Minutes of the CCL-CCTF Frequency Standards Working Group Meeting
BIPM, 10th – 11th September 2012
Agenda:
- Welcome to attendees.
- Approval of the agenda, appointment of rapporteur(s).
- Consideration of returns from NMIs to WG questionnaire.
- Consideration of additions/updates to the revised single list of frequencies including secondary representations of the second and frequencies for the realisation of the definition of the metre.
- Report on key comparison CCL-K11.
- On optical frequency ratios.
- Status of the publication of the list on the web and in Metrologia
- Preparation of WG recommendations to CCL and CCTF.
- Other business.
- Welcome to attendees.
Dr Riehle opened the meeting by welcoming the delegates, the observers and the BIPM representatives to the meeting.
The delegates and observers briefly introduced themselves.
- Approval of the agenda, appointment of rapporteur(s).
Dr Riehle presented the proposed agenda for the meeting (CCL-CCTF/12-01), which was approved by the delegates.
Dr Margolis was appointed as rapporteur for the meeting.
- Consideration of returns from NMIs to WG questionnaire.
Dr Robertsson presented an overview of the summary of the responses to the WG questionnaire. Several proposals had been made for updates to existing values and/or uncertainties in the list, as well as for the inclusion of new values and for new secondary representations of the second. Dr Robertsson had developed a series of spreadsheets as a starting point for the discussion, one for each wavelength to be considered. These spreadsheets included preliminary calculations based on the results from each laboratory, including different possibilities for weighting and expansion of uncertainty.
- Consideration of additions/updates to the revised single list of frequencies including secondary representations of the second and frequencies for the realisation of the definition of the metre.
6.8 GHz ground-state hyperfine transition in 87Rb
Discussion of this secondary representation began with a presentation from Dr Petit of BIPM, based partly on a presentation he gave at CPEM 2012. The LNE-SYRTE group had carried out 13 evaluations of their Rb fountain, which were published in June 2012. These were submitted for review to the CCTF WGPFS, in the same way as for a new primary frequency standard. Three methods were used to estimate the frequency of the Rb standard:
a)Local comparisons to the SYRTE Cs fountains (suggest frequency is lower than recommended value by 1.48×1015).
b)Comparison to TT(BIPM11) (suggests frequency is lower than recommended value by 1.67×1015 but uncertainty is difficult to estimate).
c)Comparison to the best estimate of the worldwide ensemble of primary frequency standards (suggests frequency is lower than recommended value by 1.67×1015 , and gives less dispersed values than method b).
Dr Arias reminded the delegates that there should be two steps in the evaluation of secondary representations of the second. In the first step, the FSWG decides on the frequency value and uncertainty for each secondary representation of the second included in the list of recommended frequencies. In the second step, reports should be submitted to the WGPFS (expected soon to become the WG on primary and secondary frequency standards). In the first instance the secondary standards would not be used for steering TAI; instead, the reporting mechanism provides a means of validating the value and uncertainty in the list.
Since more data had been reported to BIPM since the analysis presented, Dr Petit agreed to process the additional data and to provide an updated value to the working group. Further discussion of this frequency value was therefore postponed until the second day of the meeting. However the additional data was found to make very little difference to the best estimate of the Rb frequency value(1.74×1015 lower than the recommended value rather than 1.67×1015).
After discussion, it was agreed to use a weighted mean of the results published by SYRTE and to expand the uncertainty by a factor of three taking into account that the value is based on measurements from a single institute (although it was noted that a measurement campaign was currently underway at NPL). Dr Petit was thanked for his detailed analysis that provides additional confidence in this value and uncertainty.
The recommended frequency to be put forward to the CCTF was therefore
f87Rb= 6 834 682 610.904 312Hz
with an estimated relative standard uncertainty of 1.3×1015. This radiation is already endorsed by the CIPM as a secondary representation of the second.
1H, 1S–2S two-photon transition (243 nm)
Measurements of this transition frequency have only been reported by a single group (at MPQ). Previously the published uncertainty was expanded by nearly a factor of ten, because it did not reflect the scatter of the observed data. Two new measurements had been carried out, one using the SYRTE transportable Cs fountain, the other using an optical link to PTB. However the second was not yet published.
