Physicists experimentally demonstrate one-way quantum steering

Sep 17, 2012 by Lisa Zyga feature
Schematic of the experimental set-up, in which Alice can steer Bob, but not vice versa. Image credit: Händchen, et al. ©2012 Macmillan Publishers Limited

(Phys.org)—When Einstein described the interaction between two distant objects as "spooky interaction at a distance," he was referring to the quantum phenomenon called steering. Steering can occur in strongly entangled systems and implies a direction between the two parties involved, while entanglement without steering generally has no direction. In 1992, physicists experimentally demonstrated steering in both directions – that is, both parties could steer each other. Now for the first time, physicists have demonstrated steering in one direction only, a feat that gives new insight into the quantum world and that could have applications in quantum information.

The physicists, from the University of Hannover's Center for and Space-Time Research-QUEST, the Max Planck Institute for Gravitational Physics ( Institute), and the Institute for , all in Hannover, Germany, have published their study in a recent issue of .

As the scientists explain, steering forms the core of the famous Einstein-Podolosky-Rosen (EPR) argument put forth in 1935. As an attempt to criticize , the three scientists theoretically pointed out an odd effect of two strongly correlated particles: measuring a quantity (such as the position or the momentum) of particle A will cause the related quantity of particle B to become certain, even though the Heisenberg Uncertainty Relation predicts a minimum for individual systems. This odd effect exists even if the particles do not come in contact with each other. To explain this finding, Einstein, Podolosky, and Rosen assumed that the outcome of all possible measurements must be encoded in both particles as a hidden parameter, and that quantum mechanics is incomplete because it does not account for this parameter. Otherwise, the instantaneous interaction would conflict with the .

However, other physicists of the day saw that the interaction was actually a new . Erwin Schrödinger was the first to call the effect "entanglement" as well as describe its stronger form, steering. In steering, the two parties (Alice and Bob) have certain roles that are not interchangeable. Specifically, Alice has the role of choosing an observable to be measured, which somehow changes the states in Bob's system. If Bob chooses the observable to be measured, then the roles would change, and he would be the one steering Alice.

Because of the two different roles, physicists have wondered about the possibility of one-way steering. That is, are there any physical states that Alice could send that allow her to steer Bob, but not vice versa?

One-way steering occurs when the second vacuum mode used to generate the steering states is between 39% and 70% (white region). When the value for one steering direction is less than 1.0 (red histogram) and the value for the other steering direction is above 1.0 (blue histogram), one-way steering occurs. Image credit: Händchen, et al. ©2012 Macmillan Publishers Limited

To demonstrate that the answer is yes, the physicists here entangled two laser beams that they prepared using an intricate system that involved mixing the beams with two vacuum modes. One output mode was sent to Alice and one to Bob. If the beams were prepared with the vacuum contribution in a certain intermediate range, the researchers observed that Alice could make a measurement on her side that influenced Bob's measurements on his side – but Bob's measurements did not influence Alice's. In other words, they observed one-way steering.

However, changing the vacuum contribution could change the results. The researchers found that, for a vacuum contribution smaller than 39%, two-way steering was allowed. And for a vacuum contribution greater than 70%, no steering was allowed. In between these values, only Alice could steer.

"From the very beginning in 1935, EPR and Schrödinger focused on the question of whether a single subsystem (of an entangled state) can be described by a classical model, given some measurement results on the other subsystem," coauthor Roman Schnabel of the University of Hannover and the Max Planck Institute for told Phys.org. "In our paper, we produce for the first time an entangled state that allows for a purely classical description of one subsystem but not of the other. Our result supports the relevance of the original question by EPR and Schrödinger since we demonstrate that the answer is more diverse than one might expect."

As he explained further, the results also provide insight into the extent to which quantum mechanics differs from classical mechanics, one of the fundamental questions in physics.

"Our experiment pinpoints the effect of decoherence," he said. "Our state is produced by discarding information from one subsystem (due to the extra vacuum contribution). The one-sided information loss results in the new situation that this subsystem can be described by a classical model whereas the other subsystem still requires a quantum model. There is a long debate on how the transition from a distinct (entangled) quantum system to a system that can be described by a classical model actually happens. Our experiment is thus a nice model system that shows how decoherence produces the transition from the quantum to the classical world."

The researchers expect that the experiment can be extended to a tripartite situation involving a third party, Charlie. In the current experiment, one output mode of the light beam was dumped, but if it had been sent to Charlie, the researchers predict that Alice could also steer Charlie simultaneously to steering Bob. Neither Bob nor Charlie could steer anyone.

As the physicists explain, one-way steering is yet another counterintuitive effect of quantum mechanics. They explain that one-way steering leads to the peculiar situation in which two experimenters performing measurements on the same shared state reach different conclusions. Future research into this implication and others could have applications in areas such as quantum key distribution for secure communication and information science in general.

