Deterministic entanglement swapping: First successful implementation of a technique for quantum computers

Oct 26, 2008
Entanglement Swapping with 4 ions. Image: University of Innsbruck

( -- Scientists led by Rainer Blatt, Markus Hennrich and Mark Riebe of the Institute for Experimental Physics at Innsbruck University recently succeeded for the first time in realizing a deterministic transfer of entanglement in their lab. They reported this important technique for future quantum computing in the online edition of the acclaimed science journal Nature Physics.

“Transfer of entanglement, also known as entanglement swapping is an important technique for quantum information processing and has been demonstrated in labs before. What we managed to achieve here for the first time was a targeted transfer that is called deterministic entanglement swapping,” explained Mark Riebe and Markus Hennrich of the Institute for Experimental Physics at Innsbruck University.

Entanglement is a specific connection between two individual quantum objects. For their experiment the Innsbruck scientists lined up four ions in an electro-magnetic trap and prepared them with laser beams. In a first step the ions were entangled into two pairs. Then the researchers carried out a “Bell measurement” on one ion of each pair which resulted in an entanglement of the previously unentangled ions. Depending on the result of the measurement the ions were manipulated in such a way as to produce a specific entangled state. “The quantum-mechanical entanglement can be transferred in this way by entangling two particles without a joint history,” stated Riebe and Hennrich.

Linking the building blocks of a quantum computer efficiently

This technique would be applied in future quantum computers. Entanglement is the key feature which allows quantum computers to calculate more efficiently than existing computers. The transfer of entanglement also enables the high-quality entanglement of two particles over distances. Rainer Blatt, who leads the group of researchers, explained, “The entangled particles may be separated from each other and are still linked via what Einstein called 'spukhafte Fernwirkung' (spooky action at a distance). With other methods it is very difficult to separate entangled particles without losing the entanglement.”

Entanglement swapping is of particular significance for the next generation of quantum computers. The individual building blocks of a quantum computer would then be put on small microchips and the particles would be shuttled between processing, storage and transfer elements. Rainer Blatt emphasized, “This only works if the individual ions as carriers of the qubits can be deterministically entangled and separated. We have now succeeded for the first time in proving this experimentally. It will be possible to link the different areas of a quantum computer efficiently”.

Financial support for this research project came from the Austrian Science Fund FWF within its special research area Coherent Quantum Systems, from the European Union and the Institut für Quanteninformation GmbH.

Publication: Deterministic entanglement swapping with an ion-trap quantum computer, M. Riebe,T. Monz, K. Kim, A. S. Villar, P. Schindler, M. Chwalla, M. Hennrich, and R. Blatt, Nature Physics, Advance Online Publication 26/10/2008

Provided by University of Innsbruck

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User comments : 10

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3.3 / 5 (3) Oct 26, 2008
!!! I've been eagerly anticipating a "valid" study like this one. This is quite exciting(IMO)
2.4 / 5 (5) Oct 27, 2008
Finally all the naysayers within this field of research have been proven WRONG... Classical information IS transmittable over distance. It looks like it just took someone who thought into 2nd and 3rd order effects to figure out how to measure without measuring so to speak.
3 / 5 (2) Oct 27, 2008
I had to come back to view this one again, I'm surprised there aren't more posts in this thread.
1.5 / 5 (2) Oct 27, 2008
Many people don't get the implications Phystic... I've gotten numerous stares from my colleagues (communications engineers mostly) trying to explain...
2 / 5 (1) Oct 27, 2008
Dear phystic and QubitTamer...

Please shed some light (and provide some reference links) on quantum computing and why this is such a great breakthrough. As a retired software engineer and student of physics, I share with your communications engineering colleagues a great deal of doubt and confusion as to how this all plays out. Please explain.
1 / 5 (1) Oct 27, 2008
"Dear phystic and QubitTamer...

Please shed some light (and provide some reference links) on quantum computing and why this is such a great breakthrough. As a retired software engineer and student of physics, I share with your communications engineering colleagues a great deal of doubt and confusion as to how this all plays out. Please explain."

It's not a giant breakthrough at all. They are simply doing a proof of concept for one of many mechanisms by which a quantum computer might function. Unfortunately, quantum computers are still in what for normal computers was the 1960s - no real implementation, and no easy way to build them. We're still waiting for a good solid state device that will allow this stuff to scale, but we're nowhere near anything useful yet. I (after following this stuff for a while now) would put useful quantum computers at 15-30 years out from now.

