The Quest for the Ultimate Measurement: Single Atom Timepieces and Femtosecond frequency Combs covering the Optical Spectrum

Since the beginning of recorded history, humankind has turned to natural phenomena in search for regularity and perfection. For centuries, the motion of celestial bodies, such as the earth's rotation or the orbit of the earth around the sun, seemed to be traced out with such consistency and precision that they were thought to be the "perfect" reference from which we could measure time and base our science of physical measurements. Today, we know that these phenomena are limited in their accuracy. In fact, the earth's rotational period fluctuates at the level of 10 parts per billion per year. To satisfy the ever-increasing need for accuracy in science and technology, we have delved into the quantum world of the atom and created atomic frequency standards whose accuracy has surpassed the methods of astronomical time keeping by a factor of over a thousand.

Dr. Alan Madej from the Frequency and Time Group at NRC-INMS pioneered the use of laser cooling of atoms in Canada and was responsible for the development of a new optical frequency standard based on a single trapped strontium ion. His work is of great importance to the development of the next generation of atomic clocks based on optical, rather than microwave, frequencies. We have invited Dr. Madej as our guest columnist on IntraResearch this month.

Dr. Alan Madej at input optics of the strontium Ion trap system. Inset: Ion trap vacuum chamber and photon counting system used to detect light from the single ion.

Today, labs around the world are doing work that promises to further revolutionize the measurement of time and how we manipulate and control optical radiations or, simply put, light. The goal is to create the most perfect timepiece by using a single atomic ion as a clock source, and to count out time by using the field oscillations of a laser locked to an ultra narrow spectral line in the ion. New technologies using mode-locked femtosecond lasers allow us to create a series of regularly spaced light frequencies, or combs, across the optical spectrum.

By tying the clock's light frequency to these combs, we transfer the accuracies obtained through our ion optical transition anywhere in the optical region, and we can step down the optical frequency to the range of electronic signals where they can be compared to time signals from other atomic clocks.

Recent experiments place the limit of the optical comb system at the 10-19 level, which is several orders of magnitude better than our current measures of the unit of time. For now, it seems that limits in reaching the perfect unit of time will be imposed by the single ion itself. In current experiments, the ion is suspended in an electro-dynamic trapping field, slowed down by light pressure to motional energies corresponding to a temperature approaching absolute zero. Although these single ions are the closest approximation to an isolated unperturbed quantum entity, there still remain minute subtle effects, which tend to smear or pull the true frequency of the ions away from the ideal.

At NRC-INMS, we have been working with a single ion of Strontium (Sr⁺) and have seen shifts at the level of one part in ten trillion (10^-13). This is equivalent to measuring the distance from the earth to the moon using the width of a human hair. By using a unique method of measuring pairs of spectral lines, we evaluated the effect of what is considered one of the main limitations in these next-generation timepieces: the quadrupole moment shift of the frequency. This occurs when small amounts of electrical charge build up on components in the trap holding the single ion. The gradient of the electrical field then interacts with one of the energy levels in the ion's spectral line causing this shift. The Frequency and Time group managed to cancel this effect, which means that the potential for measuring the ion to higher levels of accuracy is greatly improved, opening the door for improving the unit of time by another factor of 1000.

New technologies using mode-locked femtosecond lasers allow us to create a series of regularly spaced light frequencies, or combs, across the optical spectrum.

Frequency comb apparatus generating a comb of reference frequencies across the near InfraRed (IR) and visible spectrum

The NRC has been a leading player in the field of single ion measurements for a number of years. In 1998, NRC-INMS achieved the first direct frequency measurement at visible optical frequencies of a single ion with an accuracy of 200 Hz for a transition at 445 trillion Hz (445 THz). This led to the ion transition's vacuum wavelength being internationally selected as a realization of the metre at the highest accuracy.

Work around the world on several ions has now achieved accuracies down to a few Hz for transitions at 100-1000 THz. At NRC, we have measured the ion transition to a level of 1 part in a hundred trillion (1 × 10^-14). Since 1967, the realization of the Système International (SI) second has been defined as a known number of cycles of microwave radiation exciting a ground state transition in Cs. This is the level at which Canada can realize the second, using its ensemble of atomic clocks based on a microwave transition in Cs atoms. Limitations in Cs atomic clocks and the new laser-cooled Cs atomic fountain clocks mean that the single atom is on the verge of becoming the most accurate measurement of time, length and other physical quantities.

Each time science pushes back a frontier in measurement and physics, new applications arise. The atomic clock enabled Global Positioning System (GPS) technology: it also allowed us to probe Einstein's theory of relativity, and to see the first evidence of the presence of gravitational waves and extra-solar planets. The new technologies of frequency combs and single ions open up a world where electromagnetic radiation from the radio to the optical domain can be controlled with atomic-clock precision. There is the potential to control optical radiations in the same way we now control radio- and micro- waves. We can now control the frequency and phase nature of light at extremely high levels. Phase-stable optical oscillators can be used for improvements in science, telecom technologies and fibre communications. These new technologies also allow us to control the phase of ultra-short pulses of light.

Advances made by the Frequency and Time group have the potential for measuring the ion to higher levels of accuracy, opening the door for improving the unit of time by another factor of 1000.

Artist's rendition of ion trap holding a single atomic ion (fluorescing at center) and being excited by cooling and clock laser laser light (courtesy of Dr. L. Marmet)

These studies impact on the study of structures and dynamics of atoms and molecules and control the pathways of chemical reactions, enhancing our understanding of chemical phenomena. They could lead to significant advancements in chemical and biotechnology processes. Some groups now compare these new generation timepieces to the traditional SI second in the search for the time variation of fundamental constants. Observing such changes would mark a significant departure from our present model of the universe and lead to emerging theories in the field.

In the short term, the impact on everyday life will be the improvement of measurement and control techniques that will accelerate the technology of harnessing light. This could range from advancements in astronomy and gravity measurements, to improvements in global positioning technology and more practical applications for photonics.

The new levels of perfection afforded by the technologies of single ions and optical combs will lead to new ways of realizing our most accurate and precise physical measurement, that of the second. Last March, the world time metrology community met in Paris to study the introduction of new secondary realizations of the second. These realizations will prepare the ground for an eventual redefinition of the second.

The goal is to create the most perfect timepiece by using a single atomic ion as a clock source, and to count out time by using the field oscillations of a laser locked to an ultra narrow spectral line in the ion.

Monolithic optical cavity made of ultra-low expansion glass used to stabilize the laser probing the single ion reference transition.

At present, there is a profusion of candidate ion and atom systems, each with attractive features and drawbacks. We will have to wait to see which system eventually comes out on top.

Both NRC-INMS and the UK's National Physical Laboratory are studying the ⁸⁸Sr⁺ single ion system. So far, this is the only single ion system being studied at high resolution by multiple labs. The relative simplicity of the required lasers and equipment make this system a suitable choice for replication.

The first fundamental change in time metrology in 40 years may soon be heralded by moving from the microwave region to the optical oscillations of a single atom whose pulse is taken at femtosecond time periods with laser light. The advances made available by this leap forward are now transforming the way we work with light and are sure to change associated technologies and science in years to come.

Dr. Alan Madej
Senior Research Officer
Frequency and Time
NRC Institute for National Measurement Standards
Ottawa

November 2004