Chip-scale atomic clock unveiled by NIST

An atomic clock is a type of clock that uses an atomic resonance frequency standard as its timekeeping element. Circadian Locomotor Output Cycles Kaput, or Clock is a gene which encodes proteins regulating Circadian rhythm. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny A frequency standard is a stable oscillator used for Frequency Calibration or reference They are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, and to control the frequency of television broadcasts and GPS satellite signals. Frequency is a measure of the number of occurrences of a repeating event per unit Time. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth

Atomic clocks do not use radioactivity, but rather the precise microwave signal that electrons in atoms emit when they change energy levels. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J A quantum mechanical system or particle that is bound, confined spacially can only take on certain discrete values of energy as opposed to classical particles which Early atomic clocks were masers with attached equipment. A maser is a device that produces coherent Electromagnetic waves through amplification due to Stimulated emission. Today's best atomic frequency standards (or clocks) are based on absorption spectroscopy of cold atoms in atomic fountains. Absorption spectroscopy refers to a range of techniques employing the interaction of electromagnetic radiation with matter An atomic fountain is a cloud of atoms that are tossed upwards by Lasers in the Earth's gravitational field

National standards agencies maintain an accuracy of 10^-9 seconds per day (approximately 1 part in 10¹⁴), and a precision set by the radio transmitter pumping the maser. The clocks maintain a continuous and stable time scale, International Atomic Time (TAI). International Atomic Time ( TAI, from the French name Temps Atomique International) is a high-precision atomic Time standard that tracks For civil time, another time scale is disseminated, Coordinated Universal Time (UTC). UTC is derived from TAI, but synchronized using leap seconds to UT1, which is based on actual rotations of the earth with respect to the mean sun. A leap second is a one- Second adjustment that keeps broadcast standards for time of day close to mean solar time. Solar times are measures of the apparent position of the Sun on the Celestial sphere.

History

The first atomic clock was built in 1949 at the U. S. National Bureau of Standards (NBS). The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. A caesium standard is a primary Frequency standard in which Electronic transitions between the two hyperfine Ground states of caesium-133 Caesium or cesium (ˈsiːziəm is the Chemical element with the symbol Cs and Atomic number 55 Louis Essen ( September 6, 1908 &ndash August 24, 1997) was an English Physicist whose most notable achievements were in The National Physical Laboratory (NPL is the national Measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington This led to the internationally agreed definition of the second being based on atomic time. The second ( SI symbol s) sometimes abbreviated sec, is the name of a unit of Time, and is the International System of Units

Since the beginning of development in the 1950s, atomic clocks have been made based on the hyperfine (microwave) transitions in hydrogen-1, caesium-133, and rubidium-87. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Rubidium (ruːˈbɪdiəm /rəˈbɪdiəm/ is a Chemical element with the symbol Rb and Atomic number 37 For decades, scientific-instrument companies such as Hewlett-Packard have been making caesium-beam clocks and hydrogen masers for entities like NIST and USNO, at prices rivalling those of cars. A Hydrogen maser, also known as hydrogen Frequency standard, is a specific type of Maser that uses the intrinsic properties of the Hydrogen atom to serve The United States Naval Observatory ( USNO) is one of the oldest Scientific agencies in the United States.

In August 2004, NIST scientists demonstrated a chip-scaled atomic clock. ^[1] According to the researchers, the clock was believed to be one-hundredth the size of any other. It was also claimed that it requires just 75 mW, making it suitable for battery-driven applications. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. This device could conceivably become a consumer product. It will presumably be much smaller, consume less power, and be much cheaper to produce than the traditional caesium-fountain clocks used by NIST and USNO as reference clocks. The United States Naval Observatory ( USNO) is one of the oldest Scientific agencies in the United States.

In February 2008, physicists at JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado at Boulder, demonstrated a new clock based on strontium atoms trapped in a laser grid. JILA, formerly known as the Joint Institute for Laboratory Astrophysics, is one of the leading physical-science research institutes in the United States. The University of Colorado at Boulder ( CU-Boulder, UCB officially Colorado and CU colloquially is the Flagship University Strontium (ˈstrɒntiəm /ˈstrɒnʃiəm/) is a Chemical element with the symbol Sr and the Atomic number 38 A laser is a device that emits Light ( Electromagnetic radiation) through a process called Stimulated emission. The new clock is more than twice as accurate as the best clock up to now, the NIST-F1, and has an inaccuracy of less than one second in 200 million years (compared to 1 second per 80 million years for the F1). NIST-F1 is a Caesium fountain Atomic clock that serves as the United States ' primary Time and frequency standard ^[2]

How they work

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. This definition makes the caesium oscillator (often called an atomic clock) the primary standard for time and frequency measurements (see caesium standard). A caesium standard is a primary Frequency standard in which Electronic transitions between the two hyperfine Ground states of caesium-133 Other physical quantities, like the volt and metre, rely on the definition of the second as part of their own definitions. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International ^[3]

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. Microwaves are electromagnetic waves with Wavelengths ranging from 1 mm to 1 m or frequencies between 0 A maser is a device that produces coherent Electromagnetic waves through amplification due to Stimulated emission. In Atomic physics, hyperfine coupling is the weak magnetic interaction between Electrons and nuclei. 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.

