Experiment opens the door to multi-party quantum communication (Update)

Mar 24, 2014
Trailers parked more than 600m from the researchers lab on the University of Waterloo campus were used to separate three photons, proving the theory of quantum nonlocality. Credit: University of Waterloo

In the world of quantum science, Alice and Bob have been talking to one another for years. Charlie joined the conversation a few years ago, but now by enforcing the space-like separation of the three parties, scientists have demonstrated full quantum nonlocality with more than two entangled photons.

For the first time, physicists at the Institute for Quantum Computing (IQC) at the University of Waterloo have demonstrated the distribution of three entangled photons at three different locations (Alice, Bob and Charlie) several hundreds of metres apart, proving for more than two entangled photons.

The findings of the experiment, Experimental Three-Particle Quantum Nonlocality under Strict Locality Conditions, are published in Nature Photonics today.

Once described by Einstein as "spooky action at a distance", this three-photon entanglement leads to interesting possibilities for multi-party quantum communication.

Nonlocality describes the ability of particles to instantaneously know about each other's state, even when separated by large distances. In the quantum world, this means it might be possible to transfer information instantaneously – faster than the speed of light. This contravenes what Einstein called the "principle of local action," the rule that distant objects cannot have direct influence on one another, and that an object is directly influenced only by its immediate surroundings.

Photons generated in the lab were beamed to separate trailers in field on the University of Waterloo campus. The experiment proved quantum nonlocality. Credit: University of Waterloo

To truly test that the hidden local variables are not responsible for the correlation between the three photons, IQC scientists needed the experiment to close what is known as the locality loophole. They achieved this separation of the entangled photons in a way that did not allow for a signal to coordinate the behaviour of the photons, but beaming the to trailers parked in fields several hundred meters from their lab.

"Correlations measured from quantum systems can tell us a lot about nature at the most fundamental level," said co-author Professor Kevin Resch, Canada Research Chair in Optical Quantum Technologies and recent winner of the E.W.R. Steacie Fellowship from the Natural Sciences and Engineering Research Council of Canada (NSERC). "Three-particle entanglement is more complex than that of pairs. We can exploit the complex behaviour to rule out certain descriptions of nature or as a resource for new quantum technologies."

The project team studied the correlations of three photons in a Greenberger-Horne-Zeilinger (GHZ) state – a type of entangled quantum state involving at least three particles.

First, photon triplets were generated in Resch's lab – the Alice in the experiment. Then, the first photon was delayed in a 580m optical fibre in the lab while the two other photons travelled up 85m of optical fibre to the rooftop where they were sent through two telescopes. Both photons were then sent to two trailers, Bob and Charlie, about 700m away from the source and from each other.

To maintain the spacelike separate in the experiment, a fourth party, Randy, located in a third trailer randomly selected each of the measurements that Alice was to perform on her photons in the lab.

Each trailer contained detectors, time-tagging devices developed by IQC spin off company Universal Quantum Devices (UQD), and quantum . To ensure the locality loophole was closed, the random number generators determined how the photon at each trailer would be measured independently. The UQD time tagging devices also ensured the measurements happened in a very small time window (three nanoseconds), meaning that no information could possibly be transmitted from one location to the other during the measurement period ¬– a critical condition to prove the non-locality of entanglement.

"The idea of entangling three photons has been around for a long time," said Professor Thomas Jennewein, a co-author of the paper. "It took the right people with the right knowledge to come together to make the experiment happen in the short time it did. IQC had the right mix at the right time."

The experiment demonstrated the distribution of three entangled particles, which can eventually be used to do more than pairwise communication where only one party can communicate with another. It opens the possibility for multipartite quantum communication protocols, including Quantum Key Distribution (QKD), third man cryptography and quantum secret sharing.

"The interesting result is that we now have the ability to do more than paired ," said the paper's lead author Chris Erven, a former IQC PhD student who is now a research assistant at the University of Bristol. "QKD, so far, has been a pairwise system – meaning that it works best and with less assumptions when you're only talking with one other person. This is the first experiment where you can now imagine a network of people connected in different ways using the correlations between three or more ."

Explore further: Scientists open a new window into quantum physics with superconductivity in LEDs

More information: Experimental three-photon quantum nonlocality under strict locality conditions, DOI: 10.1038/nphoton.2014.50

add to favorites email to friend print save as pdf

Related Stories

Quantum physics mimics spooky action into the past

Apr 23, 2012

Physicists of the group of Prof. Anton Zeilinger at the Institute for Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Vienna Center for Quantum Science and Technology (VCQ) ...

Quantum physics secures new cryptography scheme

Mar 12, 2014

The way we secure digital transactions could soon change. An international team has demonstrated a form of quantum cryptography that can protect people doing business with others they may not know or trust ...

