The physicists, from the University of Waterloo and the University of Calgary, have published their paper on three-photon energy-time entanglement in a recent issue of *Nature Physics*.

As the physicists explain, this new form of entanglement is the three-photon version of the famous EPR correlations for continuous variables (e.g., position and momentum) between two particles. The EPR thought experiment, published in 1935, raised questions about the fundamental concepts underlying the young theory of quantum mechanics.

"The Heisenberg uncertainty principle forbids one from simultaneously discovering both the position and momentum of a particle with arbitrary accuracy," lead author Krister Shalm of the University of Waterloo told *Phys.org*. "EPR pointed out that, if you create a pair of entangled particles, it is possible to measure both the position and momentum of both of them with arbitrary precision. It is still impossible to learn both the position and momentum of each of the individual particles, but, instead, we can learn information about the total position and momentum they share. Entangled particles, in some sense, are the ultimate team players. They lose their own individual identity with all the information in the system contained in the correlations."

In the original experiment, EPR tried to demonstrate that the correlations between two particles were so strong that there must be some hidden parameter to explain them that quantum mechanics does not account for. This conclusion seemed to uncover some inadequacies in quantum mechanics.

"The original arguments made by EPR in 1935 were designed to show that quantum mechanics, by itself, is not sufficient to describe reality," Shalm said. "This inspired John Bell, who showed that if you take the arguments of EPR that relied on hidden variables to their logical conclusions, you arrive at a contradiction with quantum mechanics. Since then, much work has been devoted to using Bell's work to test quantum mechanics, and extensions of the work have profoundly shaped our understanding of the quantum world."

Building on these studies over the next several decades, physicists have demonstrated many different types of entanglement, which are defined by the number and type of objects that are entangled and the properties of the objects that are entangled. These properties can fit into one of two categories: discrete or continuous, which describe the variable's domain. For example, spin is a discrete variable since its value can only be an integer or half-integer, while emission time is continuous. Entanglement has previously been demonstrated between the discrete variables of 14 ions and the continuous variables of three light beams, but until now entanglement among the continuous properties of three individual particles has remained an open challenge.

"What is exciting about our work is that we can take the original arguments made by EPR for two particles and extend them to three particles," Shalm said. "The kind of entanglement that EPR first proposed pertained to continuous variables, like position and momentum, as opposed to discrete variables, like polarization or spin. Discrete variables in photons have traditionally been easier to manipulate. With our system we finally have a viable way to explore the entanglement of continuous variables between three particles."

To achieve continuous-variable entanglement among three photons, the physicists split a photon into a pair of daughters using a process called cascaded spontaneous parametric downconversion. Since energy is conserved, each daughter photon has a frequency that is roughly half that of the pump photon. When one of the daughter photons is split, the two granddaughter photons each have a frequency that is about half that of the daughter photons. Although the frequency of each individual photon may vary slightly from exact halving, the total energy of the three photons combined is exactly equal to the energy of the pump photon. In addition, because the splitting process is instantaneous, the three photons must arrive at photon detectors at the same time.

Under these production conditions, the three photons share strong spectral correlations and, in theory, possess genuine tripartite energy-time entanglement. This means that the energy values and the emission times of the three photons share correlations that are stronger than those allowed by classical physics.

To verify that the three photons possess energy-time entanglement, the physicists had to confirm that the three photons violate a set of inequalities that are an extension of the EPR arguments for two particles. These tests require measuring and comparing the arrival times of the three photons at a single-photon detector. One way to do this is to directly measure each photon's frequency; however, current technology doesn't provide sufficient precision for direct frequency measurements. Instead, the scientists measured the frequency of the pump photon and ensured that energy was conserved in the downconversion process. They also used detectors to measure the arrival times of the photons, but noted that the detectors have a timing jitter of several hundred picoseconds that limited the precision.

