In search of precise time

Masao Takamoto, a researcher at the Quantum Metrology Laboratory

10−18 of a second. With this degree of accuracy a clock would be only one second too early or too late over a period of 30 billion years, more than twice the age of the universe. This is the challenge of a new type of atomic clock in development since 2003: an optical lattice clock.

The cutting edge construction of this type of clock was presented by Masao Takamoto, a researcher at the Quantum Metrology Laboratory of the Institute of Physical and Chemical Research (RIKEN) during the Physics Workshop of the second phase of the Intercontinental Academia (ICA), on March 9.

At the conference Precision Metrology with Optical Lattice Clocks, Takamoto said that atomic clocks are the reference for accurate measurements with 15 digits (10-15 of a second) and emphasized their importance to infrastructure sectors as they allow a greater accuracy in services such as systems Global Positioning (GPS) and the synchronization of high speed networks. He added that they are also very important for measurements in physical experiments, such as precisionspectroscopy in quantum physics.

The international standard of the second duration was defined in 1967 by cesium atomic clocks. The International Atomic Time (TAI, from the French name Temps Atomique International) is established by the average of such interconnected clocks. According to Takamoto, the best results in terms of accuracy so far have been obtained by cesium clocks of the SYRTE (Space-Time Reference System), in France, and of the NIST (National Institute of Standards and Technology), in the USA, which reached 3 x 10-16 of a second.

The search for an even greater precision and greater stability has motivated researchers to design optical atomic clocks. There are two types of them trying to occupy the role of reference in second measurement, according to Takamoto:

• a single-ion clock in an electric field with ability to achieve a precision of 10-18 s (proposed by Hans Dehmelt in 1982);
• an optical lattice clock, in which the potential of the optical lattice captures about 1 million atoms in separated "traps". It is able to achieve a precision of 10-18 s and its stability is provided by the simulation of 1 million single-ion clocks in parallel (proposed by Hidetoshi Katori in 2001).

The RIKEN and the Katori Laboratory of the University of Tokyo's School of Engineering have developed optical lattice clocks. Takamoto is the first researcher and assistant director for research of the laboratory.

The first demonstration of an optical lattice clock took place in 2003. In 2005 one of them had its absolute frequency measured. In 2006 the frequency measurement was made by three groups:. SYRTE, JILA (USA) and the National Metrology Institute of Japan. From these results, a new definition for the second has been proposed, Takamoto said.

In 2008-2009 experiments were carried out to measure the absolute frequency of the strontium optical lattice clock using optical fiber between Tokyo and Tsukuba (an actual distance of 50 km that required 120 km of optical fiber). "This and other international experiments have shown an excellent agreement between the clocks with a degree of accuracy close to 6 x 10-16 s," according to Takamoto.

In September 2006 the Consultative Committee for Time and Frequency (CCTF) adopted four types of optical clocks as "secondary representations of the second," according to the researcher: the strontium optical lattice clock and the single-ion clocks of strontium, mercury and ytterbium.

The Time and Frequency Department of the International Bureau of Weights and Measures (Bureau International des Poids et Mesures - BIPM), which is responsible for the TAI, will discuss criteria for the redefinition of the second in the next 5 to 10 years, Takamoto said. Atomic clocks to serve as a reference shall:

• be fully described and have advanced research available on them;
• be developed by various groups and laboratories;
• be marketed (preferably).

The chemical element and the clock scheme will be chosen from the performance of the various existing types. However, after the second is redefined by a clock with a precision of 10-18 s it will be necessary to find a way to share time with 18 digits under the influence of the Earth's gravitational potential: "According to theory of general relativity, time goes faster on higher ground. The height difference of 1 cm makes a difference between two clocks with a precision of 10-18 s. This is a problem from the point of view of a standard establishment."

Among the applications that this type of clock with extreme precision will allow, Takamoto cited:

• the production of precise proof for the gravitational potential by using the Theory of General Relativity;
• the demonstration of relativistic geodesy by comparing clocks connected by very long optical fiber;
• the geopotential mapping for the search for mineral resources;
• the monitoring of the variation in the gravitational potential time due to tidal effects;
• the detection of the Earth's crust movements and of volcanic activity.

For measuring the gravitational potential, research aims at the development of transportable clocks with stable long-term operation and clocks with hollow core photonic crystal fiber.

Currently nine countries have optical lattice clocks: Japan - strontium, ytterbium, mercury and cadmium; United States and Italy - strontium and ytterbium; France - strontium and mercury; Germany - strontium and magnesium; UK and China - strontium; and Korea and Australia - ytterbium.