Jul. 12, 2013 (TSR) – The spinning Earth gives our most basic measurement of time – the day – and for thousands of years it was our most stable timekeeper.
Time measurement has become a basic part of everyday life and accuracies of the nearest minute or a few seconds are usually good enough for most human activities, but highly accurate timing plays a vital role in many other aspects of the modern world.
For years, a team of physicists at the Paris Observatory have been testing an advanced form of atomic clock that is so accurate it can measure time to within a single second in 300 million years.
Writing in Nature Communications, the team said that clock now exists and is poised to redefine the ‘second’ itself. Because of its unerring precision, it could ultimately be used to establish a new and more accurate standard for the length of a ‘second’.
Presently, the ‘second’ is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
The new timekeeper, called an optical lattice clock, uses lasers and the oscillations of neutral strontium atoms to parcel out time.
“In our clocks we use laser beams. Laser beams oscillate much faster than microwave radiation, and in a sense we divide time in much shorter intervals so we can measure time more precisely,” Dr Jerome Lodewyck, from the Paris Observatory, said.
It is important to measure both accuracy and stability for many technologies such as telecommunications, satellite navigation and the stock markets rely on ever-better time measurements.
“For instance, if you have your wristwatch, and one day you are one second late, and one day one second early, then your clock is not stable. But it could still have good accuracy if over a million days the time is correct,” Dr Lodewyck explained.
Optical frequency standards offer a step improvement of one to two orders of magnitude in the future ability to realise the SI second.
The United Kingdom’s National Physical Laboratory (NPL) strontium ion optical clock project is targeted on creating systems with better performance than microwave clocks, for both space and terrestrial environments.
Strontium optical clock are expected to soon provide a better accuracy than in the best atomic fountains. It is so impressively unerring in its accuracy that if such a clock had been ticking since the instant of the Big Bang, some 13.8 billion years ago, it would have lost only about 46 seconds in all that time.
Strontium was discovered by Adair Crawford and William Cruickshank at 1790 and was named after the village of “Strontian” in Scotland where it was found. The village name in Gaelic, Sròn an t-Sìthein, translates as the nose [i.e. ‘point’] of the fairy hill, meaning a knoll or low round hill inhabited by the mythological sídhe.
Strontium is a chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a grey silver-white or yellowish metallic element that is softer than calcium and highly reactive chemically towards water. The metal turns yellow when exposed to air.
Optical Clocks: The Clocks of the Future
We have come a long way from the sundial (3500 B.C.E.) which relied heavily on the movement of the sun.
From the 17th Century until the present, all clocks have been based on regular oscillations – traditionally the swing of a pendulum, tick of clockwork or pulse of quartz crystal. However, they were not accurate as we ‘lost’ many seconds.
Pendulum clocks, which were invented in 17th Century, measure ± 10 seconds per day.
In 1762, the Harrison’s chronometer came, improved our clocks and ‘saved’ us time, measured ± 2 seconds per day.
When the quartz came in the 1930s, we became better in keeping time with ± 1 second in 30 yrs, because physicists discovered that the Earth does not rotate steadily but wobbles!
In 1955, we leaped. Louis Essen built the world’s first caesium atomic clock at NPL and paved the way for a new and better definition of the second based on the fundamental properties of atoms. Essen’s clock measured ± 1 second in 300 yrs.
We improved that in the 1980s and managed to measure ± 1 second in 300 000 yrs.
In 2004, we invented a new form of atomic clock, the caesium fountain. In this clock, a cloud of atoms is projected up into a microwave chamber and allowed to fall down under gravity.
These highly accurate atomic clocks, which makes us only lose ± 1 second in 60 million yrs., rely on switches between energy states of an atom’s electron shell, induced by light, laser or maser energy – if you force atoms to jump from one particular energy state to another, it will radiate an associated microwave signal at an extremely stable frequency.
Atomic fountains using cold cesium atoms, currently defining the second from a microwave atomic transition, will soon reach their ultimate accuracy.
As proven by recent development, optical clocks based on narrow atomic transitions in the visible spectrum are better frequency standards than fountains, and will certainly become the norm.
How do we Keep Time Around the World?
Everyone around the world needs to keep to an agreed timescale.
