Measuring length and time with light

In everyday life aspects we face the need to know the value of the different features of the objects we use, acquire, eat, etc., like checking the time, our body weight, the vehicle fuel level, our home electricity consumption, and many other physical, chemical and biological quantities. This need we have to quantitatively know our world is addressed by the metrology, the scientific discipline devoted to measurements; a measurement being a comparison of a given property against a known reference of the same nature, commonly known as “measurement standard”.

The seven SI base units and their links to fundamental physical constants. Credits: http://www.bipm.org/en/si/si_constants.html

The seven SI base units and their links to fundamental physical constants. Credits: http://www.bipm.org/en/si/si_constants.html

Metrology, source of confidence and wellbeing

This is how metrology becomes an activity of the highest economical importance, since this allows the countries to establish technically supported and high accuracy measurement standards, that are used to grant confidence to their commerce transactions inside the country and with other international partners; in this way for example, the right exchange rates can be applied at the exact instant when money is being wired among financial institutions around the world, the correct huge oil volumes can be known when being transferred between ships, containers and pipes, the hundreds of thousands of tons of food we daily commercialize everywhere in the world can be weighted, the several telecommunication fares can be determined, etc. As a result, the use of high accuracy measurement references in everyday life fosters the efficient use of energy, the environmental protection, the increment of the nations productivity and therefore the society welfare. To develop all these required measurement standards, the international community has adopted the International System of Units (SI), conformed with seven base units: the candela (cd), the meter (m), the second (s), the kelvin (K), the mol (mol), the ampere (A) and the kilogram (kg); in addition to all its multiples (kilo, mega, giga, etc.) and sub-multiples (centi, milli, pico, nano, etc.), derived units (Newton, Pascal, m·s-2, etc.) and accepted units (plane degree, degree Celsius, etc.); which are permanently realized, maintained and compared among the several National Metrology Institutes around the world in order to provide those with guaranteed high accuracy measurement references; and it is precisely here where the light plays a central role in the modern metrology by means of the use of highly stable laser sources able to provide the base units realizations with accuracy levels never reached otherwise.

Laser light produce exact meters

The laser sources insertion in modern metrology took place when this was accepted by the metrology international community as the optimum tool to realize the meter, perhaps one of the most widely used SI base units, in terms of one of the most famous physical constants: c, the speed of light when travelling in vacuum. At its beginning in 1799 the meter was realized by a platinum-iridium rule supposed to be equal to a ten-billionth of the Earth meridian, but then in 1893 the meter was redefined by the first time in terms of light wavelengths (a wavelength being the distance between two consecutive crests or valleys in an wave of light, and which is also linked to its oscillation frequency, commonly related to the color this appears) in this occasion corresponding to that of the cadmium red emission line. By 1960, when the laser came to life, the definition of the meter changed to be linked to the wavelength of the electromagnetic radiation emitted as a consequence of one of the transitions of the krypton isotope 86, but a few years later the wavelength and frequencies measurement experiments performed with lasers produced more accurate estimations of the value of c, resulting in this physical fundamental constant being adopted as the key for a new meter definition issued in 1983, which then considered five laser emission lines wavelengths, passing to eight in 1992 and to twelve 1997. Nowadays there are twenty different laser sources being used in meter realizations all around the world.

The Mexican meter realization, based on He-Ne stabilized lasers operating at 632.991 398 22 nm. Credits: René Pichardo Vega, Centro Nacional de Metrología, the National Metrology Institute of Mexico

The Mexican meter realization, based on He-Ne stabilized lasers operating at 632.991 398 22 nm. Credits: René Pichardo Vega, Centro Nacional de Metrología, the National Metrology Institute of Mexico

The advance of the laser sources technology has allowed the scientists to know better the values for the stable transitions wavelengths for several atoms and molecules, thus providing the meter realizations with levels of accuracy that could have never been reached without the laser; besides the scientific community has managed to built laser systems able to emit light in any wavelength along wide portions of the electromagnetic spectrum, known as optical combs, that undoubtedly will provide even more accurate meters in the near future.

Optical clocks produce seconds

The laser light also influenced the definition of the second, the SI base unit for the time. Even though its present definition relates it to the time it takes for a particular transition of the cesium isotope 133 to occur, the technologies under development, such as optical clocks, use lasers and thus rely again in the physical constant c. After the prediction by Bose and Einstein of a new matter aggregation state, known as the Bose-Einstein condensate, and the invention of the laser, those have been used to construct magneto-optical traps where several laser beams point a tiny atoms cloud from counter propagating directions and force this to stay quiet thus lowering its temperature closely to the absolute cero (0 K). This new techniques based in laser light, are being used to “cool” atoms or molecules thus allowing a more refined control of its quantum properties defining its emission wavelengths, the key property to built accurate clocks. The most recent results suggest that in the near future those optical clocks may be able to produce the most accurate seconds ever.

Typical aspect of a magneto-optical trap. The circular windows allow the incident laser light beams to slow down ("cool") the trapped atoms. Credits: Luis A. Orozco.

Typical aspect of a magneto-optical trap. The circular windows allow the incident laser light beams to slow down (“cool”) the trapped atoms. Credits: Luis A. Orozco.

The meter and the second are two of the SI base units which definitions and realizations have remarkably evolved after the incorporation of laser light sources, but in a similar manner other SI units like the candela, the mol and maybe even the kelvin, may take advantage of the laser light in the near future, in order to produce more accurate realizations and sophisticated definitions based in fundamental physical constants.

To learn more visit:

  1. http://www.bipm.org
  2. http://www.codata.org
  3. http://www.cenam.mx
  4. http://www.quantumcandela.net
  5. http://www.laserfest.org
  6. http://www.amo-ac.mx

Eric_RosasEric Rosas is the Appointed Vice President of the International Commission for Optics (2014-2017), member of The Optical Society International Council (2015-2017), 2010-2013 President of the Red Iberoamericana de Óptica, and 2008-2010 President of the Academia Mexicana de Óptica and the Mexico Territorial Committee for Optics of the International Commission for Optics. From 2001 to 2012 he was the Optical Sources Leader at the Centro Nacional de Metrología, the National Metrology Institute of Mexico; and since 2013 he is the Chief Intellectual Property Officer at the Centro de Investigaciones en Óptica.

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