Anyhow, quantum computing involves reducing the exponent on the amount of time it takes to perform certain tasks, such as searching lists or factoring numbers. While in normal computing the time required to factor a number grows exponentially compared to it's size, in quantum computing the relationship is merely linear. This is great - in some situations.

Of course, quantum computers are by their nature not deterministic (in my mind), requiring one to take many measurements of an outcome to be sure you have the answer. This is something inherient in quantum computing, and cannot ever be done away with.

I am too lazy to give links - google it yourself or glance at wikipedia. Have a nice day.

1 / 5 (2) Oct 27, 2008

Basics: Observation at the quantum level inherently causes quantized state to change, i.e. by the time you look and measure it, the act of measuring it made it change from whatever state it was in (non-deterministic).

The breakthrough is in being able to deterministically send information in the classical sense via entanglement without corrupting the data. The article basically describes a way to use predictable 2nd and 3rd order effects to set a data state, 'transmit' it and read it and vice versa. More significantly, the experiment now allows for refinement of a technique of doing this transfer of classical information over distance.

It means that beyond new capabilities in quantum computing which up to now has really been about quantum data storage and quantum encryption, REAL computing as we understand it now will become possible AND eventually the ability to digitize audio and video data streams and transmit them instantaneously over very long distances becomes possible.

If that's still not dawning on you think 'real-time video conference with zero time lag between a notional person on mars with a person on earth'.

Let the naysayers and crowd followers now pile on because i dared challenge the hive-mind view that quantum entanglement can't ever lead to portable instantaneous communications...
1 / 5 (2) Oct 27, 2008
Oh i just had a better visualization...

Imagine if you were watching the waves in a harbor that had a lot of various sizes of boats at various speeds traversing along a shoreline. Your viewpoint would be facing down, with a good ability to measure the wave heights, frequency, etc.

Eventually if you were a diligent enough observer you could work out the size, speed, and number of big ships, medium ships, and little ships passing by you in a given time period.

You would have had classical information transmitted to you (size of ships, speed, schedule of movement etc) via observation of ONLY one of their Nth order effects (wave action on a spot of shoreline)
3 / 5 (1) Oct 27, 2008
QubitTamer: I fairly certain that you are misinterpreting the the results of the story.

Entanglement swapping example:
If Alice has a particle which is entangled with a particle owned by Bob, and Bob teleports it to Carol, then afterwards, Alice's particle is entangled with Carol's.

A more symmetric way to describe the situation is the following: Alice has one particle, Bob two, and Carol one. Alice's particle and Bob's first particle are entangled, and so are Bob's second and Carol's particle.

Now, if Bob performs a projective measurement on his two particles in the Bell state basis and communicates the results to Carol, as per the teleportation scheme described above, the state of Bob's first particle can be teleported to Carol's. Although Alice and Carol never interacted with each other, their particles are now entangled.

In other words, no classical information has been transmitted over a distance, only the entangled state has been transmitted: From Bob to Carol.
not rated yet Oct 30, 2008
Information cannot be transmitted faster than light because of Special Relativity (SR). Special Relativity still applies in Quantum Physics (QP), it is a property of space-time and is not due to gravity. Thus SR should be a part of any valid theory of the universe (dealing with large velocities.)

Special Relativity forbids anything from traveling faster than light (unless it has imaginary mass, in which case it would have negative (kinetic) energy and thus not be observable anyway-objects such as this are called tachyons and are believed not to exist/be observable.) Since information would be conveyed by something with (kinetic) energy (an object with 0 (kinetic) energy would not interact, it would have no effects whatsoever, thus it is simpler to presume such objects don't exist,indeed, in QP they don't), and objects with negative (kinetic) energy do not exist (What is imaginary mass?), information cannot travel faster than light.

Things are muddied somewhat in QP, since one must take into account all possible paths when one computes probabilities of interactions. Some paths would require a particle to travel faster than light for some period of time. However, though they must be included to get the correct answer they never are the observed path (due to SR, I suppose.)

Entanglement cannot transmit information faster than light because one would never know if the state one measures is due to the other particle or vice versa. The only was to know would be to send a signal to the other person by non-entanglement methods, and thus no faster than light. Thus, neither in QP nor in General Relativity (GR) (which presumes SR) may one transmit information faster than light. Since both QP and GR are well-tested theories, any future theories must give the same answers as they do over the scales for which they are a good approximation of reality.