After some discussion about whether the expansion factor could be reduced, a sub-group was delegated to review the MPQ publications and to report back on the second day of the meeting with a recommendation for the frequency value and uncertainty that should be used.
This sub-group, after reviewing the published work, considered that there were some questions about systematic frequency shifts and parameter sensitivities arising in the 1997 and 2000 measurements, discussed by the MPQ group themselves although not explicitly in refereed publications. The 2010 measurement with the SYRTE fountain used a new laser system which eliminated previous problems with laser instabilities and reduced the scatter of the data. A proper measurement of the atom velocity distribution was also carried out in this case. Based on the lower scatter and the more complete uncertainty analysis the sub-group therefore had a higher level of confidence in the most recent data.
It was therefore agreed to use only the most recent published measurement and to expand the uncertainty by a factor of three since there was only a measurement from one laboratory. It was noted that there were no significant discrepancies between the values obtained over the years, giving extra confidence that the reduced expansion factor was reasonable.
The recommended frequency to be put forward to the CCTF was therefore
f1H = 1 233 030 706 593 518 Hz
with an estimated relative standard uncertainty of 1.2×1014. It was noted that, since the 1S – 2S transition is a two-photon transition, this frequency corresponds to half of the energy difference between the 1S and 2S states.
171Yb+, 6s 2S1/2 (F=0, mF=0) – 5d 2D3/2 (F=2,mF=0) transition (436 nm)
New measurements of this transition frequency had been reported by both NPL and PTB since the last meeting of the FSWG. These are in good agreement but the uncertainty of the NPL measurement (the first of this transition at NPL) is significantly larger than that of the PTB value. PTB have made an extensive series of measurements over more than one decade. All are consistent within their uncertainties, but PTB’s more recent(and more accurate) values appear to be more than 1 Hz below the previous recommended value. It was therefore agreed that the value and uncertainty of this transition frequency should be updated in the list.
It was agreed to use a weighted mean of the values from the two laboratories. Since the uncertainty of the NPL value was more than ten times larger than that of the PTB value, it was also agreed to expand the uncertainty of the weighted mean by a factor of three.It was, however, noted that two Yb+ standards had been compared at PTB and agreed at the parts in 1016 level.
The recommended frequency to be put forward to the CCTF was therefore
f171Yb+ (quadrupole) = 688 358 979 309 307.1 Hz
with an estimated relative standard uncertainty of 3×1015.This radiation is already endorsed by the CIPM as a secondary representation of the second.
171Yb+, 2S1/2 (F=0, mF=0) – 2F7/2 (F=3, mF=0) transition (467 nm)
The 2009 recommended value for this frequency was based only on measurements carried out at NPL, with the uncertainty expanded by a factor of three. Since then much more accurate measurements have been carried out by both PTB and NPL, and these are in good agreement. It was therefore agreed that the value and uncertainty of this transition frequency should be updated in the list.
It was agreed to use the weighted mean of all available values (including old data) and to enlarge the uncertainty by a factor of two since the values are from only two laboratories.
The recommended frequency to be put forward to the CCTF was therefore
f171Yb+ (octupole) = 642 121 496 772 645.6 Hz
with an estimated relative standard uncertainty of 1.3×1015.
It was agreed that this radiation should be put forward to the CCTF as a secondary representation of the second.
171Yb neutral atom, 6s21S0 – 6s6p 3P0 transition (578 nm)
Accurate measurements of this transition frequency are now available from both NIST and NMIJ (accepted for publication) and are in good agreement. A measurement has also been made at KRISS, but this value is not yet published.It was noted that a number of other groups are also developing ytterbium lattice clocks, for example in Italy, Germany, China and Japan.
It was agreed to use the weighted mean of the published values and to enlarge the uncertainty by a factor of two since these values are from only two laboratories.
The recommended frequency to be put forward to the CCTF was therefore
f171Yb = 518 295 836 590 865.0 Hz
with an estimated relative standard uncertainty of 2.7×1015.
