Quantum entanglement with notification

Jul 16, 2012
Quantum entanglement with notification
An experimental set-up, consisting of a beam splitter and two photo detectors, registers the photons emitted by the rubidium atoms (red beam) and generates a signal whenever the two atoms are in an entangled stated (illustrated by violet beams). Graphic: Wenjamin Rosenfeld

(Phys.org) -- German scientists have demonstrated a method of heralding entanglement between distant stationary quantum systems without destroying this particular state. The experimental set-up paves the way towards quantum communication over large distances.

At present, theoretical and think of as made of stationary nodes between which the quantum information is mediated by photons via . Quantum mechanical entanglement between the stationary quantum systems plays a key role for large distance communication. However, due to the loss of photons in the glass fibres the extension of such networks is limited. A solution to this problem is offered by a so-called quantum repeater which passes the entanglement on over a sequence of many small sections, thus extending an entangled state over a long way.

A team of scientists around Dr. Wenjamin Rosenfeld (Max-Planck-Institute of in Garching, MPQ), Dr. Markus Weber (Ludwig-Maximilians-Universität München, LMU) and Prof. Harald Weinfurter (LMU and MPQ) has now developed a fundamental component of such a device. In their experiment two rubidium atoms, 20 metres apart, are entangled in such a way that a signal is generated each time entanglement is achieved.

In contrast to classical objects that are, for example, either black or white, quantum particles take on both “colours” at the same time. It is at the very last moment, at the process of measurement that the particles decide on one of the two possible properties. This peculiar behaviour becomes even more surprising, when two quantum objects form one in which their properties are strictly connected, i.e. quantum correlated. When the state of one particle, e.g. the polarization state of a photon, is determined in a measurement, we know immediately which one of the two states will be observed in the partner particle, independent of the distance between the particles. This seeming “action at a distance” is incompatible with the classical communication of information which obeys the rules of local causality. In 1964 the Irish physicist John Bell described this incompatibility in the form of a mathematical inequality.

Entanglement can be used to create communication channels between two nodes in a quantum network that are safe against eavesdropping: a bugging operation would immediately destroy the entanglement which would be noticed by decreasing correlations at the two nodes. Entanglement could also serve as a resource for the teleportation of over very large distances. The difficulty to overcome is the fact that a substantial part of the photons which are supposed to mediate the entanglement are lost due to scattering and absorption processes in the glass fibre. Now scientists around Harald Weinfurter have developed a first component of a quantum device which will help to compensate for these losses.

The basic concept is to create entanglement between two atoms, which are separated in space, with the help of photons, and to transfer this state onto succeeding atoms (stationary respectively). Hence, two rubidium atoms, each captured inside an optical dipole trap, are stimulated by light pulses from a control laser to emit a photon. In this process the quantum state of the atom is entangled with the polarization state of the photon. Travelling through separate glass fibres the two photons reach a beam splitter where they are brought to interference. The simultaneous detection of two photons at different output ports of the beam splitter gives notice of successful entanglement. In absence of a signal the whole procedure is repeated. “We have to try for about a million times”, Professor Weinfurter explains the experimental difficulties which are mainly due to the loss of photons during their coupling to the glass fibre. “The confirmation of entanglement makes it easier to connect several such systems in series, like a chain, thereby extending the entanglement over the whole chain. Without such a signal we would have to use much more complex methods for the generation of entanglement.”

As the two parts of the experiment are already operating independently the whole experimental set-up is scalable to larger dimensions. The team hopes to achieve of two stationary systems up to distance of about 400 metres in a few years. Maybe, this experiment will also help to settle a controversy started by Albert Einstein in 1935. The renowned physicist had his doubts about the ambiguity of quantum particles and the influence of the observer on their properties. Einstein proposed, together with Boris Podolski and Nathan Rosen, that the properties are “hidden” in the particle, even before measurement. The new experiment has the potential of testing the validity of this theory.

Explore further: Superabsorbing ring could make light work of snaps

More information: Julian Hofmann, Michael Krug, Norbert Ortegel, Lea Gérard, Markus Weber, Wenjamin Rosenfeld and Harald Weinfurter, Heralded Entanglement between Widely Separated Atoms, Science, July 6, 2012; DOI:10.1126/science.1221856

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gwrede
1 / 5 (3) Jul 16, 2012
Einstein proposed, together with Boris Podolski and Nathan Rosen, that the properties are hidden in the particle, even before measurement. The new experiment has the potential of testing the validity of this theory.
I so wish that Einstein's proposal is confirmed. I simply cannot bring myself to believe in Spooky Action at a Distance.

