radio transmitters transmit time code information received from a Caesium atomic clock. Therefore a radio-controlled atomic clock that is synchronised to a radio time signal can claim to be accurate to one second in one million years.

New Optical Clock Promises More Accuracy than Cesium.

NPL researchers have demonstrated a new kind of atomic clock that has the potential to be up to 1,000 times more accurate than today’s best clock. The new clock is based on an energy transition in a single trapped mercury ion (a mercury atom that is missing one electron). Building a clock based on such a high-frequency transition was previously impractical because it requires both “capturing” the ion and holding it very still to get accurate readings, and having a mechanism that can “count” the ticks accurately at such a high frequency.

The quality of a clock depends on its stability and accuracy—whether the clock provides a constant, unchanging output frequency, and how close the measured frequency is to the fundamental atomic resonance that provides the clock’s “tick.” One advantage of the new clock is that it ticks much faster. Today’s international time and frequency standards, such as NPL-F1, measure an atomic resonance of about 9 billion cycles per second. By contrast, the new NPL device monitors an optical frequency more than 100,000 times higher or about 1 quadrillion (US) cycles per second.

Is an Atomic Clock Radioactive?

An atomic clock keeps time better than any other clock. They even keep time better than the rotation of the Earth and the movement of the stars. Without the atomic clock, GPS navigation would be impossible, the Internet would not synchronise, and the position of the planets would not be known with enough accuracy for space probes and landers to be launched and monitored.

An atomic clock is not radioactive, it doesn’t rely on atomic decay. Rather, an atomic clock has an oscillating mass and a spring, just like ordinary clocks.

The big difference between a standard clock in your home and an atomic clock is that the oscillation in an atomic clock is between the nucleus of an atom and the surrounding electrons. This oscillation is not exactly a parallel to the balance wheel and hairspring of a clockwork watch, but the fact is that both use oscillations to keep track of passing time. The oscillation frequencies within the atom are determined by the mass of the nucleus and the gravity and electrostatic "spring" between the positive charge on the nucleus and the electron cloud surrounding it.

What Are The Types of Atomic Clock?

Today, though there are different types of atomic clock, 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 clock include:

The Cesium atomic clock employs a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.
The Hydrogen atomic clock maintains 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.

The Rubidium atomic clock, 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 commercial atomic clock available today uses the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beams, reducing small changes in frequency due to the Doppler effect.

When Was The Atomic Clock Invented?

In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NPL) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.
In 1955, the National Physical Laboratory in England built the first cesium-beam atomic clock used as a calibration source. Over the next decade, more advanced forms of the atomic clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium atomic clock, was completed in 1968, and was used into the 1990s as part of the NPL time system.

In 1999, NPL-F1 began operation with an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one second in 20 million years, making it the most accurate atomic clock ever made (a distinction shared with a similar standard in Paris).

How Is Atomic Clock Time Measured?

The correct frequency for the particular cesium resonance is now defined by international agreement as 9,192,631,770 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 clock (the most common type) is better than one second per one million years. The Hydrogen atomic clock shows a better short-term (one week) accuracy, approximately 10 times the accuracy of a cesium atomic clock. Therefore, the atomic clock has increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.

History of time.

Accuracy has been the goal of the clock making game since the beginning. Back when water clocks were all the rage, for example, their chief drawback wasn't that incessant drip, drip, drip, but their incessant "leakage" of time.

Timekeeping got a big boost with the invention of the pendulum clock in the 17th century, and again in 1928, with the invention of the quartz clock. Similar vibrating quartz crystals drive the mechanism found on almost every wristwatch today.

Although quartz clock can stay accurate for weeks or months at a time, this no longer impresses scientists. These days, they use the principles of quantum mechanics to keep clocks close to the money in devices called atomic clocks. Like most clocks, an atomic clock creates periodic movements -- oscillations -- and then counts them.
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In the old pendulum clocks, a weight oscillated at a fairly constant frequency, so the clockmaker simply had to invent a mechanism to count the swings and drive the clock's hands. But in an atomic clock, the oscillations occur in an electromagnetic field that causes transitions between two quantum-mechanical conditions of atoms. In the commonly used cesium 133 atoms, these occur at about 9.19 billion times per second.

