Theorists propose globally networked entangled atomic clock

Jun 16, 2014 by Bob Yirka report
The concept of world-wide quantum clock network. Credit: Nature Physics (2014) doi:10.1038/nphys3000

(Phys.org) —A small team of physicists from the U.S. and Denmark has published a paper in the journal Nature Physics outlining the idea of a collection of atomic clocks located around the world—all networked via entangled particles. They propose that such a system of clocks would be far more accurate than anything that exists today.

The idea of networking clocks involves two areas of research—atomic clocks and entanglement. Atomic clocks are of course the most accurate time devices available today—they track time by measuring the resonance frequency of atoms—generally caesium. And entanglement is where pairs of particles are linked in ways that are still not fully understood—what happens to one automatically happens to the other, regardless of distance. To make a global networked clock, the researchers propose, would involve setting up a bunch of atomic clocks at various sites around the globe (and one or more in space) and then entangling particles between each of them, one after the other. The result would be a single clock that would be more precise than any of its component clocks. That would be possible, the team notes, because entanglement would allow for reduced measurement noise in all of the clocks.

If such a network could be built, it would mean an unprecedented level of accuracy, which would in turn mean improvements in devices that rely on time accuracy, such as GPS. It could also serve as a platform for physicists studying gravitational waves.

The researchers note that a shared global clock would be more stable than clocks used today because they would be constantly updating—the more clocks in the network, the more stable the clock as a whole would be. It would also be secure, they add, because anyone trying to tap into it would be instantly discovered, courtesy of the laws of quantum mechanics. They believe such a global clock would be up to 100 times more accurate than anything we have today, which is saying a lot—a modern is so accurate that it would take 300 million years to gain or lose just one second.

Building such a networked clock, even if a plan and funding were available, is still a long way off, of course, scientists still have a lot to learn about , both how it works, and how to use it.

Explore further: Accuracy of the NPL caesium fountain clock further improved

More information: A quantum network of clocks, Nature Physics (2014) DOI: 10.1038/nphys3000

Abstract
The development of precise atomic clocks plays an increasingly important role in modern society. Shared timing information constitutes a key resource for navigation with a direct correspondence between timing accuracy and precision in applications such as the Global Positioning System. By combining precision metrology and quantum networks, we propose a quantum, cooperative protocol for operating a network of geographically remote optical atomic clocks. Using nonlocal entangled states, we demonstrate an optimal utilization of global resources, and show that such a network can be operated near the fundamental precision limit set by quantum theory. Furthermore, the internal structure of the network, combined with quantum communication techniques, guarantees security both from internal and external threats. Realization of such a global quantum network of clocks may allow construction of a real-time single international time scale (world clock) with unprecedented stability and accuracy.

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antialias_physorg
5 / 5 (1) Jun 16, 2014
what happens to one automatically happens to the other

That's not correct. Just the behavior of one upon measurement is what you also measure at the other. You cannot SET one and expect the other to conform to that setting. That would be FTL information transfer.
ab3a
not rated yet Jun 16, 2014
I wonder how such entangled systems could handle phase and altitude differences of the entangled particles.
Modernmystic
1 / 5 (1) Jun 16, 2014
what happens to one automatically happens to the other

That's not correct. Just the behavior of one upon measurement is what you also measure at the other. You cannot SET one and expect the other to conform to that setting. That would be FTL information transfer.


What if you simply were able to tell a change happened, regardless of the state after changing its partner? The fact that there was a change at a particular time is itself information. A change (of any kind at a particular time) could be a 1 and no change could be a zero. That would seem to qualify as information transfer, unless time itself can muck up the works, but it should be easy enough to correct for that with the kind of precision we have with modern clocks.

I'm assuming you HAVE to be able to at least tell a change happened, otherwise I find it difficult to see how they proved entanglement in the first place.
Tektrix
not rated yet Jun 16, 2014

What if you simply were able to tell a change happened, regardless of the state after changing its partner? ...


How do you tell the change happened? You need to make a measurement to do so, which will collapse the state of the system if it isn't already collapsed by the 'sender'. So, when the time comes to see if you have a 0 or a 1, when you look (take a measurement), you have no way of knowing if it was in that state before you looked, or if your looking at it made it change states. You need a classical channel to find out who looked first.
Modernmystic
1 / 5 (1) Jun 16, 2014


How do you tell the change happened? You need to make a measurement to do so, which will collapse the state of the system if it isn't already collapsed by the 'sender'. So, when the time comes to see if you have a 0 or a 1, when you look (take a measurement), you have no way of knowing if it was in that state before you looked, or if your looking at it made it change states. You need a classical channel to find out who looked first.


OK, I think I partially understood that...

Here's my problem. If you can't tell a change happened without looking though then how can you demonstrate entanglement to begin with? If you can tell the initial particle was in "state A" and then entangled it, moved the two apart, made a measurement of the separated particle at a specific time and found it was in "state B" you'd know it was a 1, if no change, 0.

