[time-nuts] 88Sr+ ion-clock live stream

[Magnus Danielson]

Hi,

On 2019-12-07 21:46, Anders Wallin wrote:
> In Cs my understanding is that a transition mF=0 to mF=0 transition is used
> - so it is insensitive to the magnetic field[1].
> There is no magnetic field insensitive component of the 88Sr+ clock
> transition (other optical clocks will vary!)
> See e.g. around page 10 here for an energy-level diagram:
> http://resource.npl.co.uk/docs/networks/time/meeting3/klein.pdf

Let me make a somewhat more accurate model.

The mf values of +3, +2, +1, -1, -2, -3 transitions have a relatively
strong sensitivity to magnetic field, with a strong linear term on the
magnetic field strength, where as the 0 transition has a much weaker
quadratic sensitivity, assuming weak magnetic field which is fair
assumption. This is true for any atomic transition, so it is not unique
to Cesium. Cesium has however the second weakest (of classical neutral
atom microwave frequency, only Thallium being better) magnetic
sensitivity for it's hyper-fine transition. Even with cesium, you can
tweak the frequency of transition with the change of the magnetic field
(B-field in the lingo). All such cesium clocks is really secondary
standards, even if marketing have been boasting their contribution a lot.

Later the +1 and -1 transitions with their much stronger dependence on
the magnetic field, as being measured can be used to servo the magnetic
field to lock the +1 to -1 difference and with that the magnetic field
is stabilized and the offset caused by the magnetic field on the 0
transition used for clock steering and then it starts to approach real
primary clock behavior.

So, insensitive is overstating it, more much less sensitivity than the
other transitions, but the details of them is very useful. At some time
I should re-read it to learn these things deeper.

> To measure 88Sr+ line-center (where there is no peak at nonzero B-field!)
> the mid-point between a Zeeman pair is a good approximation, but one gets
> rid of the electric quadrupole shift by measuring the center of three pairs
> of components and calculating line-center from that. The servo-loop will
> thus need to probe the left and right side of multiple peaks in sequence.
> Our pulse-sequence now does 100 probe-pulses in about 7 seconds. If we
> probe left/right side of three pars (twelve frequencies in total) the
> line-center can be computed about once per minute. The ultra-stable clock
> laser acting as local oscillator needs to maintain stability on its own
> during those 1-2 minutes.
>
> The BIPM SRS document has more references
> https://www.bipm.org/utils/common/pdf/mep/88Sr+_445THz_2017.pdf

Sounds like lots of fun and would be cool to see in reality. Thanks for
the links.

If one had time and resources... and spare cubic meters in the lab.

Cheers,
Magnus