On the 1st January 1972, Coordinated Universal Time (UTC) was adopted as the official time for the world.
The International Bureau of Weights and Measures (BIPM) acts as the official time keeper of atomic time for the world.
There are 65 laboratories with over 230 clocks contributing to the international timescale.
As BIPM counts the seconds astronomers still continue to measure time by the rotation of the Earth about its axis. This is compared to UTC, and if these measurements differ by more than 0.9 seconds a leap second is added or subtracted to keep the timescales together.
The Time and Frequency group is responsible for operating the national time scale UTC (NPL) and disseminating time throughout the UK. Their caesium fountain primary frequency standard measures the SI second at the highest accuracy, and they contribute to the generation of the world’s reference time scale UTC. We also maintain and disseminate another of the seven base SI units: the metre.
The replacement for Greenwich Mean Time, UTC is part of all our daily lives: it is the timing used for the internet, banking and aviation standards as well as precise scientific experiments.
The Bureau International de Poids et Mesures (BIPM) computes UTC based on inputs from collections of atomic clocks maintained by institutions around the world, including NPL and ESA’s ESTEC technical centre in Noordwijk, the Netherlands.
How an Atomic Clock Works (Theory)
An atomic clock works like a conventional clock but the time-base of the clock, instead of being an oscillating mass as in a pendulum clock, is based on the properties of atoms when transitioning between different energy states.
The atom can be pictured as a mini solar system, with the heavy nucleus at the centre surrounded by electrons in a variety of different orbits.
The orbits correspond to energy levels, and electrons can only move between levels when they absorb or release just the right amount of energy.
An atom, when excited by an external energy source, goes to a higher energy state. Then, from this state, it goes to a lower energy state. In this transition, the atom releases energy at a very precise frequency which is characteristic of the type of atom. This is like a signature for the type of material used. All that is needed for making a good clock is a way of detecting this frequency and using it as an input to a counter. This is the principle behind an atomic clock.
The transitions between energy states can take place by releasing or absorbing energy at optical or microwave frequencies. An atomic second corresponds to 9 192 631 700 counts of the frequency of the energy detected in the transition of the Cesium 133 isotope when exposed to suitable excitation.
This energy is absorbed or released in the form of electromagnetic radiation, the frequency of which depends on the difference in energy between the two levels.
This transition is the source of the term “quantum jump”, quantum referring to the tiny but precise amount of energy needed to allow the electron to jump to a different level.
By measuring the frequency of the electromagnetic radiation, like counting the number of pendulum swings on a pendulum, we can measure the passage of time.
Quantum standards are based on fundamental properties of matter.
In the case of the atomic clock this is the energy released when electrons move between energy levels of a caesium atom.
Using the movement of the Earth to define the second is a problem because this varies unpredictably with time, so the length of a second defined this way would not be constant.
But quantum standards, as far as we know, will be stable forever, no matter when or where it is measured.
The caesium fountain uses laser beams to slow down the atoms. The slow movement of the atoms allows a more accurate measurement of the transition between energy levels and hence the frequency of radiation.
Thousands of years in the future, or in a distant galaxy, the energy levels of the caesium atom are exactly the same, and so is the length of the second defined in this way.
Redefining the new ‘Second’ for the 21st Century
Lattice clocks, which hold the clock atom in a dipole lattice trap during measurement, represent a potential leap forward for frequency standards research, and are candidates for a future redefinition of the second.
Over the past few years, optical frequency standards based on cold atoms and cold trapped ions have benefited significantly from the advent of the self-referenced femtosecond comb, with the result that the most accurate optical frequency measurements are limited by the caesium primary standard itself. Optical frequency standards are also likely to demonstrate reproducibilities below that of the caesium standard, raising the prospect of an optical redefinition of the second in the future.
To cool the atoms or ions, physicists use Doppler cooling methods, which exploit laser-atom interactions to reduce the temperature. This works for all types of atom-based clocks being used and developed at NPL, including microwave atomic fountain standards, trapped ion optical frequency standards, and neutral atom optical frequency standards.
