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

[Anders Wallin]

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