Researchers explore quantum entanglement

Feb 08, 2013

Albert Einstein called quantum entanglement—two particles in different locations, even on other sides of the universe, influencing each other—"spooky action at a distance."

Recommended for you

Controlling core switching in Pac-man disks

Dec 24, 2014

Magnetic vortices in thin films can encode information in the perpendicular magnetization pointing up or down relative to the vortex core. These binary states could be useful for non-volatile data storage ...

World's most complex crystal simulated

Dec 24, 2014

The most complicated crystal structure ever produced in a computer simulation has been achieved by researchers at the University of Michigan. They say the findings help demonstrate how complexity can emerge ...

Atoms queue up for quantum computer networks

Dec 24, 2014

In order to develop future quantum computer networks, it is necessary to hold a known number of atoms and read them without them disappearing. To do this, researchers from the Niels Bohr Institute have developed ...

New video supports radiation dosimetry audits

Dec 23, 2014

The National Physical Laboratory (NPL), working with the National Radiotherapy Trials Quality Assurance Group, has produced a video guide to support physicists participating in radiation dosimetry audits.

Ultrasounds dance the 'moonwalk' in new metamaterial

Dec 23, 2014

Metamaterials have extraordinary properties when it comes to diverting and controlling waves, especially sound and light: for instance, they can make an object invisible, or increase the resolving power of ...

User comments : 28

Adjust slider to filter visible comments by rank

Display comments: newest first

TheKnowItAll
5 / 5 (1) Mar 24, 2014
If they built another system in parallel to the existing one, synchronized with it but not entangled with it. I wonder if one system might have any influence on the other. Have they ever tried such a configuration?
antialias_physorg
5 / 5 (5) Mar 24, 2014
In the quantum world, this means it might be possible to transfer information instantaneously – faster than the speed of light.

Argh. I wish people would get this right.
Information is defined as a correlation of a-priori knowledge of a state with a-posteriori knowledge of a state. (You must know what you send and the receiver must be able to infer what you sent by what he received)
But with entanglement you do NOT know what you send. If you were to look/force the entangled entity into a defined state beforehand you'd break entanglement. You can only get the a-posteriori knowledge.

So it is action at a distance. But not INTERaction (which would be information transmission).

Note:
It opens the possibility for multipartite quantum communication protocols,

Note the word 'PROTOCOL' (not: CONTENT). You can use this for encryption (as encryption does not constitute information transmission)
bluehigh
not rated yet Mar 24, 2014
Information is defined as a correlation of a-priori knowledge of a state with a-posteriori knowledge of a state.
- antialias

That's not a definition of "information". It's a partial definition of "communication".
bluehigh
not rated yet Mar 24, 2014
But with entanglement you do NOT know what you send.
- antialias

However, you do know information was sent. You also know information was received. (Not by confirmation at light speed, but through knowledge of system functionality).

antialias_physorg
5 / 5 (6) Mar 24, 2014
That's not a definition of "information".

However, you do know information was sent. You also know information was received.

You might want to read up on information theory and then start over.

You can only send information if you encode something first, send it, and then decode. Entanglement does not allow you to encode and send, because to encode you have to either:
a) set a property
or
b) measure a property and then only send those that have a desired property

Either of these two breaks entaglement.

Taking two entagled entities and separating them gives you no information transmission - as you have no way to predict what the property is until you measure it. You can say that both will be complementary (which is why it's good for perfect bitwise modulo 2 encryption) - but you can't say WHAT you sent.
Taking one and forcing it into a state does nothing to the other. You just break entaglement
bluehigh
not rated yet Mar 24, 2014
I distinguish between "information" and "information transmission". Perhaps antialias you are correct in that you can use these terms interchangeably.

It is not always necessary to measure a property to have knowledge. Information ( or knowledge) can be successfully deduced or inferred. Therein lives entanglement.
bluehigh
5 / 5 (1) Mar 24, 2014
Just for fun, here's that silly mind game again. Information can travel FTL as demonstrated that I can with absolute certainty KNOW that the sun will rise at 7:02am tomorrow morning. A full 21 hours before the light arrives. Mind speed is FTL.

Ok, ok ... I go take my meds now ....
arjuna_scagnetto
5 / 5 (3) Mar 24, 2014
"this means it might be possible to transfer information instantaneously"

WRONG! You don't know in which state the particles are, then you can't use them to send any information! The only damned thing you know is that if you measure the spin of your particles and you find is up then the other particle will have spin down.! it's not possible to use this phenomenon for information transmission. No WAY!
shavera
5 / 5 (1) Mar 24, 2014
Actually, it's quite possible for information transmission. Just that it's limited to c. I really hate the lazy clickbait journalism that goes into entanglement->ftl messaging. There's no new useful information transferred through entanglement at any rate faster than c. None at all.