Even accounting for this uncertainty, the results showed that the three photons do indeed violate the EPR inequalities and are therefore energy-time entangled. Future improvements in detector precision would provide improvements in the measured values of the inequalities by over two orders of magnitude. In addition, new technologies to enhance the observed effects could potentially allow this scheme to be scaled up to larger photon numbers.

In terms of applications, this entanglement scheme could be useful in quantum communications because it provides the opportunity to entangle multiple degrees of freedom, generating "hyper-entanglement." If one of the entangled photons could be interfaced with an atomic storage medium while the other two photons are transmitted over telecom fibers to remote quantum nodes, then scientists could create new possibilities for storing and distributing quantum information. Other modifications of the scheme could lead to new fundamental tests of quantum mechanics.

"Three particle states that are entangled in their continuous degrees of freedom may allow for a new class of tests for quantum mechanics that could further our understanding of quantum theory and entanglement," Shalm said. "This is also an important technological step. It is a system that lets us exploit optical nonlinearities at the single-photon level. This may have important applications in creating the gates needed in a quantum computer, or in distributing quantum information over a network."

In the future, the scientists plan to try to combine the position and momentum entanglement among the three photons with more traditional types of entanglement based on angular momentum and polarization. This kind of combined entanglement could lead to the creation of hybrid quantum systems that possess multiple unique properties of light at the same time.

**Explore further:**
Quantum physics mimics spooky action into the past

## Lurker2358

This is not even "entanglement". This is just addition, I think.

A plus B equals C

B plus A equals C.

If C is constant then A and B are mutually determined, by the Commutative property.

It works for multiplications too, so halving frequencies and finding that they are in fact halved is just confirming the Commutative property of addition and/or multiplication, and is not necessarily "entanglement" at all, even if it is entanglement, it is a logical entanglement, rather than a physical entanglement.

## eloheim

"Even accounting for this uncertainty, the results showed that the three photons do indeed violate the EPR inequalities and are therefore energy-time entangled."

This means that doing something to one of the photons has an *instant* effect on the other two. It's almost like the photon's a "hole" in reality, with the far-off location of the other particle(s) immediately on the other side. Somehow those two spots are right next to each other, touching, even though there's nothing in between.

I've also found one thing that always gets left out when trying to impress the significance of this process is exactly what "instant" means in context of the finite speed of light.

## Lurker2358

The experiment they've described does not exhibit that "spooky action at a distance" behavior.

The only thing it does, as described, is divide a known quantity into two or more quantities which sum back to the original value, and then measures those values.

## Noumenon

If a particle's position is measured accurately, because of the momentum-position uncertainty relation, ...that position state represents a linear sum of many momentum basis states. The momentum is unknown to the degree the position is determined.

Likewise there is an uncertainty relation between energy and time of the photon. Since energy is conserved there must be a correlation between the times and frequencies (energies).

It is not like splitting up a coin, one heads so the other must be tails.

## antialias_physorg

Not quite. If that were the case you could transmit information that way (which you can't). Only determining a property of one (forcing it into a choice of one of its uncertain properties) will instantly tell you everything about the property of the other.

Subtle, but crucial.

## Lurker2358

Temporal entanglement implies that you somehow can transmit information via entanglement, particularly since the "past" in the experiments is also displaced in space by the distance the Earth moved during the allotted time between the start of the experiment and the end.

More than one article has been written on here showing both temporal entanglement and reverse causality were found via experiment in an Earth laboratory. This requires information to move from particle B in the future at position B' to particle A in the past at position A' without crossing the space or time between them. Considering the Earth moves very fast this would be a large distance.

## Ray W

Intercept a signal between Alice & Bob, split the signal, store one while doubling the other and sending it on to Alice or Bob. Monitor changes in your stored signal as the reciever decodes the signal. Use this information to break the security between Alice & Bob.

The trick is doubling the signal without loosing the original entanglement.

## Lurker2358

## Telekinetic

That is one of the most insightful statements I've read on this forum. You're a thinker.

## ROBERTOF

## TheGhostofOtto1923

"When a measurement is made and it causes one member of such a pair to take on a definite value...the other member of this entangled pair will at any subsequent time be found to have taken the appropriately correlated value In Quantum entanglement, part of the transfer happens INSTANTaneously..."

