Keeping Time with Light
n international team of researchers based at the universities of Vienna, Duisburg-Essen and Tel Aviv have succeeded in using polarized laser light to rotate a nanorod in a controlled fashion, providing a stable micromechanical oscillator for an electronic timekeeper. With the aid of laser beams, the group led by Stefan Kuhn, James Millen and Markus Arndt of the University of Vienna trapped a silicon nanorad, less than one-thousandth of a millimeter long, in a vacuum. The two counter-propagating light beams effectively keep the rod in suspension, and a third laser is used to rotate the rod by means of pulses of polarized light. Since the rotation is locked to the pulse frequency, the rotation period is sufficiently stable to act as a high-precision clock. Over a period of 4 days, this clock loses no more than a millionth of a second.
Tick…Tock… Stable oscillators provide the basis for the timekeepers that play such a significant role in our everyday lives. The invention of the first robust ship’s chronometer in the 18th century, enabled clocks to be used for navigation, and precise timing is still a crucial element in modern satellite-based positioning systems. Highly accurate electronic pulse generators keep the internet ticking along smoothly – and determine the speed with which information can be exchanged. Oscillation frequencies – and thus, time – can be measured with greater accuracy than any other physical quantity, allowing the tiniest deviations between timekeepers to be detected and quantified. By observing the motions of physical oscillators that can act as timekeepers, such as the pendulum of a grandfather clock, and synchronizing them to a reference clock, one quantitate the effects of external factors such as vibrations on a clock’s accuracy.
Stefan Kuhn of Vienna University and his colleagues have now developed an extremely stable watch hand – a silicon nanorod less than 1 micron long, which is trapped in suspension in a vacuum in the valley of a standing light wave generated with the aid of two counter-propagating laser beams. The electromagnetic fields associated with precisely timed pulses of linearly and circularly polarized laser light are then used to rotate the nanorod orthogonally to the beam axis at a rate of more than a million times per second. Thus the rotation period is locked to the frequency of polarization switching. “Over a period of four days, our clock lost no more than a millionth of a second,” says James Millen. The precision of comparable systems is limited by contact with the external environment. Because the nanorod is held in suspension by the standing wave created by the trapping lasers, the nanorotor has no physical contact with the outside world. This factor accounts for its enormous stability.
Nevertheless, since the nanorotor does not require an ultra-high vacuum for its operation, it to be used for extremely precise measurements of the local environment, such pressure differences over very short distances. The suspended rod could therefore be exposed to a flow of gas in order to measure turbulence, or exposed to a beam of light or atoms in order to probe their properties. It might even be possible one day to use this system to probe the limits of the quantum theory. “At high rotation rates, the nanorod is an extremely sensitive and extraordinarily precise sensor. However, at low frequencies, the system could also be used to perform entirely new experiments on the quantum mechanical behavior of rotating objects,” says Markus Arndt.
Illustration: James Millen / Universität Wien
Optically driven ultra-stable nanomechanical rotor
S. Kuhn, B. A. Stickler, A. Kosloff, F. Patolsky, K. Hornberger, M. Arndt and J. Millen, Nature Communications