Optical standards are unable to demonstrate lower absolute uncertainty until such time as a redefinition might occur, even though they may be shown to be more reproducible, for example by direct comparisons between two optical standards. This situation has led to the establishment of secondary representations of the second. These secondary representations, whether optical or microwave standards, may be used to realise the SI second, albeit with uncertainty no better than that of the caesium primary standard.
There are currently five different standards which may be used as secondary representations of the second:
- The 6.8 GHz rubidium microwave fountain standard
- The 5d106s 2S1/2 (F=0) – 5d96s2 2D5/2 (F=2) quadrupole transition in 199Hg+ at 282 nm (1065 THz)
- The 5s 2S1/2 – 4d 2D5/2 transition in 88Sr+ at 674 nm (445 THz)
- The 6s 2S1/2 (F=0) – 5d 2D3/2 (F=2) transition in 171Yb+ at 436 nm (688 THz)
- The 5s2 1S0 – 5s5p 3P0 transition in 87Sr at 698 nm (429 THz)
It is considered that the establishment of these secondary representations will help with the detailed evaluation of reproducibility at the highest level, and will significantly aid the process of comparing different standards in the preparation of a future redefinition of the second.
At present there are two competing methods for frequency metrology at the highest level: single trapped ions and large ensembles of neutral atoms, both using narrow transitions that can be excited by optical frequencies. Ion-based standards are currently ‘in the lead’, as single trapped ions are extremely insensitive to external perturbations. Neutral-atom standards, however, come a close second. What they lose in larger systematic shifts of the transition frequency (mostly collision shifts and velocity effects due to the fact that the atoms cannot be held in their trap during the clock measurement) they gain in statistics. With a system consisting N atoms (where N is often greater than 10 million), it is as if we are able to do N single-atom experiments simultaneously, leading to an improvement by a factor of the square root of N.
The concept of a neutral atom lattice clock is a way to map the environmental insensitivity of the ion standards onto a system made up of millions of atoms. Interrogating the clock transition while the atoms are held in a trap made from a laser beam would normally cause large shifts in the energy levels of the atoms due to the Stark effect. However Katori et al. showed that there are certain calculable ‘magic’ frequencies for which the ground and excited energy levels of the clock transition shift by precisely the same amount, leaving unchanged the energy difference that we are trying to measure. The potential for this type of clock is so great that every major national metrology laboratory in the world is now working towards a neutral-atom lattice clock. NPL’s lattice clock is based on neutral strontium atoms.
Current optical frequency standards are complex research-oriented systems and cannot readily be transported. Optical frequency standards have already demonstrated similar accuracy and superior stability to caesium fountain primary frequency standards, with excellent prospects for further improvement by several orders of magnitude. This creates a pressing need for new methods of frequency transfer with unprecedented levels of stability to enable optical frequency standards in remote locations to be compared.
Optical fibres provide a promising alternative for stable distribution of either microwave or optical frequency references, since they are low loss, readily scalable, and can be environmentally isolated. Furthermore there is an extensive existing infrastructure in place in the form of optical telecommunications networks.
Several distinct methods for distribution of ultrastable reference frequencies have been demonstrated over the past few years. The most extensively studied technique is the transfer of a microwave frequency reference using an amplitude-modulated cw laser. More appropriate for remote comparison of optical frequency standards is to directly transfer an optical frequency using a cw laser. The improved timing resolution available from the higher frequency optical carrier offers the potential for several orders of magnitude reduction in instability.
The ability to compare remote optical frequency standards in this way will be of vital importance to the international metrology community in assessing their suitability for a potential future redefinition of the ‘second’. However techniques for high stability frequency transfer will also be beneficial for a variety of other applications including tests of fundamental physical theories, precision spectroscopy and remote synchronization.
The Galileo Satellites use UTC to tell the world’s time
Galileo is Europe’s satellite navigation (satnav) system and, like all similar systems, relies on the highly precise measurement of time.
Galileo’s highly-accurate clocks are at the heart of the system. Each satellite emits a signal containing the time it was transmitted and the satellite’s orbital position. Because the speed of light is known, the time it takes for the ‘satnav’ signal to reach a ground-based receiver can be used to calculate the distance from the satellite.
There are four Galileo satellites already in orbit and the atomic clocks on board them are now providing time accurate to a few nanoseconds, or billionths of a second.
