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

[Anders Wallin]

Today (a mere 4-and-a-half months since my post below) we managed to lock
the ultra-stable laser to 3 pairs of Zeeman-components of the ion.
As previously, this is a complex experiment so it may fail at any time,
enjoy it while it lasts: https://www.youtube.com/watch?v=hZQtlHXECQ4

The servo that steers the clock-laser onto the clock-transition of the ion
measures quantum jumps on the red and blue side of each Zeeman-peak, and
strives for the same amount of jumps on both sides.

The overlay image cycles between a few different images:
- The clock-transition line-center estimates, based on the mid-point
between a Zeeman pair. Unfortunately this is still in units of the final
double-pass AOM that shifts the clock-laser frequency before the ion. I'll
try to improve this so it relates to the actual optical frequency at 445
THz in the future.
- an estimate of the magnetic field (1.2 uT or so), again three separate
estimates based on the Zeeman-pairs.
- The 'quantum-jumps' observed on the red and blue side of each transition.
The servo strives for equal amount of jumps on the blue and red side - then
we know we are centered on the peak. You can note how noisy the data is
(with Poisson statistics) when we only have one atom to work with! (Compare
to a Ceasium clock with a SNR of 1e6 (or so I hear))

We roughly checked the observed line-center against the drift-model for one
of our active Hydrogen-masers and so far they agree to about 3e-15. The
BIPM SRS recommended frequency is given with an (conservative?) uncertainty
of 1.5e-15 [1]

regards,
Anders

[1] https://www.bipm.org/utils/common/pdf/mep/88Sr+_445THz_2017.pdf

On Fri, Dec 6, 2019 at 5:39 PM Anders Wallin <anders.e.e.wallin at gmail.com>
wrote:

> Hi all, you may find our live-stream from the lab amusing:
> https://www.youtube.com/watch?v=z9VFbs4FogY
>
> The central bright dot is fluorescence at 422nm from laser cooling a
> single trapped 88Sr+ ion. The ion emits about 1e7 photons/s at most and we
> currently detect about 500 of those in a 20ms detection window (using a
> Hamamatsu PMT module).
> The bar-chart shows the clock-transition spectrum at 445 THz (674nm). The
> X-axis is a drive-frequency to the last AOM that shifts the clock-laser
> frequency to coincide with the ion frequency. The frequency around 75.9MHz
> should be doubled to get the real optical frequency-shift (double-pass AOM).
> We 'shoot' 100 pulses at each clock-laser frequency towards the ion, and
> detect how many times we are able to drive the ion into the 'dark'
> clock-state. The clock state is long-lived (400ms or so), and detection is
> by turning on the cooling and noticing that the ion is dark. In theory the
> ion should blink in the camera-view also, but the exposure-time is now long
> enough that there is not much visible blinking.
>
> The height of the bars show that we are able to excite the
> clock-transition with about 20-30% probability at the moment.
>
> The clock-transition splits into five symmetric Zeeman pairs, four of
> which are observed in this scan range. We have two magnetic shields and
> compensating electromagnets to reduce the DC magnetic field to 0.4 uT or
> so. This splits the outermost components by about 20kHz (10kHz AOM-range in
> the figure, doubled). The line-center is around 75.945 MHz on the AOM. This
> corresponds to the clock-transition center, 444 779 044 095 486.5 Hz. Maybe
> we should make this more obvious, an AOM number like 75 MHz is not so
> impressive...
>
> The peaks are now around 500Hz wide. This is about 1e-12 fractional. There
> is room for improvement as the 88Sr+ clock-state natural lifetime of 400ms
> only limits linewidth to 4Hz or so... Line-center (the middle of all zeeman
> peaks) can be determined more precisely than linewidth.
>
> The control system (ARTIQ) is now just repeating the same scan over and
> over, takes around 1h per scan (12kHz range with 25Hz resolution IIRC), and
> each new spectrum is plotted with a new color.
>
> Enjoy it while it lasts - this is a live stream and anything may happen
> (cooling laser unlocks from Rb-cell, clock-laser unlocks from ULE-cavity,
> ion disappears, whatever....). Ion storage times have been in the ~4 days
> range previously - so I am hoping it will run nice now for 24h or so.
>
> Final clock-operation will not scan this thoroughly over all the peaks,
> just three of them, and just one frequency on the left/right side of each
> peak - and then a numerical servo locks on to the center of each peak.
>
> If you run an optical clock I hereby challenge you to live-stream it and
> post here!
>
> cheerio,
> Anders
>