I Applications of the proposed global quantum network of clocks

[phoenix-anna]

TL;DR Summary

One of the proposed early applications of the Quantum Internet is clock synchronization. What would be the benefits of improving clock synchronization accuracy by one or more orders of magnitude?

Physicists have proposed linking a global network of cesium clocks in a phase-coherent entangled state, for example in the article A Quantum Network of Clocks (arXiv:131045v1). My audience would like to know how better synchronization or more accurate timekeeping would lead to advances in our understanding of the laws of nature or would provide other benefits. I can imagine a few: tests of general relativity, for example. But after searching I could find any articles mentioning the motivation for this proposal and I am unsure about it.

[Better link to the paper added by a Mentor]
https://arxiv.org/abs/1310.6045

 

[berkeman]

It looks like that arXiv paper is from 2013. Have you been able to find anything more recent?

 

[dlgoff]

It looks like that arXiv paper is from 2013. Have you been able to find anything more recent?

I'm guessing you found this:
[Better link to the paper added by a Mentor]
https://arxiv.org/abs/1310.6045
Interesting. Thanks

 

[Twigg]

Physicists have proposed linking a global network of cesium clocks in a phase-coherent entangled state

First, I'd like to clarify that the authors of this paper are famous for their work on optical atomic clocks, not microwave atomic clocks (the cesium fountain clock is a microwave clock). This article discusses a network of optical clocks. They don't seem to specify any particular atom in the article, but the second to last author Jun Ye has two very impressive strontium lattice clocks.

I can imagine a few: tests of general relativity, for example.

It's a bit of misconception that you need more sensitive clocks to test GR. If you just want to test GR, your best bet isn't a super sensitive clock, but rather a portable clock. You can put the portable clock on a plane or on a satellite and observe a much larger redshift than you would with a stationary clock. The bigger the redshift effect, the easier it is to measure deviations from GR. This is what made the Hafele-Keating experiment such a smart choice. Another example is the GREAT (Galileo gravitational Redshift test with Eccentric SAtellites) project, where some smart physicists utilized some poorly-launched GPS satellites with hydrogen maser clocks on board. To the best of my knowledge, GREAT is the most stringent test of gravitational redshifts to date, and the clocks used are far from cutting edge.

That's not to say that more sensitive clocks aren't helpful in testing GR, but being portable is the most important criteria by far.

The prospect of having a network of satellite clocks like those used in the GREAT experiment, but with quantum coherence added as in the arXiv paper, would likely improve their sensitivity to beyond-GR effects. However, it's not clear to me if that gain in sensitivity would be worth the investment (time, money, politics, etc.). Also, I don't think there's enough money in the field to fund a fleet of satellites. GREAT only happened because the eccentric-orbiting satellites were already up in the sky floating around as expensive space junk until they were put to work collecting data. It was a "one person's trash is another person's treasure" kind of moment.

(second post incoming, this one got really long)

 

[phoenix-anna]

Putting aside the question of whether it is politically feasible, why would we do it if we could?

 

[Twigg]

Putting aside the question of whether it is politically feasible, why would we do it if we could?

Sorry about the rambling. Here are some I can think of:
1. If the network consisted of entangled clocks on satellites, it would be an excellent platform for gravitational wave (GW) detection, competitive with LISA and eLISA. See this 2016 paper (arXiv 1606.01859 / Phys Rev D 94, 124043) for a full reference. If nothing else, it would provide independent confirmation of GWs as seen by a fundamentally different apparatus (clocks instead of an interferometer). If I understand the 2013 paper you linked correctly, it would offer a $\sqrt{2}$ reduction in the noise floor, for a two-clock network.
2. Having multiple satellite clocks entangled would (I think?) make it much easier to detect a dark matter domain wall. See this Nat. Comm. article (arXiv: 170.06844) from 2017 for a proposal using GPS satellites (without entanglement). My gut feeling is that if one of your satellite clocks moved through a domain wall, the other clocks entangled with it would go nuts (more so than if they were only classically coherent by a common local oscillator phase).
3. The entangled network would grant you a higher sensitivity (by a factor of $\sqrt{K}$, where $K$ is the number of clocks in the network) on a gravitational redshift experiment like GREAT with a network of satellite-based clocks. This would in principle grant a higher sensitivity to redshifts effects beyond GR.

This list only applies to a quantum network of clocks in space. I can't think of any novel uses for quantum network of ground-based clocks, at least nothing as high-impact as the above list.

The advantage of ground-based clocks is that they tend to be more advanced than what you can put into space, because you have fewer design constraints. So, a ground based network might be a much better time-keeping network, but there isn't a high demand for this. When/if the SI redefines the second based on an optical transition, that definition will be strictly a local (not global) definition of the second. That's the difference between the SI second and UTC. In contrast, there just isn't a lot of demand for a better global time standard than UTC, commercially or academically, at least not to the best of my knowledge.

 

[phoenix-anna]

Thanks for your detailed and focused response; it was exactly what I needed. Thanks, too, for correcting my misconception that they were Cesium clocks. This is terrific!

 

[phoenix-anna]

I'm not sure what local standard would mean?

 

[Twigg]

The abstract of this Nature Photonics article from 2020 explains it better than I could. Unfortunately I couldn't find a preprint online. Sorry :/ The abstract was well written though!

Since clocks at different elevations won't be synchronized, the best you can do is to define the second at one given location (and at a given velocity, if you want to take Doppler shifts into consideration too). That's what I mean by a "local" standard. Technically, the current SI definition of the second is a local measurement, in which you need to measure the transition frequency of cesium at your location. However, modern cesium clocks aren't sensitive enough (by about 2ish orders of magnitude) to pick up gravitational redshifts over small elevation changes (bottom floor of a building to top floor of a building). This is kind of important when you consider that at least 5 (that I can think of) of the more promising optical clocks are at 1 mile elevation in Boulder Colorado.

Another exciting experimental demonstration of gravitational redshifts over short distances (down to a millimeter!) came out this year: Nature article and arxiv preprint (https://arxiv.org/abs/2109.12238). This one is a bit different, because it's measuring the difference in frequency between the "bottom half" of the atoms and the "top half" of the atoms due to gravity. Unlike Katori's experiment with the two portable clocks, Jun's result looks at the frequency differences within a single atomic sample (a single clock).

Edit: Maybe a better way to explain what I mean by "local" is that clocks in different reference frames (altitude, velocity) will appear desynchronized.