Cryptographically, it's a brilliant system. Possibly impossible to eavesdrop on without alerting the other members.

But not faster than light. Stop with that.
Whydening Gyre
not rated yet Mar 24, 2014
Information is defined as a correlation of a-priori knowledge of a state with a-posteriori knowledge of a state.
- antialias

That's not a definition of "information". It's a partial definition of "communication".

I knew you were gonna say that. FTL?
Whydening Gyre
not rated yet Mar 24, 2014
"this means it might be possible to transfer information instantaneously"

No. We just don't have the equipment to measure faster than light.
bluehigh
not rated yet Mar 24, 2014
I knew you were gonna say that. FTL?

... Excellent WG thanks for the laugh.

Just a another silly thought ... When an entangled photon property is measured, how much time elapses before the entanglement is broken? Is the break instantaneous or does coherence gracefully degrade?

I read that entanglement can be detected without breakage, so ... If i measure Alice's photon then how much time elapses until I can detect the decoherence of Bobs photon. So my next attempt at an FTL comms system would simply rely on ... Entanglement = true/false.

I need another pill.
bluehigh
not rated yet Mar 24, 2014
It could be as simple as a polarising filter modulated to "measure" Alice's photons and a receiver to detect the entanglement state of Bobs photons and recover the modulated signal.

Keep in mind that the photon state (eg: polarisation) is not the information. It's only the coherence value. Entanglement = true/false.

FTL communication solved. That was easy.
antialias_physorg
not rated yet Mar 25, 2014
It could be as simple as a polarising filter modulated to "measure" Alice's photons

Look, it's rather simple:
1) If you entangle 2 photons, measure them in the way you propse, and then send the other partner off it will be sent at c (so you have achieved data transmission at c - like everyone else. No entanglement necessary for that)
2) if you send them off to Alice and Bob and AFTER RECEPTION modulate the signal at Alice it does NOTHING to the signal at Bob. You're just breaking entanglement.
3) If you do like 2) but measure and select only those that would constitute a message then Bob is none the wiser - as he doesn't know which ones you selected.

Without modulation (or selection) you aren't encoding a message. So: no go.

I read that entanglement can be detected without breakage,

It's called weak measurement. But it gives you no way out of the "no FTL data transmission" limit.
MrVibrating
not rated yet Mar 25, 2014
It's called weak measurement. But it gives you no way out of the "no FTL data transmission" limit.
Further, AFAIK this has only been used to constrain Heisenberg uncertainty - if weak measurement were able to establish a future collapse state then communication would be a big step closer... however it must be stressed that this is quite a different proposition to simply narrowing local spatiotemporal properties.
bluehigh
not rated yet Mar 25, 2014
AA your items 1,2,3 relate to observing the quantum state and completely miss the point of my suggestion.

It's the breaking of entanglement that is the "information" carrier.

Furthermore it's not my contention that "weak measurement" is the process of detection. Rather i propose a modification to the four beam path apparatus.

http://www.caltec...ocations

If the polarising filter path is excluded then three paths remain that potentially contain the "information" as to entanglement state. Not the property, just the value .. Entanglement = true/false.

Admittedly, in principle there would be a signal to noise ratio that would be extreme. However, even with high error rates, the communication would be FTL.

That is, of course, if quantum entanglement and superposition are physical and not just spooky action at a distance.
bluehigh
not rated yet Mar 25, 2014
More here for heretics ...

http://www.ipod.o...ment.asp

And here ...

http://arxiv.org/...59v2.pdf

antialias_physorg
not rated yet Mar 25, 2014
Furthermore it's not my contention that "weak measurement" is the process of detection. Rather i propose a modification to the four beam path apparatus.

Same problems as in the 1) case. Now you have to just decide whether you send an entangled or unentangled photon as your 'message'. But that still means after you decide the photon speeds with c to its target. Note that the reserchers themselves do not mention FTL information transmission. You could be sure they would if they could do it.

As for pilot wave theory: the jury is still out on that. But since a number of loopholes of Bell's theorem have been closed recently it's not looking good.
jalmy
not rated yet Mar 25, 2014
So an internet based on this system would allow the NSA to copy literally every bit of information being relayed by quantum routers. Completely undetectable because there would be no lag or delay.
jalmy
not rated yet Mar 25, 2014
This definitely means you can communicate (transfer information) faster than the speed of light. If you think of it that way, but it is a matter of perspective. First of all you have to move your entangled pairs to their locations, and you have to obey the laws of physics to do that. What this is telling us is that the information is already there. It does not have to travel because the object already made the trip (obeying standard speed limits) and contains all the information it needs to be "aware" of its partner. There is no information exchange. As far as these particles are concerned they are "one" and occupy the same space.at the same time.
shavera
1 / 5 (1) Mar 25, 2014
jalmy. No. It doesn't. It really really really really doesn't mean you can transfer information faster than light. It really doesn't. I promise.