This adjustment process is where most of the work and complexity of the clock lies. The adjustment tries to correct for unwanted side-effects, such as frequencies from other electron transitions, temperature changes, and the "spreading" in frequencies caused by ensemble effects. One way of doing this is to sweep the microwave oscillator's frequency across a narrow range to generate a modulated signal at the detector. The detector's signal can then be demodulated to apply feedback to control long-term drift in the radio frequency. Lock-in amplifierjpg|thumb|250px|and one from Stanford Research Systems In this way, the quantum-mechanical properties of the atomic transition frequency of the caesium can be used to tune the microwave oscillator to the same frequency, except for a small amount of experimental error. When a clock is first turned on, it takes a while for the oscillator to stabilize.

In practice, the feedback and monitoring mechanism is much more complex than described above.

Historical accuracy of atomic clocks from NIST.

A number of other atomic clock schemes are in use for other purposes. Rubidium standard clocks are prized for their low cost, small size (commercial standards are as small as 400 cm³) and short-term stability. A rubidium standard is a Frequency standard in which a specified hyperfine Transition of Electrons in Rubidium -87 atoms is used to They are used in many commercial, portable and aerospace applications. Hydrogen masers (often manufactured in Russia) have superior short-term stability compared to other standards, but lower long-term accuracy.

Often, one standard is used to fix another. For example, some commercial applications use a Rubidium standard periodically corrected by a GPS receiver. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth This achieves excellent short-term accuracy, with long-term accuracy equal to (and traceable to) the U. S. national time standards.

The lifetime of a standard is an important practical issue. Modern rubidium standard tubes last more than ten years, and can cost as little as US$50. Caesium reference tubes suitable for national standards currently last about seven years and cost about US$35,000. The long-term stability of hydrogen maser standards decreases because of changes in the cavity's properties over time.

Modern clocks use magneto-optical traps to cool the atoms for improved precision. A magneto-optical trap (abbreviated MOT) is a device that cools down non-charged atoms to temperatures near Absolute zero and traps them at a certain place using magnetic

Application

Atomic clocks are used to generate standard frequencies. They are installed at sites of time signals, LORAN-C, and Alpha navigation transmitters. A time signal is a visible audible mechanical or electronic signal used as a reference to determine the time of day LORAN ( LO ng R ange A id to N avigation is a terrestrial Radio navigation system using Low frequency Radio transmitters Alpha (also called RSDN-20) is a Russian system for long range Radio navigation. 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.

Further, atomic clocks are used for long-baseline interferometry in radioastronomy. Interferometry is the technique of using the pattern of Interference created by the superposition of two or more Waves to diagnose the properties of Radio astronomy is a subfield of Astronomy that studies celestial objects at radio frequencies.

Atomic clocks are the basis of the GPS navigation system. Basic concept of GPS operation A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS Satellites high above the Earth The GPS master clock is a weighted average of atomic clocks at the ground stations and on-board the GPS satellites, each of which has several atomic clocks.

Power consumption

Power consumption varies enormously, but there is a crude scaling with size. Chip scale atomic clocks can use power on the order of 100 mW; NIST F1 uses power orders of magnitude greater. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second.

Research

Most research focuses on ways to make the clocks smaller, cheaper, more accurate, and more reliable. These goals often conflict.

New technologies, such as femtosecond frequency combs, optical lattices and quantum information, have enabled prototypes of next generation atomic clocks. These clocks are based on optical rather than microwave transitions. A major obstacle to developing an optical clock is the difficulty of directly measuring optical frequencies. This problem has been solved with the development of self-referenced mode-locked lasers, commonly referred to as femtosecond frequency combs. A frequency comb is the graphic representation of the spectrum of a mode locked laser. Before the demonstration of the frequency comb in 2000, terahertz techniques were needed to bridge the gap between radio and optical frequencies, and the systems for doing so were cumbersome and complicated. Electromagnetic waves sent at terahertz frequencies, known as terahertz radiation, submillimeter radiation, terahertz waves, terahertz With the refinement of the frequency comb these measurements have become much more accessible and numerous optical clock systems are now being developed around the world.

