SECONDARY REPRESENTATION OF THE SI SECOND. Dale Henderson

Transcription

1 Dale Henderson

2 to provide an ultra-high stability microwave frequency standard to underpin the noise analysis of the primary standards of time and length. main deliverable will be a high-flux rubidium-87 fountain with a cryogenic sapphire-ring resonator with a target frequency stability of 5 parts in at one second averaging time and 5 parts in at 100 seconds averaging time.

3 A transition at optical frequency is likely to replace caesium via a new definition of the SI second Higher Q and similar S/N (better stability) Possibility of smaller type B ( systematic ) uncertainty

4 Many groups working on optical standards Single trapped ion Neutral atom cloud Trapped neutral atoms (lattice)

5 NPL has single trapped Yb ion and Sr standards (optical transitions) These can be measured directly against caesium fountain using femtosecond comb

6 Microwave fountain in rubidium has advantages over caesium Frequency shift due to collisions of atoms in the cloud is 2 orders less

7 A Rb fountain can have higher short term stability, without compromising long term stability because of the reduced cold collision shift

8 can combine loading a magneto-optical trap MOT from a 2-D MOT with our cryogenic local oscillator Our cryo-oscillator has 3 in stability seconds

9 5 parts in at 100 seconds averaging time is needed to compare directly the ability of the femtosecond comb to transfer stability from optical standards

10 Strategic issue A list of possible replacements of caesium has been started

11 CIPM 2001 Recommendation(I) Joint WG CCL/CCTF 1. There will be a single list of recommended radiations, within which some radiations would relate to the realization of the meter, and others would relate to the secondary representations of the second. 2. The joint CCL/CCTF working group periodically reviews and discusses the uncertainty budgets of suitable standards. The working group would evaluate the validity of the uncertainty budget before making their recommendation to the CCTF that the standard be added to the list of frequencies as appropriate for secondary representations of the second. The joint working group would keep the CCL informed about its activity. 24

12 Purpose of the list of secondary representations of the second 1. to facilitate the process of choosing wisely the best candidates for a potential future re-definition of the second; 2. in this respect to document the results of measurements of selected frequency standards with respect to primary frequency standards and their uncertainties; 3. to document the suitability of certain new frequency standards for monitoring of the long-term stability of International Atomic Time TAI; 4. to allow laboratories which operate a non-cs based frequency standard with an estimated uncertainty of realising the unperturbed transition frequency of the particular atomic transition the attribution of the transition frequency previously measured elsewhere - as documented in the list - to their standard and its use as a secondary frequency reference. Joint WG CCL/CCTF Acc. to the minutes of the meeting of the CCTF WG in Tampa 25

13 Recommendation of the JWG (Part 1) CCTF2004 The CCL/CCTF Joint Working Group (JWG) for the secondary representations of the second was originally formed as a working group of the CCTF at its 15th session in 2001 (Recommendation CCTF1(2001)). It was first given the task of establishing a list of secondary representations of the second. At the suggestion of the CIPM (CIPM 2001) its terms of reference were re-formulated to include the interests of the CCL. The JWG subsequently considered the results of its questionaire and the ongoing discussions between IEN, BNM-SYRTE, NMIJ, NIST, NPL, NRC, PTB, and VNIIFTRI. And as a result of its sessions on 9th and 10th September 2003 and 30th March 2004 it

14 Recommendation of the JWG (Part 2) CCTF 2004 recommends that the unperturbed ground-state hyperfine quantum transition of 87 Rb may be used as a secondary representation of the second with a frequency of f Rb = Hz and an estimated relative standard uncertainty (1σ) of 3 x and recognizes that several optical frequency standards have been reviewed by the JWG. Although none has been proposed at this stage, the JWG believes that the rapid progress with these optical frequency standards requires that they should again be reviewed at its next meeting for their possible use as secondary representations of the second.