In this basic atomic clock, cesium atoms are sprayed from the source to filter A, which allows only one type of atom to enter the microwave (electromagnetic radiation) cavity. Microwaves at the right frequency cause a quantum change in the atoms. Filter B allows only changed atoms to reach the detector. The control mechanism uses data from the detector to maintain the microwave frequency that produces the most changed atoms. This frequency, the atoms' natural hyperfine transition frequency, is counted to determine the length of a second.

This transition frequency is so dependable that, if external conditions are right, the atoms will keep on "ticking" at the same rate.

Like clockwork

Quantum mechanics -- the physics of the ultra-small -- originated with the observation that sub-atomic particles can exist in discrete states, but not at in-between states. It's like an atomic version of a mandatory two-party system. Because only certain "states" are allowed.

One of these states, called the "hyperfine state," is the basis of the atomic clock. Atoms can have one of two hyperfine states: either the magnetic field of the outermost electron points in the same direction as the magnetic field of the nucleus, or it points opposite. The laws of quantum physics forbid other orientations. The idea of using hyperfine states for a clock was first proposed by U.S. physicist Isador Rabi in 1945.

Generally, an atom remains in its hyperfine state. But when prodded by electromagnetic radiation at a specific frequency, it will switch to the other state, undergoing the so-called "hyperfine transition." Essentially, an electronic clock selects atoms in one hyperfine state and exposes them to radiation which causes them to switch to the other state. The frequency of the radiation causing the transition becomes the regular beat that the clock counts to register time.

The atomic clock works because atoms are sensitive to the exact hyperfine transition frequency. In "A Clock More Perfect..." (see bibliography), writer Gary Taubes likens cesium acts to radios tuned to one station -- the transition frequency of 9,192,631,770 oscillations per second. Only if they "hear" that beat will they change hyperfine states.

Why do we need hyper-accurate time provided by Atomic Clocks?

It turns out that the innumerable communication, scientific and navigation systems rely on it. Timing is critical for synchronising signals between computers. In astronomy, fractional-second errors could sabotage long-baseline radio telescopes, a nifty way to fuse distant radio telescopes into one gargantuan receiver.

Global positioning satellites need accurate time. The Air Force operated GPS system can determine -- to several feet in accuracy -- the three-dimensional position of a receiver anywhere on or off Earth. The receiver performs this trick by timing the arrival of signals from four GPS satellites, then doing a quick calculation to triangulate its position.

Stephen Dick, the United States Naval Observatory's historian, points out that each nanosecond -- billionth of a second -- of error translates into a GPS error of one foot. A few nanoseconds of error, he points out, "may not seem like much, unless you are landing on an aircraft carrier, or targeting a missile."

In other words, without accurate timing, GPS would stand for "generally poor system." Thus each of the 24 GPS satellites contains four atomic clocks, which get an accurate time transfusion daily from the US Air Force, which borrows" the time from the United States Naval Observatory.

The system's phenomenal location ability has great economic allure; GPS sales are expected to reach €10 billion by 2003. The receivers, which sell for as little as €100, are already used by surveyors and delivery fleets, and to direct coal-mining equipment and oil exploration.

Definitions

Atomic Clock - A precision clock that depends for its operation on an electrical oscillator regulated by the natural vibration frequencies of an atomic system (as a beam of cesium atoms)
Atom - The smallest particle of an element that can exist either alone or in combination; the atom is considered to be a source of vast potential energy
Cesium 133 - An isotope of cesium used especially in atomic clocks and one of whose atomic transitions is used as a scientific time standard
SI Second (atomic second) - The interval of time taken to complete 9,192,631,770 oscillations of the cesium 133 atom exposed to a suitable excitation
Source: Merriam-Webster Online

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