If I don't believe in god, then it would be incongruous to jump to the side of Believers in science. And too many things in quantum physics are just that.
Deathclock
2.9 / 5 (8) Jul 16, 2012
"I so wish that Einstein's proposal is confirmed. I simply cannot bring myself to believe in Spooky Action at a Distance."

Agreed. The only reasonable answer is that the particles are not identical as we think they are, that they possess traits that we don't yet understand, making their fate predetermined, prior to measurement.
Deathclock
2.9 / 5 (8) Jul 16, 2012
I don't really buy this "they exist in all possible states simultaneously until measured" stuff either... Think of the propeller of an old airplane spinning at several thousand RPM... that also appears to exist in all possible states simultaneously, but if you take a picture of it (with a fast shutter...) you'll see that the propeller blades are really in only one state at a time, and what we see is an illusion due to the limitations of our vision. Likewise, I suspect that what we see of the quantum world is mostly illusory due to the limitations of our measurement instruments.

TL;DR - You cannot claim that something exists in all states simultaneously unless you are positive that you have found the smallest possible increment of time and have analyzed the problem on that scale... I don't think we've observed these particles in steps of one Planck time... or anywhere even close to it.
dk_1
3.5 / 5 (4) Jul 16, 2012
The last paragraph in this article, discussing the EPR Paradox, is about 75 years out of date. Bell's theorem confirms that hidden variables are not possible. Further, it demonstrates that no version of any hidden variables explanation could explain the behavior observed over the last 80 odd years.
Deathclock
2.7 / 5 (7) Jul 16, 2012
"Bell's theorem confirms that hidden variables are not possible."

In a particular interpretation of QM...
Sean_W
1 / 5 (2) Jul 16, 2012
I find it very odd that entanglement can occur when two events--atoms emitting photons--result in a third, later event--the measurement after interference. Seems like you are getting your entanglement sent back from the future. Odd.
Deathclock
2.3 / 5 (6) Jul 16, 2012
If you accept determinism you escape Bell's inequality due to not having to accept the no conspiracy assumption...
javjav
1 / 5 (1) Jul 16, 2012
I think this technology could improve the Bell test in two ways:

- Testing it at much greater distances, which avoid hidden (unknown) signals that could be communicating both particles.

- It removes statistical error. Now you can remove quantum noise and ensure that all particles used for the test where really entangled.
Deathclock
2.3 / 5 (6) Jul 16, 2012
If you accept determinism you escape Bell's inequality due to not having to accept the no conspiracy assumption...


I mean really, you have to assume non-determinism to accept the results of Bell's inequality experiments, which people then use as evidence against determinism... makes all kinds of sense.
Tausch
1 / 5 (2) Jul 16, 2012
'Entanglement' A strange translation.
Not a translation that is readily available, apparent or obvious to those raised bilingually.

Just saying.
'Closure' approaches the concept better.
Begging the question...
'Closure' of what?

The original label:
Verschränkung.

Not a first for misconceptions due to 'translations-never-do-justice' to original expressions of human language.

Too late.
'Entanglement' is entrenched ...for better or worst.
nevermark
2 / 5 (1) Jul 16, 2012
I simply cannot bring myself to believe in Spooky Action at a Distance.


Action at a distance makes more intuitive sense if space is ultimately a network of connected points. In our universe these points have happened to sort themselves out into a smooth 3D (or 4D with time) network so we have become accustomed to thinking of space having the properties of smooth geometry, but at the smallest levels space may be a rats nest of connected points so that the concept of "distance" does not mean anything.

Points of space may be connected by many links along one path and few on another. In that context, entanglement would just be an extreme case of that situation. Think of a huge fishnet which looks smooth on the whole, but contains many skips. Entanglement would be just creating a longer skip, not changing the nets properties in any way.

Speculation. But there is evidence space and distance are very different at the Plank scale.
Torbjorn_Larsson_OM
4.5 / 5 (2) Jul 16, 2012
@ qwrede:

"I so wish that Einstein's proposal is confirmed. I simply cannot bring myself to believe in Spooky Action at a Distance."

