Physicists Revive 1990s Laser Concept To Propose a Next-Generation Atomic Clock 15
Physicists have proposed a new kind of atomic clock based on a revived superradiant laser concept that could produce an extraordinarily stable signal with a linewidth around 100 microhertz, potentially the narrowest ever for an optical laser. "The implications of this result could stretch well beyond timekeeping," reports Phys.org. "A laser immune to environmental frequency shifts would be a powerful tool in optical interferometry -- using interference patterns in light to make ultra-precise measurements." From the report: In a conventional laser, a mirrored cavity bounces light back and forth between atoms, building up a bright, coherent beam. A superradiant laser works differently: rather than relying on the cavity to maintain coherence, the atoms themselves act as single coordinated emitters, collectively synchronizing their light emission. Following early theoretical ideas emerged in the 1990s, the concept didn't gain concrete traction until 2008, when researchers at the University of Colorado proposed that superradiant lasers could serve as a new kind of atomic clock.
Atomic clocks work by using laser light to probe a very precise transition in an atom, causing electrons to transition between energy levels at an extraordinarily stable frequency. Because a superradiant laser stores its coherence in the atoms rather than the cavity, its output frequency is far less vulnerable to environmental disturbances like vibrations or temperature fluctuations. Yet although this concept was first demonstrated experimentally in 2012 in a pulsed regime, the influence of heating has so far held superradiant lasers back from their full potential. To keep the laser running continuously as an atomic clock requires, atoms must be constantly replenished with energy. Doing this atom-by-atom delivers random kicks that heat the atomic sample and disrupt the lasing process, confining it to brief pulses rather than a steady beam.
In their study, Reilly's team considered whether a modification to earlier theoretical concepts could make a continuous laser suitable for an atomic clock. In almost all previous studies, atoms were treated as simple two-level systems: an electron sitting in a ground state, occasionally jumping up to an excited state and back again. The team proposed that the heating problem could be solved by adding one extra ground state to the picture. In a two-level system, if both the pumping (re-energizing) and decay processes happen collectively through the cavity, the mathematics constrains the system in a way that prevents stable, continuous lasing. But with three levels available, pumping and decay can operate on entirely separate transitions, breaking that constraint and allowing the collective approach to work. The findings have been published in the journal Physical Review Letters.
Atomic clocks work by using laser light to probe a very precise transition in an atom, causing electrons to transition between energy levels at an extraordinarily stable frequency. Because a superradiant laser stores its coherence in the atoms rather than the cavity, its output frequency is far less vulnerable to environmental disturbances like vibrations or temperature fluctuations. Yet although this concept was first demonstrated experimentally in 2012 in a pulsed regime, the influence of heating has so far held superradiant lasers back from their full potential. To keep the laser running continuously as an atomic clock requires, atoms must be constantly replenished with energy. Doing this atom-by-atom delivers random kicks that heat the atomic sample and disrupt the lasing process, confining it to brief pulses rather than a steady beam.
In their study, Reilly's team considered whether a modification to earlier theoretical concepts could make a continuous laser suitable for an atomic clock. In almost all previous studies, atoms were treated as simple two-level systems: an electron sitting in a ground state, occasionally jumping up to an excited state and back again. The team proposed that the heating problem could be solved by adding one extra ground state to the picture. In a two-level system, if both the pumping (re-energizing) and decay processes happen collectively through the cavity, the mathematics constrains the system in a way that prevents stable, continuous lasing. But with three levels available, pumping and decay can operate on entirely separate transitions, breaking that constraint and allowing the collective approach to work. The findings have been published in the journal Physical Review Letters.
Standard location for 'absolute' time measurement (Score:5, Interesting)
With highly accurate time measurement the location matters as there is one environmental factor that cannot be shielded: gravity. So depending how far you are in the earth's gravity well time will flow at different speeds. So: is there a standard position or do we just accept that these clocks will measure slightly different times ?
This does not matter for almost all of us, it will not affect my decision as to when I eat my breakfast - I will not notice.
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Re:Standard location for 'absolute' time measureme (Score:5, Informative)
I think the SI definition accounts for this. This is the kind of problem metrologists love to solve.
The SI standard specifies mean sea level as the base, and gravity compensation is done in different ways depending on the clock type. The clocks are now so precise that they can detect changes in height (gravity) of around 1mm (it has probably gotten better since the last time I talked to some true time geeks).
Gravitational time dilation (Score:5, Informative)
The clocks are now so precise that they can detect changes in height (gravity) of around 1mm (it has probably gotten better since the last time I talked to some true time geeks).
Wow, I hadn't realized that measurements of the gravitational time dilation were now so precise as to be measurable on such a sub-centimeter scale! Amazing!
https://physicsworld.com/a/gra... [physicsworld.com]
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I think the SI definition accounts for this. This is the kind of problem metrologists love to solve.
The SI standard specifies mean sea level as the base, and gravity compensation is done in different ways depending on the clock type. The clocks are now so precise that they can detect changes in height (gravity) of around 1mm (it has probably gotten better since the last time I talked to some true time geeks).
Wow, sounds like they have to do a lot of adjustment, sea level rise, and gravity fluctuations involved. https://academo.org/demos/grav... [academo.org] Now you have me very interested. what a way to measure height and change! Off to do some research! ;^)
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The SI definition of the second just involves cesium atoms. I don't think the SI has a definition of terrestrial time, if they do I'd be interested in reading about it. There is a primary definition of terrestrial time, TAI, which estimates time on the Earth's geoid, which is similar to but not quite the same thing as mean sea level.
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The lasers will go wherever the sharks take them.
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The lasers will go wherever the sharks take them.
And, finally, there will be a shark I can set my watch by.
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Here you go:
https://en.wikipedia.org/wiki/... [wikipedia.org]
Even more gory details in the third reference:
https://webtai.bipm.org/ftp/pu... [bipm.org]
It's based on over 450 atomic clocks in very precisely characterized locations, corrected so the result approximates proper time on the Earth's geoid as closely as possible.
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Yes, there's a spot in Fort Collins, CO where NIST has marked and surveyed and dictated it to be "the spot". All the clocks are compensated to that location even though they can
This will allow us to more accurately calibrate (Score:2)
...the Doomsday Clock.
Someone needs to say it (Score:3)