Helen S. Margolis

Frequency ratio of two optical clock transitions in 171Yb+ and constraints on the time variation of fundamental constants.

Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6×10(-16). Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, μ/μ=0.2(1.1)×10(-16)  yr(-1), along with an improved constraint on time variation of the fine structure constant, α/α=-0.7(2.1)×10(-17)  yr(-1).

High-resolution microwave frequency transfer over an 86-km-long optical fiber network using a mode-locked laser

We demonstrate the transfer of an ultrastable microwave frequency by transmitting a 30-nm-wide optical frequency comb from a mode-locked laser over 86 km of installed optical fiber. The pulse train is returned to the transmitter via the same fiber for compensation of environmentally induced optical path length changes. The fractional transfer stability measured at the remote end reaches 4×10(-17) after 1600 s, corresponding to a timing jitter of 64 fs.

Dissemination of an optical frequency comb over fiber with 3 × 10 −18 fractional accuracy

We demonstrate that the structure of an optical frequency comb transferred over several km of fiber can be preserved at a level compatible with the best optical frequency references currently available.

Absolute frequency measurement of a 1.5-µm acetylene standard by use of a combined frequency chain and femtosecond comb

We have developed and characterized a pair of Doppler-free acetylene-stabilized diode laser frequency standards as optical communications references. The Allan deviation sigma/f of an individual system reaches a minimum of 4 x 10(-14) at a sampling time of 5000 s, and the long-term lock-point repeatability is found to be 0.4 kHz (one standard uncertainty), corresponding to a fractional uncertainty of 2 x 10(-12). Using a combination of a frequency chain and a self-referenced femtosecond comb, we have measured the frequency of line P(16) of the v1 + v3 overtone band of 13C2H2 to be 194,369,569,385.9 (3.0) kHz. The uncertainty of this number is limited solely by the reproducibility of the standards.

Trapped ion optical frequency standards

Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.

An optical frequency standard based on the electric octupole transition in /sup 171/Yb/sup +/

The frequency of the /sup 2/S/sub 1/2/(F=0,m/sub F/=0)-/sup 2/F/sub 7/2/(F=3,m/sub F/=0) transition in a single, trapped, laser cooled ion of /sup 171/Yb/sup +/ has been measured with an improved narrow probe laser and a femtosecond laser frequency comb generator. Our best estimate of the frequency is 642 121 496 772.3 /spl plusmn/ 0.6 kHz by comparison with earlier measurements. The uncertainty is limited by measurement statistics and by the ac Stark shift.

Absolute frequency measurement of the 2S1/2?2F7/2 electric octupole transition in a single ion of 171Yb+ with 10?15 fractional uncertainty

An absolute frequency measurement has been made of the 2S1/2?2F7/2 electric octupole transition in a single ion of 171Yb+. The implementation of a diode-based probe laser stabilized to this highly forbidden transition has resulted in an improvement of more than one order of magnitude upon the lowest published uncertainty. After correcting for systematic shifts, the frequency was determined to be 642?121?496?772?646.22?(67)?Hz. This corresponds to a fractional uncertainty of 1.0???10?15.

Absolute frequency measurements of 633 nm iodine-stabilized helium–neon lasers

The frequency of a helium–neon laser stabilized to the hyperfine component f (alternatively denoted a16) of the 127I2 11-5 R(127) line at 633 nm has been measured with respect to the SI second using a femtosecond optical frequency comb generator based on a mode-locked Ti : sapphire laser and microstructure fibre. The standard uncertainty of this measurement is 0.6 kHz. The same laser was taken to BIPM mid-way through the measurements and its frequency compared to that of the BIPMs continuously maintained iodine-stabilized helium–neon laser BIPM-4. From this comparison, the frequency of BIPM-4 when stabilized to the same iodine hyperfine component f is 473 612 353 608.1 (0.7) kHz.

Test of special relativity using a fiber network of optical clocks

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson-Mansouri-Sexl parameter |α|≲1.1×10^{-8}, quantifying a violation of time dilation, thus improving by a factor of around 2 the best known constraint obtained with Ives-Stilwell type experiments, and by 2 orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this Letter will improve by orders of magnitude in the near future.

Suppression of amplitude-to-phase noise conversion in balanced optical-microwave phase detectors

We demonstrate an amplitude-to-phase (AM-PM) conversion coefficient for a balanced optical-microwave phase detector (BOM-PD) of 0.001 rad, corresponding to AM-PM induced phase noise 60 dB below the single-sideband relative intensity noise of the laser. This enables us to generate 8 GHz microwave signals from a commercial Er-fibre comb with a single-sideband residual phase noise of -131 dBc Hz(-1) at 1 Hz offset frequency and -148 dBc Hz(-1) at 1 kHz offset frequency.