-Instantaneously indicates no delay in change of state whatsoever regardless of separation distance. The speed of light indicates a delay proportionate to the separation distance. FAIL

"...the steps required to read out the teleported quantum state ensure that no information can be communicated faster than the speed of light"

-Again, and as always, words are inadequate to describe these things. Thats why physicists use numbers.

## Telekinetic

Ghost, you really must stop relying on Wikipedia for proof.

They let anyone contribute erroneous information. You better do your homework on entanglement. By the way, are you "lite"?

## TheGhostofOtto1923

"The researchers found that when each photon reached its destination, it could instantly sense its twin's behaviour without any direct communication. The finding does not violate the laws of quantum mechanics"

"The experiment shows that in quantum mechanics at least, some things transcend space-time, says Terence Rudolph, a theorist at Imperial College London."

-This is exactly what the wiki article says. You should not reject wiki excerpts solely on the facts that you do not understand them, nor fail to cross-check their veracity.

And no I am not lite. Lite frequently downrates me. I state this on my profile page, as well as exposing at least one accuser for the lying stalking dimwit that he is.

## Noumenon

## TheGhostofOtto1923

http://medicalxpr...nal.html

-You need to update your lexicon. And your profile page.

## Pressure2

## Pressure2

## Pressure2

## StarGazer2011

This seems to sum it up for me; can anyone explain why 'action at a distance' is actually required to explain this correlation?

And before someone rattles on about 'uncertainty' be aware that argument is just putting the cart before the horse; perhaps the existance of corellation between quantum states (entanglement) is actually evidence that the copenhagen interpretation is wrong and that the 'uncertainty' is an artifact of interaction, not a property of the particles (or the universe) themselves. I.e. that postiion and momentum are actually simultanously defined as actual discrete concrete values but not simultaneously measurable due to interaction at the moment of measurment.

## Estevan57

Yup. Hit the nail on the head with that one. Good call.

## Lurker2358

In the temporal entanglement experiment which was done in an article on here a while back, if the entanglement is what it claims to be, of which I'm not entirely sure, rather than a mere classical causality correlation, then information absolutely must have been communicated between the test object in the "future" set back to the test object in the "past" set. Since the past position of the Earth was very far away from the future position of the Earth, this information must have made quite a decent sized trip instantaneously.

The reason is that the two events are otherwise unrelated except that they are both part of the same experiment, and that the experiment is designed to try and entangle the outcomes of events which happen at different times. It appears to work regardless of the delay, even across several days apparently.

## Estevan57

Amazing how similar the two profiles have become.

## Telekinetic

Above is the profile page of lite. The only poster that uses or has ever used the term "pussytard" is GhostofOtto1923. Ladies and Gentlemen of the jury...

## Noumenon

Why?

## Telekinetic

And that, fellow commenters, is the origin of GhostofOtto's anti-religious ravings. He sees himself as wearing the mantel of Martin Borman et al. I'll bet he's got a nice little collection of S.S. daggers. They're fakes, sucker.

## Judgeking

## AmritSorli

By quantum entanglement quantum vacuum itself in which move photons is a direct information medium between photons. The introduction of "hidden variables" at the beginning of last century was unnecessary, physicists has forgotten that 3D universal space is a physical reality which has origin in a 3D quantum vacuum. With such an approach introduction of hidden variables would not be needed. Finally time in this view is merely a numerical order of photons motion in a 3D quantum vacuum.

http://dx.doi.org...25.1.141

## vacuum-mechanics

This seems not so much help to understanding of quantum theory and entanglement. The reason behind is that we still do not know how the mechanism of quantum mechanics work! Knowing the mechanism below may help!

http://www.vacuum...17〈=en

## kochevnik

## Mumrah

If that's all there was to it then you'd be right but its not. Consider the double slit experiment. The photon travels through the left (1/2) or right (1/2) slit. Except it doesn't work like that does it? As long as you don't observe the slits it can do both and these eventualities can interfere.