Explore further: UCI team is first to capture motion of single molecule in real time

More information: Vitus Händchen, et al. "Observation of one-way Einstein-Podolsky-Rosen steering." Nature Photonics. DOI: 10.1038/NPHOTON.2012.202

Journal reference: Nature Photonics search and more info website

4.7 /5 (10 votes)

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Pressure2
1.7 / 5 (6) Sep 17, 2012
Steering or no steering what difference does it make? Einstein was and is still right.

What I would like to know is if one does not know which phase goes in which direction, how can any information ever be sent?
And if one knows what they are sending in one direction they can know the other. It would be just classical physics then.
nuge
4 / 5 (3) Sep 17, 2012
No information can be sent. That's actually an important point.
Pressure2
2 / 5 (4) Sep 17, 2012
No information can be sent. That's actually an important point.

So what would be the purpose of spending time and money on this if no information can be sent? It seems like and exercise in futility.
FantomX1
2 / 5 (4) Sep 17, 2012
i think, however not sure, that you can set up phase, and read it in one place, and on other side, reading it wont change the information that had come.

it could be perfect for intergalactic communication without lags, or immediate space communication if i understand well, teleportation, so problem is already solved?
rah
3.7 / 5 (3) Sep 17, 2012
Steering or no steering what difference does it make? Einstein was and is still right.
No. He was not right about this. He never accepted quantum theory as being complete or even correct.
Pressure2
1 / 5 (2) Sep 18, 2012
Steering or no steering what difference does it make? Einstein was and is still right.
No. He was not right about this. He never accepted quantum theory as being complete or even correct.


That is what made Einstein right. While the quantum theory of the atom has merit, the quantum theory of entanglement is today's alchemistry and has had 75 years to be proven. I'm still waiting and watching for proof.
Pkunk_
1 / 5 (1) Sep 18, 2012
Steering or no steering what difference does it make? Einstein was and is still right.

What I would like to know is if one does not know which phase goes in which direction, how can any information ever be sent?
And if one knows what they are sending in one direction they can know the other. It would be just classical physics then.

It's very simple actually - have two sets of entangled photons or other quantum objects. One for sending on one end and vice versa on the other end. It is actually the basis of all electronic communication.
And time coherence doesn't matter one bit when it comes to high speed protocols. They just rely on stop/start bits.
drhoo
not rated yet Sep 18, 2012
""the quantum theory of entanglement is today's alchemistry and has had 75 years to be proven. ""

Bells inequality is violated.
This is proof of entanglement eh ?
http://en.wikiped..._theorem
Torbjorn_Larsson_OM
not rated yet Sep 18, 2012
The interesting thing was not that obviously Einstein was wrong* but _how_ he was it. His own general relativity (not that he didn't make quantum mechanics possible too) predicts that spacetime is incomplete, because it forces black holes and vacuum transitions of eternal inflation on us. Penrose's solution is combining SR and QM into an underlying twistor space, which has some points as disjoint line segments in spacetime. What happens locally in twistor space doesn't need to look locally in spacetime.

That "two experimenters performing measurements on the same shared state reach different conclusions" is not very peculiar, it happens already in relativity and it is why it got its eponymous name. What observers can agree on is the physics, not the observations.

* Yes, Bell test experiments show how QM and SR combines (so we can have QFT), without any hidden variables. Those are the best test in physics btw, some are ~ 25 sigma. 3 decades old info.
Pressure2
3.7 / 5 (3) Sep 18, 2012
Steering or no steering what difference does it make? Einstein was and is still right.

What I would like to know is if one does not know which phase goes in which direction, how can any information ever be sent?
And if one knows what they are sending in one direction they can know the other. It would be just classical physics then.

It's very simple actually - have two sets of entangled photons or other quantum objects. One for sending on one end and vice versa on the other end. It is actually the basis of all electronic communication.
And time coherence doesn't matter one bit when it comes to high speed protocols. They just rely on stop/start bits.

It really doesn't make any difference how many pairs one uses, if you do not know the phases going in, there is no way to get any information out the receiving end.
By the way today's electronic communications are based on classical physics.
Pressure2
2.3 / 5 (3) Sep 18, 2012
There is an easy way to settle this. Have three people conduct these experiments, one sending the entangled particles, the other two each receiving one of them. Let the sender decide which phase he is sending (steering) to each receiver. If entanglement is real he would not be able to predict what phase each receiver detects because in theory the particles are in both phases until detected. If entanglement is not real he would be able predict which receiver will detect which phase.
Job001
not rated yet Sep 18, 2012
I think it is safe to say that no one understands Quantum Mechanics. (Richard Feynman)