Or is that just not the way it works?
Jantoo
not rated yet Jun 16, 2014
They propose that such a system of clocks would be far more accurate than anything that exists today
Wouldn't it increase the uncertainty in the pace of the clock instead? The entangled system of multiple particles would become more classical.
antialias_physorg
5 / 5 (1) Jun 16, 2014
What if you simply were able to tell a change happened, regardless of the state after changing its partner?

That's what I'm saying in my first post. If you change one particle it doesn't do anything at the other end (you just break entanglement). And the other side doesn't know whether the particle he has is entangled or not - so you can't even transfer information by breaking/not breaking entanglement.

I find it difficult to see how they proved entanglement in the first place.

By measuring the particles at either end and then just recording what they got and comparing notes.
Modernmystic
not rated yet Jun 16, 2014
And the other side doesn't know whether the particle he has is entangled or not - so you can't even transfer information by breaking/not breaking entanglement.


How could he not know? I'm very confused. If you both are "in on the experiment" then you both know you have entangled particles. Or is it just a probability game? We MIGHT have entangled them or we might not have?

By measuring the particles at either end and then just recording what they got and comparing notes.

If you don't know if the particles are entangled or not then what good would comparing notes do? If you do know then it seems you could tell if something changed or not. Unless you're blind to the state of both particles AFTER they are entangled. Which then again raises the question of how did you know there was a change if you didn't or couldn't know the initial state.
antialias_physorg
5 / 5 (1) Jun 16, 2014
If you both are "in on the experiment" then you both know you have entangled particles.

If you're sending messages via entangled/not entangled then the other side can't know (otherwise he already knows the message beforehand and you aren't sending anything)

If all are entangled then that tells you nothing, either. If you break entanglement on some then and the other side doesn't know beforehand which ones (which constitutes an enconding) then that doesn't help since the other side can't tell without asking. in which case you just have classical communication and can obliterate all the entanglement effort in any case.

it seems you could tell if something changed or not.

No. The distribution is the same. A polarized photon looks the same as an entangled polarized photon. You could only tell by bringing them back together (at the speed of photons)

how did you know there was a change if you didn't or couldn't know the initial state.

You can't.
Modernmystic
not rated yet Jun 16, 2014
You can't.


If you can't know there was a change, then how do you know if there was entanglement?

Thanks for taking the time to walk me through this BTW.
antialias_physorg
5 / 5 (1) Jun 16, 2014
If you can't know there was a change, then how do you know if there was entanglement?

As I said:
Alice creates entangled photons.
She sends half to Bob
Bob measures his (and Alice measures hers). They both jot down what they got.
Then Bob travels back to/calls Alice and they compare notes - and lo and behold: they got same sequence.

That's why you CAN use this for secure quantum encryption, BTW.
Alice creates entangled photons (for encrytion she doesn't care which way they are polarized.)
Then she sends one of each off to Bob.
Then she encrypts a text by measuring her photons (getting 1s and 0s) and bitwise adding them to her message.

She then starts to send this off to Bob.
Bob decrypts by bitwise adding (or subtracting...same thing) his measurements.

If anyone was eavesdropping they'd have measured the photons en route to Bob - breaking entanglement(suddenly Bob gets gibberish - he knows now Eve is eavesdropping)

Note: encryption does not constitute information transfe
Modernmystic
not rated yet Jun 16, 2014
As I said:
Alice creates entangled photons.
She sends half to Bob
Bob measures his (and Alice measures hers). They both jot down what they got.
Then Bob travels back to/calls Alice and they compare notes - and lo and behold: they got same sequence.


OK, now I'm getting it. I think the sticking point for a lot of people on this one is they're thinking that Alice is looking at a SINGLE photon and Bob is looking at another SINGLE photon that are entangled. When Alice measures hers, Bob's changes, BINGO FTL communication. That's not the way it works though if I'm hearing you correctly. What I'm hearing is something like;

Alice and Bob are in the same room. Alice bakes brownies, but doesn't know how many are chocolate and how many peanut butter. She puts them in a box and sends half with Bob. Bob can go a trillion miles away and when he opens his box he gets the same sequence of brownies Alice got whenever she happened to open hers....

It's acting like a single system. Closer?
antialias_physorg
5 / 5 (1) Jun 16, 2014
It's acting like a single system. Closer?

Yes. (that's why Einstein said "spooky ACTION at a distance" not "spooky information transmission at a distance"). Stuff gets/is synchronized,

For the notion of information transmission you have to have
- A priori enconding
- Transmission
- A posteriori decoding

With entanglement the 'a priori encoding' part is missing

Alice can't know what she puts in the box. If she does beforehand (forcing a sequence of photons which she entangles) and then sends off the entangled photons it's just regular radio transmission at the speed of light (she might as well not have bothered entangling any).
If she rearranges the sequence of brownies in the box after having sent off a box to Bob then she breaks entanglement and Bob's box looks different from hers (i.e.: no message)
George_Rajna
Jun 17, 2014
This comment has been removed by a moderator.
Ricardo_Espinoza
not rated yet Jul 21, 2014
Wouldn't a Sun Dial be accurate. Thinking!