It was agreed that this radiation should be put forward to the CCTF as a secondary representation of the second.
88Sr+, 5s 2S1/2 – 4d 2D5/2 transition (674 nm)
A new measurement of this transition frequency had recently been made at NRC (measurement campaign in February – May 2012), and had been accepted for publication in PRL, subject to corrections (not affecting the frequency value). The main uncertainty was in the link to the SI second, which was realised through GPS common view transfer. The measured frequency is consistent with the previous NPL measurement.
It was agreed to use the weighted mean of the NRC and NPL values and to enlarge the uncertainty by a factor of two since values are only available from two laboratories.
The recommended frequency to be put forward to the CCTF was therefore
f88Sr+ = 444 779 044 095 485.3 Hz
with an estimated relative standard uncertainty of 4.0×1015. This radiation is already endorsed by the CIPM as a secondary representation of the second.
87Sr neutral atom, 5s21S0 – 5s5p 3P0 transition (698 nm)
Three new measurements of this transition frequency had been reported, from NICT, PTB and LNE-SYRTE. However the LNE-SYRTE measurement had not yet been published in a peer-reviewed journal and so could not be used to contribute to an updated value according to the FSWG rules.
It was agreed to use the weighted mean of the published values. No expansion factor would be required according to the usual procedures of the FSWG. However it was agreed that it would be unwise to reduce the previous uncertainty quoted in the list, bearing in mind that the latest PTB and NICT values differ by 1 Hz, and that recent measurements at PTB (not yet accepted for publication) suggest that the calculated value for the blackbody Stark shift (used by all groups) may have to be corrected.
The recommended frequency to be put forward to the CCTF was therefore
f87Sr = 429 228 004 229 873.4 Hz
with an estimated relative standard uncertainty of 1.0×1015. This radiation is already endorsed by the CIPM as a secondary representation of the second.
40Ca+, 4s 2S1/2 – 3d 2D5/2 transition (729 nm)
New frequency measurements had been reported from NICT and Wuhan, to add to the previous results from Innsbruck and NICT. The new NICT measurement has an uncertainty of 2.9×1015, but disagrees with the Wuhan measurement by 1.3×1014 (5.3Hz). The Innsbruck value agrees with the Wuhan measurement. NICT have also done an optical frequency ratio measurement between their Ca+ standard and Sr, which confirms the result of the absolute frequency measurement. A direct comparison of the Wuhan and NICT Ca+ standards has recently been carried out via GPS, and a difference of several Hz was observed, and so it does appear that the frequencies of the Ca+ standards are different.
Despite the discrepancies between the different values, the overall scatter of the values was 8Hz, smaller than the previously recommended uncertainty. It was therefore agreed that the frequency value and uncertainty should be updated to reflect this additional uncertainty. However an unweighted mean of the available values would have to be used since some or all of the estimated uncertainties could not be trusted. After some discussion, an expansion factor of two for the uncertainty was agreed upon.
The recommended frequency to be put forward to the CCTF was therefore
f40Ca+ = 411 042 129 776 395 Hz
with an estimated relative standard uncertainty of 1.5×1014.
27Al+1S0 – 3P0 transition (267 nm)
Although NIST had not made a specific request for the Al+ standard to be included either in the list of recommended frequencies or as a secondary representation of the second, it was felt by a number of delegates that it should not be left out. Dr Riehle consulted Dr Oates by telephone to ask what the NIST position was. After consulting his colleagues, Dr Oates confirmed that NIST would be happy for Al+ to be included in the list, and would not be against it being recommended as a secondary representation of the second.
The NIST group has made one direct absolute frequency measurement of the Al+ clock transition, but with relatively high uncertainty. Their more accurate measurement is obtained from an Al+/Hg+ optical frequency ratio measurement, combined with the Hg+ absolute frequency measurement. It was noted that NIST has also directly compared two Al+ optical clocks, observing agreement at the 1.7×1017level (statistical uncertainty 7×1018).
It was agreed to use the weighted mean of the two NIST absolute frequency values and to expand the uncertainty by a factor of three because the frequency value was derived from measurements from a single laboratory.