I think what's going on is a form of relativity. Entangled particles have a fixed state relative to one another that isn't fixed WRT the universe at large. The act of observation fixes the state relative to the observer hence you can deduce the state of the other particles. Nothing need have absolute state.

## Pressure2

## Claudius

It is simpler to explain this with the many-worlds hypothesis. No hidden parameter needed, in that case. Ockham's razor, and all that.

## Telekinetic

## TheGhostofOtto1923

## Noumenon

Umm yea, I'm not sure Ockham's razor would apply to a Many-Worlds hypothesis,... of all things.

The simplest explanation is, there not to be a problem to begin with,... i.e. that the non-intuitive nature of qm is purely na epistemological one. Qm is as complete as in can in principal be, in conforming reality so it is conceptualizable through observation.

## Claudius

That depends on which many-worlds hypothesis you are talking about. The one I mean is the one promoted by David Deutsch, for which there is physical evidence, and is actually much simpler that the classical interpretation.

## Telekinetic

Well then, let's put the question out to Vendicar Decarian, are you the user of the sock puppet "lite"? Or are you comfortable being backstabbed by TheGhostofOtto1923?

## ValeriaT

But because the vacuum is not completely empty and it's filled with many tiny curvatures of space-time, you as a observer have many tiny reference frames available. So you can observe the same event from multiple reference points and you'll get slightly different results during each independent observation, despite each observer observes the very same event. IMO it's just another interpretation of the fact, the vacuum is full of tiny fluctuations/virtual particles/space-time curvatures and nothing very special is about it.

## Telekinetic

Until you realize, according to David Deutsch (who is even more accomplished in his field than you, ValeriaT) that we have counterparts made of the same particles and structures and hence, the same phenomena apply to us on a much larger scale. Therefore, if true, it is extremely special and even spellbinding.

## ValeriaT

## antialias_physorg

Because you can do nifty stuff with entangled entities that do not requitre the transmission of information

E.g. the secure encryption of an information transmission. Encyrption does not add information and is therefore possible using entanglement.

And anyhow - as with all research inetersts: you never know what kinds of uses it may be put to in the future (think about that even Einstein regarded laser light as a "neat physical phenomenon without any possible application")

While the many-worlds hypothesis is alluring the idea of having many hidden/invisible worlds does qualify as an addition of (infinitely many) hidden variables.

## Ober

For those who doubt entanglement, I suggest you re-visit his work.

## Noumenon

Yes, there are two camps, realists (Deutsch) and positivists. Thanks for the reference, I'll check out his book.

## Telekinetic

"Deutsch invented the idea of the quantum computer in the 1970s as a way to experimentally test the "Many Universes Theory" of quantum physics -- the idea that when a particle changes, it changes into all possible forms, across multiple universes."- Wired Magazine

## Ober

I wonder if holographic theory could be applied AGAINto the 2d surface, to further reduce the amount of dimensions needed to explain the observables. Maybe the 2d surface could collapse further into 1d.

## omerbashich

## Fleetfoot

That term is used in the press reports, seldom in the original papers.

If an entangled pair is created and then both are measured but some distance apart, there is a correlation between the measurements. The statistics rule out the notion that each is given a value when they are created.

When one is measured, it's value is random. When the other is measured, its value is determined by the first. However, since they are widely separated but measured at close to "the same time", it is not possible to say which is first or second. Each is "first" in some frames and "second" in others.

It is also not possible to say whether a measurement sets the value obtained or whether it was set some time earlier by the measurement of the other particle.

In serious papers, they will only say the measurements are "space-like separated".

There is also no evidence of communication between the particles, only correlation.

## ValeriaT

Edit: if you're impressed with the possibility, you could do the mathematical operations with multiple quabits at the same moment, then you should be warned, that the stability of these entangled quabits decreases with the number of entangled states geometrically.

Memo: the physical laws are binding for everyone.