Method for delaying photon 140ms+ while maintaining entanglement?

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In summary: something. and hope that the photons will bounce back and forth between the two cavities often enough.
  • #1
criquant
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TL;DR Summary
My team is searching for a way to achieve the above. Any help appreciated.
Hi, my team and I are working to design an experiment. We are using spontaneous parametric downconversion with a fiber-optic system to create polarization-entangled photons at a rate of roughly 70k pairs/second. We need to be able to insert a delay into one of the two paths while maintaining polarization entanglement - ideally at least 140ms. We've explored a number of techniques for doing so but thus far have not identified any really strong candidates. Fiber-optic recirculating loops would be the ideal approach, but there doesn't seem to be a way to avoid extreme polarization degradation (given the long distance required to attain the required timespan) while also maintaining entanglement. Our strong preference of course is to use a technique that has been experimentally proven rather than figure out how to do something new, and of course the lower the technical complexity the better, as we do intend to actually execute this experiment. Any insight or guidance is greatly appreciated.
 
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  • #2
criquant said:
TL;DR Summary: My team is searching for a way to achieve the above. Any help appreciated.

Hi, my team and I are working to design an experiment. We are using spontaneous parametric downconversion with a fiber-optic system to create polarization-entangled photons at a rate of roughly 70k pairs/second. We need to be able to insert a delay into one of the two paths while maintaining polarization entanglement - ideally at least 140ms. We've explored a number of techniques for doing so but thus far have not identified any really strong candidates. Fiber-optic recirculating loops would be the ideal approach, but there doesn't seem to be a way to avoid extreme polarization degradation (given the long distance required to attain the required timespan) while also maintaining entanglement. Our strong preference of course is to use a technique that has been experimentally proven rather than figure out how to do something new, and of course the lower the technical complexity the better, as we do intend to actually execute this experiment. Any insight or guidance is greatly appreciated.

Violation of Bell inequalities by photons more than 10 km apart
https://arxiv.org/abs/quant-ph/9806043

High-fidelity transmission of polarization encoded qubits from an entangled source over 100 km of fiber
https://arxiv.org/abs/0801.3620
 
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  • #3
DrChinese said:
I would think about 2 inches would be enough for your application (140 ms).
ms?

I get 17000 miles or so to get 140 ms.
 
  • #4
Vanadium 50 said:
ms?

I get 17000 miles or so to get 140 ms.
A minor computation error on my part LOL. I misread it. My bad... :oldbiggrin:

Yes, that amount of delay is difficult. I think one of my references accomplished a fraction of that, but I'm sure the resources they used would not be available to most teams.
 
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  • #5
140 ms is already incredibly long. In the optical domain, this is the time range where people hope to go to in some undefined future with quantum memories.

I did not follow the literature too closely. About 10 years ago, a storage time of 6 ms was considered groundbreaking and worthy of a publication in Nature Physics:
https://www.nature.com/articles/nphys1152

Such EIT-based storage might have gotten a bit better over the years, but requires incredibly costly equipment. Cheaper alternatives might be found in electron spin transitions of e.g., Erbium-doped solids, which nowadays also reach storage times of few ms.
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.19.044029

You might also consider hollow-core fibers filled with atoms:
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.033320

Here, the polarization degradation should not be as bad as in hollow-core fibers if you tailor the system. Still, this will usually mean rather 10 than 100 ms at best and you will need to find a system that matches your wavelength and bandwidth.

In summary: If you find a method to realize a 140 ms delay in the optical regime that keeps entanglement intact, submit it to Nature. People are waiting for that.
 
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  • #6
@criquant Can you tell us (at least tangentially) why such a long delay is required? It seems outlandishly large and is pretty clearly a problem.
 
  • #7
Cthugha said:
140 ms is already incredibly long. In the optical domain, this is the time range where people hope to go to in some undefined future with quantum memories.

I did not follow the literature too closely. About 10 years ago, a storage time of 6 ms was considered groundbreaking and worthy of a publication in Nature Physics:
https://www.nature.com/articles/nphys1152
gentzen said:
Yes, they do have access to such information. And with currently existing quantum memory technology (stable for 10 seconds), they should already be able to exploit it. Quadratic improvement is definitively possible. There are also theoretical scenarios where exponential improvement seems possible, but it is still unclear how practically relevant they are.

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.090803
 
  • #8
gentzen said:
Well, that is a quantum memory inside the trapped ion architecture. They store ion-photon entanglement. This is of course interesting for ion-trap-based networks, but due to the bandwidth problem, this is unfortunately not really applicable to standard "vanilla" SPDC photons. You would have to put the SPDC crystal into a cavity resonant with the quantum memory transition of the ion, which in turn typically reduces the purity of the entangled state and results in quite slow dynamics and rates.
This can be done, see, e.g.:
https://www.nature.com/articles/s41534-023-00701-z

Unfortunately, this is far away from "I want a delay line", but rather along the lines of "I need to invest more than a million dollars".
 
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