“A billionth of a second equals a nanosecond, a time interval far beyond our own human capacity of appreciation,” explains Marco Falcone, ESA’s Galileo System Manager.
“A single lightning flash across the sky during a thunderstorm lasts about ten milliseconds, which is already 10,000,000 nanoseconds. But for high-tech applications, as well as navigation services, nanosecond accuracy is essential.”
NPL has collaborated with five other European time-measurement institutions on a ‘Time Validation Facility’ for Galileo. This centre compares European clocks and national time scales to the time provided on board the satellites and estimates how far away this time signal is from Universal Coordinated Time (UTC), the international time scale. This is then provided to the Galileo Control Centre and uploaded to the satellites, enabling their signals to be expressed in terms of UTC.
‘Galileo time’ is derived independently of UTC but is being kept close to it, with a precise ‘offset’ between the two values being calculated continuously and then disseminated through Galileo’s navigation message.
Matching the receiver and satellite clocks then multiplying the time taken by the speed of light gives the range between user and satellite, allowing the receiver to fix its own location relative to four or more satellites.
“Each navigation system has its internal reference system time used to synchronise all system clocks and maintain overall coherence,” adds Marco.
Galileo runs on Galileo System Time, GST, which is fixed on the ground at the Galileo Control Centre in Fucino, Italy, by the Precise Timing Facility, based on the average of different atomic clocks.
Strictly speaking, for navigation purposes alone this internal reference system time does not need to be in agreement with UTC at the highest level of accuracy but with this agreement being the case, it is therefore possible to immediately disseminate UTC to the users to the best accuracy and this is the aim of Galileo.
The offset between GST and UTC is currently estimated in Turin, Italy, by the Istituto Nazionale di Ricerca Metrologica (INRIM), where time measurements are performed every day with the most precise techniques available to check GST status.
INRIM has been supporting ESA’s Galileo development since the early phases of the project. INRIM has overseen the creation of a ‘Time Validation Facility’ for Galileo in collaboration with five other European time-measurement institutions: the Physikalisch Technische Bundesanstalt in Germany, the National Physical Laboratory in the UK, the Systeme de References Temps Espace/Observatoire de Paris in France, the Real Instituto y Observatorio de la Armada in Spain and Observatoire Royale de Belgique.
Each day, the most precise European clocks and national time scales are compared to GST and the offset compared to UTC is estimated and provided to the Galileo Control Centre. This offset is then uploaded to the Galileo satellites for transmission in the navigation message available to users.
As explained by Patrizia Tavella from INRIM, “The UTC value available to the user via Galileo is expected to be accurate within 26 nanoseconds, but in the last two months it was even better, with a prediction error in the last two months of less than five nanoseconds.”
How the Galileo clocks work
The Galileo Space Segment comprise a constellation of a total of 30 Medium Earth Orbit (MEO) satellites, of which 3 are spares, in a so-called Walker 27/3/1 constellation.
Each satellite will broadcast precise time signals, ephemeris and other data. The Galileo satellite constellation has been optimised to the following nominal constellation specifications:
- circular orbits (satellite altitude of 23 222 km)
- orbital inclination of 56°
- three equally spaced orbital planes
- nine operational satellites, equally spaced in each plane
- one spare satellite (also transmitting) in each plane
The Galileo satellite is a 700 kg/1600 W class satellite. The spacecraft rotates about its Earth-pointing axis so that the flat surface of the solar arrays always faces the Sun to collect maximum solar energy. The antennas, shown on the underside of the body in the above picture, always point towards the Earth. The spacecraft body will measure 2.7 m x 1.1 m x 1.2 m and the deployed solar arrays span 13 m.
The Galileo satellites carry two types of clocks: passive hydrogen masers and rubidium atomic frequency standards.
The passive hydrogen maser clock on board Galileo is the master clock on board each satellite. It is an atomic clock which uses the ultra stable 1.4 GHz transition in a hydrogen atom to measure time to within 0.45 nanoseconds over 12 hours.
