Nobel Lecture: Defining and measuring optical frequencies

Abstract

Four long-running currents in laser technology met and merged in 1999-2000. Two of these were the quest toward a stable repetitive sequence of ever-shorter optical pulses and, on the other hand, the quest for the most time-stable, unvarying optical frequency possible. The marriage of UltraFast and UltraStable lasers was brokered mainly by two international teams and became exciting when a special "designer" microstructure optical fiber was shown to be nonlinear enough to produce "white light" from the femtosecond laser pulses, such that the output spectrum embraced a full optical octave. Then, for the first time, one could realize an optical frequency interval equal to the comb's lowest frequency, and count out this interval as a multiple of the repetition rate of the femtosecond pulse laser. This "gear-box" connection between the radio frequency standard and any/all optical frequency standards came just as Sensitivity-Enhancing ideas were maturing. The four-way Union empowered an explosion of accurate frequency measurement results in the standards field and prepares the way for refined tests of some of our cherished physical principles, such as the time-stability of some of the basic numbers in physics ͑e.g., the "fine-structure" constant, the speed of light, certain atomic mass ratios etc.͒, and the equivalence of time-keeping by clocks based on different physics. The stable laser technology also allows time-synchronization between two independent femtosecond lasers so exact they can be made to appear as if the source were a single laser. By improving pump/probe experiments, one important application will be in bond-specific spatial scanning of biological samples. This next decade in optical physics should be a blast.

Experimental realization of an optical second with strontium lattice clocks

“…The titanium-sapphire laser-based frequency comb 3,4 at the heart of our measurement chain ( Fig. 1) connects two frequency domains separated by 5 orders of magnitude.…”

“…In parallel, an operational architecture was developed to establish and distribute time scales based on this microwave second, for example, via satellite systems like GPS or TWSTFT. The advent of the frequency combs 3,4 in 2000 led to a revolution in the field of frequency metrology. At once, this simple and accurate way to link the optical (B10 15 Hz) and the microwave domains made it possible to take full benefit from decades of progress in optical spectroscopy and laser cooling of atoms.…”

“…Research on optical lattice clocks (OLCs) is comparatively recent [9][10][11][12][13] , but in less than 10 years they have reached the 10 À 16 level [14][15][16] , with excellent prospects towards the 10 À 17 level. The large number of probed atoms, on the order of 10 4 , is a major asset: it already allows unprecedented statistical resolutions that translate into frequency stabilities of a few 10 À 16 at one second, demonstrated in some groups [17][18][19][20] . The key feature of this type of clocks is the trapping of neutral atoms in a deep optical potential, which allows interrogation times of several 100 ms.…”

Ultracold atoms and precise time standards

“…For optical frequency standards, this was a major challenge. This requirement is now fulfilled using an octave-spanning femtosecond frequency comb [27,28]. Finally, in order to have a stable oscillator that does not vary over time, one needs a stable atomic reference on which to perform spectroscopy, such that one can feed back to the local oscillator.…”

Dynamic steady state of periodically driven quantum systems

“…First of all, it is so-called frequency comb-generators using the periodic pulse modulation of the laser field. Such sources of the pulse radiation are actively used now in the modern atomic clocks for frequency measurements [4,5], and also they have promising perspectives for the direct frequency comb spectroscopy [6][7][8]. Also it is widely used now the phase (frequency) and/or amplitude pe- * Electronic address: viyudin@mail.ru riodic modulation of the laser field to form the reference resonances for small-size atomic clocks and magnetometers [9][10][11][12][13][14].…”