And in fact, it makes secure exchanges of information *even more secure* than without. You would fairly quickly know as soon as a "man-in-the-middle" was listening in to your communications and you could shut it down without exchanging enough data to really "leak" information. In principle at least.
taka
not rated yet Mar 25, 2014
That entanglement gets broken if measured is just a postulate. It seems true, but you cannot be sure it is always true. There exist weak measurements that might allow to probe information without destroying entanglement. So the hope that quantum communication cannot be listened in may be false.

The fact that things are entangled mean the tings are one, there is technically just one thing that touches space in different places. That mean a real Wormhole is created. So places are actually connected, there is no space between. And therefore instantaneous communication is possible. If there exist a way to change state without destroying entanglement and then probe it in another end it will work as continuous and instantaneous communication channel that really cannot be listened in and it do not require any media between. Just entangled particles must be transported once. It is just belief that this is not possible, loos of physic do not forbid it. Quantum mechanic postulates are not loos.
antialias_physorg
not rated yet Mar 25, 2014
So an internet based on this system would allow the NSA to copy literally every bit of information being relayed by quantum routers

But they can't do anything with it. (At the very least the receiving party immediatly notices when they start doing it)
This definitely means you can communicate (transfer information) faster than the speed of light.

No it does not. Where do people get this notion? If it did, don't you think the researchers were the first to point this out?
There exist weak measurements that might allow to probe information without destroying entanglement.

Weak measurement means loss of information.http://arxiv.org/...4909.pdf
You're trading knowledge with correlation strength ofthe entangled entities (which is one of the most beatiful proofs of the no FTL information 'law', IMO)
there is technically just one thing that touches space in different places. That mean a real Wormhole is created.

Huh? No. Not in any way, shape or form.
Osiris1
not rated yet Mar 27, 2014
Look at all these foolish worshipers of 'c'. Not a single one of them could ever follow Einstein's work. Or even knew there were extra terms in his energy conversion equation...published in his old book, a copy of which turned up in a yard sale near my home many years ago. I bought that book. I do not profess to follow all what he wrote, but bear in mind that 1905 was a very long time ago, scientifically. Quantum theory was yet to be born. This is new physics and is being proven as we write here. Herr Albert had PLENTY of pathoskeptiks when he published, as these willl too. But the promise of instant commo is strong in a day when we would like to be able to connect faster with potential colonies on Mars, for instance, in the near future. Even the simplest messages or data would be welcome if instantaneousity was in the offing.
bluehigh
not rated yet Mar 27, 2014
AA, perhaps I am not explaining my conjecture clearly.

Now you have to just decide whether you send an entangled or unentangled photon as your 'message'. But that still means after you decide the photon speeds with c to its target
- AA

Alice always sends an/the entangled photon. Bob has already acquired the entangled photon. Alice "measures" the entangled photon. The entanglement immediately collapses. Bob instantaneously detects the "break" in entanglement.

Seems to me that only two possible outcomes can occur.

1. FTL Comms
Or
2. Entanglement is falsified.

Please, using logic show me why this is not correct. Invoking "C" as the flight time of the photon is unrelated when both endponts have acquired entangled photons.
bluehigh
not rated yet Mar 27, 2014
That entanglement gets broken if measured is just a postulate.
- taka

Indeed, a third option and worthy of serious investigation. Any takers on partial entanglement, where individual quantum properties can be remain entangled, while others are "disconnected"?

Jizby
Mar 27, 2014
This comment has been removed by a moderator.
Jizby
Mar 27, 2014
This comment has been removed by a moderator.
jalmy
not rated yet Mar 27, 2014
@antialias Sorry to burst your bubble of ignorance but literally every argument you made has been proven wrong by experiment. Entanglement has proven to make wormholes in experiment.

http://web.mit.ed...htmlent.

Entangled particles definitely share information FTL.

http://www.extrem...an-light

Try to develop an imagination. Or at the very least, read about experiments people who have them have done.
jalmy
not rated yet Mar 28, 2014
That entanglement gets broken if measured is just a postulate. It seems true, but you cannot be sure it is always true. There exist weak measurements that might allow to probe information without destroying it.


It may not even matter. If in the process of measuring you destroy entanglement or the particle etc, but at the same time entangle a new particle that has a known state based on the original particle, which has been shown in some experiments. You essentially can "read" the particle and "clone" it at the same time. Weather or not it is the original particle is irrelevant. All we care about is the information. Which with these "soft" measurements it is looking like the information can be preserved.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.