Like in the radio range absorption spectroscopy is used to stabilize an oscillator — in this case a laser. When the optical frequency is divided down into a countable radio frequency using a femtosecond comb, the bandwidth of the phase noise is also divided by that factor. A frequency comb is the graphic representation of the spectrum of a mode locked laser. Bandwidth is the difference between the upper and lower Cutoff frequencies of for example a filter, a Communication channel, or a Signal spectrum Although the bandwidth of laser phase noise is generally greater than stable microwave sources, after division it is less.

The two primary systems under consideration for use in optical frequency standards are single ions isolated in an ion trap and neutral atoms trapped in an optical lattice. ^[4] These two techniques allow the atoms or ions to be highly isolated from external perturbations, thus producing an extremely stable frequency reference.

Optical clocks have already achieved better stability and lower systematic uncertainty than the best microwave clocks. ^[4] This puts them in a position to replace the current standard for time, the caesium fountain clock.

Atomic systems under consideration include but are not limited to Al+, Hg+,^[4] Hg, Sr, Sr+, In+, Ca+, Ca, Yb+ and Yb. WikipediaNaming Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Strontium (ˈstrɒntiəm /ˈstrɒnʃiəm/) is a Chemical element with the symbol Sr and the Atomic number 38 Strontium (ˈstrɒntiəm /ˈstrɒnʃiəm/) is a Chemical element with the symbol Sr and the Atomic number 38 Indium (ˈɪndiəm is a Chemical element with chemical symbol In and Atomic number 49 Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20 Calcium (ˈkælsiəm is the Chemical element with the symbol Ca and Atomic number 20

Radio clocks

Main article: Radio clock

Modern radio clocks can be referenced to atomic clocks, and provide a way of getting high-quality atomic-derived time over a wide area using inexpensive equipment. A radio clock is a Clock that is Synchronized by a Time code bit stream transmitted by a Radio transmitter connected to a Time standard However, radio clocks are not appropriate for high-precision scientific work. Many retailers market radio clocks as "atomic clocks", but in doing so they are misrepresenting their products.

There are a number of longwave radio transmitters around the world, in particular DCF77 (Germany), HPG (Switzerland), JJY (Japan), NPL or MSF (United Kingdom), TDF (France) and WWVB (United States). DCF77 is a longwave Time signal and standard-frequency radio station Swiss time reference system The HBG Transmitter operates on 75 kHz with 20 kW and is located in Prangins, Switzerland JJY is the Call sign of a Low frequency Time signal Radio station. The Time from NPL is a radio signal broadcast from the Anthorn VLF transmitter near Anthorn, Cumbria which serves as the United Kingdom The Time from NPL is a radio signal broadcast from the Anthorn VLF transmitter near Anthorn, Cumbria which serves as the United Kingdom Télé Diffusion de France, or TDF is a Time signal service broadcast on longwave radio by the French "Laboratoire primaire du temps et des frequences" For the Virginia based broadcast radio station please see WWVB-FM. Many other countries can receive these signals (JJY can sometimes be received even in Western Australia and Tasmania at night), but it depends on time of day and atmospheric conditions. There is also a transit delay of approximately 1 ms for every 300 kilometers (186 mi) the receiver is from the transmitter. A millisecond (from Milli- and Second; abbreviation ms is one thousandth of a Second. When operating properly and when correctly synchronized, better brands of radio clocks are normally accurate to the second.

Typical radio "atomic clocks" require placement in a location with a relatively unobstructed atmospheric path to the transmitter, perform synchronization once a day during the night-time, and need fair to good atmospheric conditions to successfully update the time. The device that keeps track of the time between, or without, updates is usually a cheap and relatively inaccurate quartz-crystal clock, since it is thought that an expensive precise time keeper is not necessary with automatic atomic clock updates. A quartz clock is a Clock that uses an Electronic oscillator that is regulated by a Quartz crystal to keep time The clock may include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours.

See also

• Network Time Protocol
• NIST-F1
• Radio clock
• Télé Distribution Française
• GPS
• Optical Clockwork
• Primary Atomic Reference Clock in Space
• Magneto-optical trap
• International Atomic Time

References

External links

• What is a Cesium atom clock?
• National Research Council of Canada: Optical frequency standard based on a single trapped ion
• United States Naval Observatory Time Service Department
• PTB Braunschweig, Germany - with link in English language
• National Physical Laboratory (UK) time website
• NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet
• NIST press release about chip-scaled atomic clock
• NIST website
• Web pages on atomic clocks by The Science Museum (London)
• Optical Atomic Clock BBC, 2005
• The atomic fountain