The Galileo passive hydrogen maser clock is also made of an atomic resonator and its associated control electronics. In this clock, a small storage bottle supplies molecular hydrogen to a gas discharge bulb. Here molecules of hydrogen are dissociated into atomic hydrogen. After dissociation, the atoms enter a resonance cavity by passing through a collimator and a magnetic state selector. This magnetic state selector is used to let only atoms of the desired energy level enter a resonant cavity. Here the atoms are confined in a quartz storage bulb. Once in this storage bulb the hydrogen atoms tend to return to their ‘fundamental’ energy state, emitting a microwave frequency as they do so.
This frequency is detected by an interrogation circuit which locks an external signal to the ‘natural’ transition of the hydrogen atoms. Locking occurs when the injected frequency is the same as the resonant frequency of the atoms; this corresponds to an amplification of the microwave signal.
The resonant frequency of the microwave cavity is approximately 1.420 GHz. The clock’s electronics includes circuitry for the control of the frequency plus the thermal control system to maintain the resonant cavity at the correct temperature.
The atomic resonator is very sensitive to the external environment (for example, magnetic field). Great care is required to keep environmental disturbances small so that the full performance potential of these sophisticated clocks can be realized.
A rubidium clock will be used as a second, technologically independent time source. It is accurate to within 1.8 nanoseconds over 12 hours. Prototype versions of these clocks have already been flown on ESA’s GIOVE missions.
The Galileo rubidium clock is also made of an atomic resonator and its associated control electronics. Inside the atomic resonator there is a rubidium vapour cell. The atoms are kept in a gaseous state at high temperature. In order to initiate the resonance, the atoms of the cell are excited to a higher state by the light of a rubidium discharge lamp located in one end of the atomic resonator. At the other end of the resonator there is a photodiode which detects the amount of light that passes through the cell.
Following the excitation, the atoms decay to a lower state. From this state, the atoms are excited back to an intermediate level by injecting microwave energy in the resonator at a given frequency. Transition to the intermediate level only occurs if the frequency corresponds exactly to the one associated with this transition. When the atoms are in the intermediate state, the absorption of light is at a maximum.
The output of the photodiode is connected to control circuitry which adjusts the microwave frequency. The correct frequency is maintained by tuning the microwave source to obtain maximum light absorption. The resonance is maintained by the energy from the rubidium lamp, since the atoms in the intermediate state are again excited to the higher state and then decay to the lower state, from which the whole process starts again.
The stability of the rubidium clock is so good that it would lose only three seconds in one million years, while the passive hydrogen maser is even more stable and it would lose only one second in three million years. However this kind of stability is really needed, since an error of only a few nanoseconds (billionths of a second) on the Galileo measurements would produce a positioning error of metres which would not be acceptable.
The spacecraft has four clocks, two of each type. At any time, only one of each type is operating. Under normal conditions, the operating maser clock produces the reference frequency from which the navigation signal is generated. Should the maser clock fail, however, the operating rubidium clock will take over instantaneously and the two reserve clocks will start up. If the problem with the failed maser clock is unique to that clock, the second maser clock will take over from the rubidium clock after a few days when it is fully operational. The rubidium clock will then go on stand-by or reserve again. In this way, by having four clocks, the Galileo spacecraft is guaranteed to generate a navigation signal at all times.
What about Errors?
Determining precise location depends on accurately measuring the distances between receiver and satellite, and that depends on very accurate measurement of the radio signal’s travel time from the satellite to the receiver. As these signals travel at the speed of light, the journey times are tiny fractions of a second.
Numerous errors can degrade the accuracy of a position measurement. For example, errors in satellite to receiver distances can creep in if conditions within the ionosphere, the electrically charged outer layer of the atmosphere, slow down the signal. Conditions within the ionosphere are influenced by the level of activity on the surface of the Sun. Inaccurate distance measurements will also occur if the signal takes an abnormally long path because it is reflected off tall buildings or other surfaces before reaching the receiver.
There are various ways of overcoming such inaccuracies. The best known is called differential satellite navigation, which uses a fixed receiver in a known position as a reference.
The time taken for the signal to travel from the satellite to the fixed receiver can be calculated precisely because the positions of the fixed receiver and the satellites (and hence the length of the travel path) are known precisely. Any difference between the calculated travel time and that actually measured reflects inaccuracies introduced by disturbances in the ionosphere.
