Picture of the Strontium ion trap chamber and detection system. (Courtesy NRC.)
FOR RELEASE: Monday, April 30, 2001, 3:06 PM
Alan Madej (613-993-9385,
alan.madej@nrc.ca)
Institute for National Measurement Standards
National Research Council of Canada Ottawa, Canada K1A 0R6
Contact number during the April Meeting:
Henley Park Hotel : 202-638-5200
Popular Version of Paper
S4.002
Monday, April 30, 2001, 3:06 PM
APS April 2001 Meeting, Washington, DC
The excitation of transitions in atoms and molecules can be considered as an incarnation of "Nature's ruler" whereby a regular, accurate, and invariant range of frequency reference values can be produced. As far as we can tell, all atoms, anywhere in the universe, which are of the same atomic isotope and charge state, are exactly identical to each other. This property provides us with a multitude of frequency references for use anywhere. We can use these atomic or molecular transitions as standard reference points in order to control the frequency or wavelength of various forms of electromagnetic radiation (such as radio, microwave, infrared, and optical). The output radiation from such a radio, microwave, or laser source then has a well-defined frequency and wavelength based on the atom reference. In a manner similar to how a grandfather clock counts the swings of its pendulum, if we have a mechanism to count the cycles or "ticks" of the stabilized radiation, we can produce an atomic clock. This is the basic idea behind our present generation of atomic clocks, which use an atomic transition in cesium atoms at the 9.2 GHz (or 9.2 billion cycles per second) to define our second. These atomic standards are the basis of scientific and legal time around the world. Moreover, atomic frequency standards form the heart of navigation systems such as those in the GPS array of satellites, which allows us to find our location on the earth to metre-level accuracy. In a similar way, by stabilizing the wavelength of radiation to an atom's fixed value, we can use light radiation, in conjunction with a device called an interferometer, for the ultra- precise measurement of physical length. Absolute physical length can now be measured for macroscopic objects with accuracy comparable to an atomic dimension and relative length changes on the order of a fraction of a nuclear diameter can be detected.
Over the last few decades, work has been devoted to controlling the frequency or wavelength of laser light using the resonances in atoms and molecules in order to create an ultra-accurate reference that would rival the best atomic clocks and push forward precision measurements beyond their present extraordinary levels. However, invariably, the perfection of the atomic resonance is in some way corrupted by the atom's environment and motion. For instance, there are problems in working with free atoms moving about at thermal velocities (>100 m/s at room temperature) and suffering Doppler effects (shifts in the transition frequency due to the atom's motion) and collisions, in addition to the effects of spatially varying fields and the finite time a laser beam can illuminate a moving atom. An elegant solution to these problems was proposed over 25 years ago by H. Dehmelt (U. Washington). He proposed the idea that a single ion, that was trapped in a time-varying electric field under ultra-high vacuum conditions and slowed down using laser light, would suffer almost no practical disturbance. A laser that was very stable in frequency could then probe the ion's transition and would then be able to read-out the undisturbed reference with an accuracy superior to any other means of physical measurement.
Picture of the Strontium ion trap chamber and detection system. (Courtesy NRC.)
A number of groups from around the world have been steadily working toward this ultimate form of reference for almost 20 years. Our group at NRC has chosen to work with a single ion of strontium. The elegance of this system is that all the transitions involved in probing and exciting the ion exist at infrared and visible frequencies where compact, tunable solid-state lasers are readily available. This significantly simplifies the experimental setup and brings closer the possibility of a working, practical realization of a device to be used as a standard or reference. The ultra-narrow reference transition excited in the atom uses red light from a diode laser at a wavelength of 0.674 micrometer (a frequency of 445 THz). Over the years, we have developed lasers at this wavelength to probe the ion with a resolution of better than 0.0005 parts per billion. In 1998, we were able to lock the laser frequency to the ion transition and count the frequency relative to a Cesium atomic clock frequency. This was the world's first direct measurement of a single ion visible transition and led to the establishment of the strontium ion as the first internationally recognized single atom reference for light frequency and wavelength.
The 674-nm diode laser system used to probe the Sr+ single ion. The laser frequency is quieted by controlling its frequency on the short term with an ultra-stable optical cavity. (Courtesy NRC)
An optical cavity made of Ultra-low-expansion material used to control the frequency of the 674-nm probe laser. Cavities like this can have average changes in length less than 0.001 part per billion per day. (Courtesy NRC.)
In recent experiments, we have sought to use the extreme accuracy provided by the nearly perfect single atom reference to improve our knowledge of other light wavelengths/frequencies of interest in the optical frequency region of the spectrum. By comparing the frequency of light referenced to the single strontium ion with other laser frequencies, we have measured an accurate laser reference in the 1.5- micron wavelength fiber-optic telecommunication band. In this way, an accurate marker of wavelength/frequency is created in this band which may be useful in the allocation and control of the increasing number of frequency channels now being used for fiber optic communications. Also, we have used the single ion reference to measure the frequency and wavelength of the widely used helium-neon laser locked to reference transitions in iodine at 633nm. These I2/HeNe lasers are used at all major national metrology labs around the world and form the basis for precise physical measurements of length. The absolute value of these standards had been measured only once before in the early 1990's and thus its reevaluation plays an important part in upgrading the world's agreed-upon values. In the current series of measurements, we were able to use the single atom to measure the 633-nm portable laser standard to its ultimate accuracy of better than 1 part in 70 000 000 000.
Recently, a radically new way of counting optical cycles of light radiation has emerged on the scientific stage. Using lasers that produce a regularly spaced series of ultra-short femtosecond pulses (< 10 -13 s) together with new optical fiber technology, it has been possible to create a regularly spaced grid of known optical frequencies that now span the entire visible region of the spectrum. These known frequencies can be controlled by existing atomic clocks and thus act as a form of "gear box" to convert radio-frequency stable radiation to optical radiation and vice versa. This can potentially result in a profound improvement over previous methods of optical frequency measurement which required several extremely stable laser systems, together with elaborate light conversion methods and electronics, to step down optical frequencies (which oscillate at a rate of approximately 500 trillion cycles per second) to frequencies that could be measured with existing electronics. What took several years of concerted effort and resources can now be done with femtosecond laser based combs in a matter of days and, moreover, any frequency within the span of the frequency comb can now be accessed. Given the promise of such an important emergent technology, we have embarked on a collaboration with a group led by Jun Ye and John L. Hall at JILA/NIST labs in Boulder to verify whether such a new frequency measurement technology is free from unknown shifts or disturbances. The JILA/NIST group measured the absolute frequency of a transportable 633-nm laser standard using the new femtosecond laser technique and then brought the laser to Ottawa. The JILA/NIST laser was then measured against the known, single strontium ion frequency. It was found that the two values agreed to better than 1 part in 1012. This provides an important confirmation as to the viability of the new method and its application to future areas of research and technology.
The femtosecond laser comb technology will allow the first real possibility of constructing an "optical clock." We are currently building a femtosecond comb system that will serve to connect our traditional atomic clocks to our strontium single ion reference. It may be, that in the next few years, international time will be based on laser light locked to the atomic transitions in single, trapped and laser cooled ions. This will open a new level of measurements of quantum physics, relativity, and the stability of what we believe to be fundamental physical quantities. As the control of light becomes more highly precise, the technological benefits may be of even greater importance. For example, important benefits may be in the allocation of optical communication channels, the provision of a multitude of known light wavelengths for precision length measurement, and the precise control of optical phase. We may also see benefits in better control of laser-induced chemistry, the generation of coherent X-ray light pulses to study matter. It is clear that the tools are now available for new and exciting developments in the years to come.