Well, you can't have it confirmed any longer. The failure of hidden variables in Bell test experiments is the best test in physics @ ~ 23 sigma in some cases.

What they are discussing is loopholes, but don't hold your breath.

And it isn't "action at a distance". Rather our common sense idea of locality doesn't fit when quantum mechanics is combined with special relativity. When Penrose came up with twistor theory combining them, it turns out events in QM & SR twistor space maps to connected points in spacetime. (See fig 2 @ http://universe-r...stor.htm )

A nonlocal space-time is local in its deeper nature. So it is "action at a point".

@ Deathclock

""Bell's theorem confirms that hidden variables are not possible." In a particular interpretation of QM..."

Bell test experiments reject hidden variables independent of interpretations.
Argiod
1 / 5 (2) Jul 16, 2012
Uhura: switch to sub-space communications and send the following message to Starfleet...
Deesky
2 / 5 (4) Jul 16, 2012
I so wish that Einstein's proposal is confirmed.

Sorry, but quite the opposite has been confirmed.

I simply cannot bring myself to believe in Spooky Action at a Distance.

That's just a human failing. QM tells us that our intuition, which evolved in the classical (or macro) world simply does not apply in the quantum world. This has been proved time and again.

If I don't believe in god, then it would be incongruous to jump to the side of Believers in science. And too many things in quantum physics are just that.

No, quite the opposite. QM isn't some unfalsifiable made up shit, like religion, but a direct result of experimentation on reality. You should read up on the history of QM starting with the puzzle of blackbody radiation - it's fascinating stuff!

Try this:
http://www.youtub...=related
Deesky
2 / 5 (4) Jul 16, 2012
The only reasonable answer is that the particles are not identical as we think they are, that they possess traits that we don't yet understand, making their fate predetermined, prior to measurement.

It may sound reasonable, but it's not how the universe works.

I don't really buy this "they exist in all possible states simultaneously until measured" stuff either.

Whether you 'buy' it or not, the universe doesn't care, it just goes on regardless. I think you would also benefit from the video I linked to in my previous post (A Brief History of Quantum Mechanics).

@Torbjorn
What they are discussing is loopholes, but don't hold your breath.

They aren't discussing 'loopholes' because they aren't talking about FTL communications, just detection of eavesdropping attempts which would lead to a disruption in the entanglement state, and thus can be detected.
Deathclock
2.6 / 5 (5) Jul 17, 2012
Deesky:

"If you accept determinism you escape Bell's inequality due to not having to accept the no conspiracy assumption..."

Don't just rate me a 1, respond to the argument...
Deathclock
2 / 5 (4) Jul 17, 2012
Have a read Deesky:

"It has long been recognized that a local hidden variable theory of quantum mechanics can in principle be constructed, provided one is willing to countenance pre-measurement correlations between the properties of measured systems and measuring devices. However, this conspiratorial approach is typically dismissed out of hand. In this paper I examine the justification for dismissing conspiracy theories of quantum mechanics."

http://philsci-ar...du/2651/
Deathclock
1 / 5 (2) Jul 17, 2012
Deesky
not rated yet Jul 17, 2012
Deesky: Don't just rate me a 1, respond to the argument


Hey, I did respond to your comment, and I didn't give you a one rating. I chose not to give you no rating at all, because your comment wasn't worthy of a rating. Instead, I pointed you to an educational reference in the hope that you might learn something.

But since you're being a douche by downranking me for no other reason than pettiness, I'll return the favor!
Deathclock
1 / 5 (1) Jul 17, 2012
You didn't respond to the argument that the conclusions drawn from Bell's theorem are only valid when making certain assumptions, assumptions that not all physicists agree with. I rated you down because you responded to others in an authoritative tone as if the science was settled when it is not, far from it in fact. Not only are there several competing interpretations of quantum mechanics that draw completely separate sets of conclusions from the observations, but the specific experiment we are talking about relies on quite a few assumptions that are far from known to be true.
gwrede
1 / 5 (1) Jul 17, 2012
Bell Test on Wikipedia ends with

some researchers point out that it is a logical possibility that quantum physics itself prevents a loophole-free test from ever being implemented
Until then I will keep my fingers crossed.