If a moving receiver, attached to an aircraft for example, is within a few hundred kilometres of the fixed receiver, then it is fair to assume that the errors experienced by the signal in reaching both receivers will be roughly the same, as variations in ionospheric conditions tend to be similar over large areas. The timing errors determined by the fixed receiver can then be used to eliminate similar errors in the moving receiver. Major users of satellite navigation, such as large airports, may decide to use the differential technique by installing their own fixed receivers.
Another technique, which makes use of two positioning signals at two different frequencies, does away with the need for differential satellite navigation. It works on the principle that each frequency will be slowed down by a slightly different amount when travelling through the atmosphere. By sending the two frequencies at the same time and recording the time difference between their arrivals, it is possible to build up a model of ionospheric conditions. This dual-frequency technique can increase positioning accuracy to better than one to two metres and will be standard for Galileo.
Finally, it is possible to determine positions to within a few tens of centimetres using a technique called Three Carrier Ambiguity Resolution (TCAR). TCAR eliminates inaccuracies by looking at the wavelike structure of the signal and determining how much it has been pushed along during its passage through the atmosphere. Such phase shifts can be measured to within a fraction of the wavelength, which is typically 20 centimetres.
The calculations needed to do TCAR are presently beyond the capabilities of standard hand-held receivers and this technique is used mainly for long-term monitoring of movement in buildings, oil pipelines, changes in sea level and even changes in the shape of the Earth’s crust.
The receiver measures travel times by comparing ‘time marks’ imprinted on the satellite signals with the time recorded on the receiver’s clock. The time marks are controlled by a highly accurate atomic clock on board each satellite.
These clocks, however, are too expensive to incorporate into standard receivers, which have to make do with small quartz oscillators like those found in a wristwatch. Quartz oscillators are very accurate when measuring times of less than a few seconds, but rather inaccurate over longer periods. The solution is to re-set the receiver’s time to the satellite’s time continuously. This is done by the receiver’s processor using an approximation method involving signals from at least four satellites.
For this system of measurement to work, all satellites need to be synchronised so that they can start transmitting their signals at precisely the same time. This is achieved by continuously synchronising all on-board atomic clocks with a master clock on the ground. These super-accurate clocks can keep time to within one second in 100 million years!
The Next Generation: Goodbye Microwave Frequency, Hello Optical Frequency
All other things being equal, the stability of an atomic clock is proportional to its operating frequency. Visible light has a frequency roughly five orders of magnitude higher than that of microwaves. This means that clocks based on narrow atomic absorptions at optical, rather than microwave, frequencies should be much more stable. They also have the potential to achieve higher accuracy.
Clocks for the 21st century are being developed in the form of ion traps.
Ions are charged atoms which can be trapped by electromagnetic fields almost indefinitely. Once trapped a laser beam can then be used to cool the ion down close to absolute zero, keeping it stationary.
At NPL the element strontium has been chosen to develop ion trap clocks as its ions can have very stable states. These clocks may have accuracies of around 1000 times higher than the best current atomic clocks.
NPL developed optical frequency standards based on transitions in single trapped ions of strontium and ytterbium, and neutral strontium atoms confined in an optical lattice.
That is equivalent to losing no more than one second in the lifetime of the universe.
The heart of an optical atomic clock is a highly stable reference frequency provided by a narrow optical absorption in an atom or ion.
Optical atomic clocks have many potential applications. These range from improved satellite navigation systems and better tracking of deep space probes to sensitive tests of fundamental physical theories.
With the latest success, we are now redefining the SI unit of time, the second, being redefined.
Although cold trapped ions or atoms provide the most reproducible frequency references, lower accuracy frequency references based on lasers stabilised to absorbers in gas or vapour cells are suitable for many applications.
NPL uses iodine-stabilised helium-neon lasers for the practical implementation of the SI unit of length, the metre. They are also developing compact and portable standards in the 1.5 micrometre region for telecommunications applications.
Another clock is also undergoing development – an ion clock. This clock loses just one second every few billion years, but because it relies on a single ion, it is not yet deemed to be stable enough for widespread use.
1) H. Katori et al., Phys. Rev. Lett. 91, 173005 (2003).