The recommended frequency to be put forward to the CCTF was therefore
f27Al+ = 1 121 015 393 207 857.3 Hz
with an estimated relative standard uncertainty of 1.9×1015.
It was agreed that this radiation should be put forward to the CCTF as a secondary representation of the second.
It was also noted that the procedure followed for the Al+ frequency meant that the Hg+ frequency value and uncertainty should also be updated for consistency.
199Hg+, 5d106s 2S1/2 (F=0) – 5d96s22D5/2 (F=2) transition (282 nm)
The value used by the NIST group for the Hg+ frequency in the Al+ frequency measurement was taken from their 2007 Applied Physics B publication on Hg+. This includes all available absolute frequency ratio measurements, spanning the period 2000 – 2006. It was therefore agreed to use this value and to expand the uncertainty by a factor of three because all frequency measurements were from the same laboratory.
The recommended frequency to be put forward to the CCTF was therefore
f199Hg+ = 1 064 721 609 899 145.3Hz
with an estimated relative standard uncertainty of 1.9×1015.
199Hg neutral atom, 6s21S0 – 6s6p 3P0 transition (266 nm)
A new measurement of this transition frequency had been made by SYRTE. This was the first carried out in a lattice configuration, and had an uncertainty of 6.4 Hz, four orders of magnitude better than their initial measurement.
It was agreed that this value should be included in the list of recommended frequencies, but that at this early stage the uncertainty was too high for it to be recommended as a secondary representation of the second. A factor of three expansion factor for the uncertainty was agreed, since there was only a single measurement available.
The recommended frequency to be put forward to the CCTF was therefore
f199Hg = 1 128 575 290 808 162 Hz
with an estimated relative standard uncertainty of 1.7×1014.
- Report on key comparison CCL-K11.
Dr Gill gave a short presentation incorporatingthe conclusions of a report from Dr Matus, who was unable to attend the meeting.
A gradual divergence in the approach being adopted by different laboratories was noted. Some are continuing to operate their iodine-stabilised lasers within the operating parameters specified in the MeP, whilst others are moving towards using the lasers as independent frequency standards which are calibrated using frequency combs. In the second case, participants choose not to correct the laser frequency for operational parameters, relying instead on the stability of the setups. This can give smaller uncertainties. Both approaches are acceptable, and the CCL-K11 key comparison is supposed to show the equivalence between the measurement capabilities of different NMIs.
The work undertaken by a sub-group to revise the CCL-K11 protocol to include comb-based calibrations of laser frequencies was reviewed. The draft protocol was available on the FSWG document site, although there were still some typographical errors to be corrected. Most of the discussion centred around the situation involving calibrations using a comb referenced to a suitable frequency reference, with uncertainty significantly better than a few parts in 1011, and whether self-confirmation via peer-reviewed publication was sufficient for demonstration of capability for key comparison and associated CMC statements. It was considered that the peer-reviewed publication should demonstrate a high accuracy frequency measurement of a particular system, preferably one that had also been measured by a different NMI.
Dr Madej gave an example of validation of a comb system at NIST Gaithersburg. This uses a GPS-disciplined oscillator as a frequency reference, which was validated by measuring the frequency of the same laser system at Gaithersburg, at NRC, and finally at Gaithersburg again. The reproducibility was better than 1 kHz, demonstrating measurement capability at this level, and the results were summarized in a peer-reviewed paper.
It was agreed that the point discussing peer-reviewed publication should be modified to suggest examples of the type of evidence that could be presented to demonstrate measurement capability. The activity to revise the protocol would then be considered completed.
Dr Madej, Dr Merimaa and Dr Gill confirmed that their institutes were happy to continue as node laboratories for the key comparison. Dr Gill would check the situation regarding BEV. Dr Hong stated that NMIJ would be prepared to continue, but would be happy to hand over the role if another laboratory wanted to take over. Dr Warrington pointed out that there was provision in the protocol for a laboratory that is not a node to act as a host, although nobody has yet taken this up as an option. This might help to reduce the load on node laboratories.