Could some entangled states be useless for quantum cryptography?

( -- One of the widely accepted properties of quantum entanglement is secrecy. Since scientists and researchers began working with quantum key distribution, entanglement has been considered an essential part of keeping communications private. What if entanglement didn't always mean secrecy, though? New work is shedding light on the nature of entanglement and quantum key distribution - and possibly proving that a high degree of entanglement does not necessarily lead to complete secrecy.

“Entanglement, or quantum correlation, is responsible for enabling quantum key distribution, a method to distribute a perfectly secret key that can be used to encrypt messages,” Matthias Christandl tells via email. “We have now discovered that not every type of entanglement is useful for quantum key distribution... [W]e showed that sometimes particles can be very strongly entangled, but at the same time be nearly useless for cryptography!”

Christandl is a scientist who recently moved from the University of Munich, Germany, to ETH Zurich, Switzerland. Christandl worked with Norbert Schuch at the Max Planck Institute for in Garching, Germany, and with Andreas Winter at the University of Bristol in the U.K. and the National University of Singapore to illustrate an example of a that is entangled, but that does not offer the secrecy needed for . Their work appears in : “Highly Entangled States with Almost No Secrecy.”

So far, the findings are only a theoretical observation. “We studied the possibilities that the world of single atoms and weak bursts of light offers when security is concerned,” explains Christandl. The two particles that are used in this theoretical exploration are in what Christandl calls an “antisymmetric state.”

Observing this in the laboratory may not be something that occurs anytime really soon, though. “This state may be difficult to create in the laboratory,” says Christandl, “as one needs a high number of maximally entangled particles in order to create the state. ...However, the insights gained in this work might offer a route to new and better protocols for quantum cryptography.”

Christandl and his colleagues are looking at ways to better understand how secrecy works in quantum physics. The idea that, theoretically, even high degrees of may not lead to complete secrecy, could change the way that scientists view quantum cryptography. It could also encourage the study of different quantum states to determine which are better for , and which should be avoided for such purposes.

While there are no plans to set up an experiment to test this idea, Christandl thinks that the theoretical findings can help advance the study of quantum physics. “A better understanding might lead to novel quantum protocols and quantum technologies in the future.”

The next step for Christandl and his peers is to look for entanglement that offers no secrecy. “This would push our findings to the extreme,” he says, “offering an analog of the famous ‘bound entanglement.’ Such a finding would conclusively revise the view that entanglement and secrecy go hand in hand.”

Explore further

Too much entanglement can render quantum computers useless

More information: Matthias Christandl, Norbert Schuch, and Andreas Winter, “Highly Entangled States with Almost No Secrecy,” Physical Review Letters (2010). Available online:

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Jul 05, 2010


How about some explanation of the actual science. This article was nearly content free.

Jul 05, 2010
"We have now discovered that not every type of entanglement is useful for quantum key distribution"

Just like not every number is useful for regular cryptography.

Jul 05, 2010
All that they need now is some mathematician to come along and show that all types of quantum entanglements are really the same type expressed differently.. would kill the idea of quantum computers much less cryptography. question does this violate the principal of Schrodinger's cat?

Jul 06, 2010
Schrodinger's cat is an "excited state" : An alive cat with poison. Poison becomes active, the state decays to become a dead cat in the box. If not, the cat is still alive.

Electron "spin": Electron has a permanent magnetic moment (MM): When applying a magnetic field, the electron's energy will increase: But it can lose this increase by aligning MM along, or opposite, the applied magnetic field. One can derive that this will be the case when an electron is a Gaussian wave with a single degree of freedom. Thus, the probability that the electron will be found with spin up or spin down is the normal probability when spinning a penny: It has a cause.

So PLEASE move back through the mirror: Alice is probably sick and tired of serving tea to theoretical physicsts.

Entangling happens when two waves merge like two drops of water. Two electrons form a holistic boson-wave with zero spin. When the latter disentangles the resultant e-spins MUST correlate. Causal physics again!

Jul 06, 2010
Not so easy. This and other such results hint that the idea (the fundament of quantum physic that is believed, but newer proved) that reading out some information from quantum state inevitably destroys this state and the rest of information in it, may be just plain wrong. This is bad news for key distribution, it mean the sent key can be read in-between even if it is quantum one.
But it is good news for communication as it men you can use just one entangled particle to create not a key but secret channel itself and use it continuously, most likely it mean also instant information transfer. Light speed may not be limit for information, it is anyway just an idea that it should be, no real arguments for pro or cons until now. Can also explain who SETI do not hear anything, this is much more effective way for communication then radio and there is no way to listen in. Basically this may be a real wormhole that gets created when particles entangle, just this allow throw only informatin.

Jul 06, 2010

This has already been discussed by myself like a year ago when I described how to create an inter-stellar and inter-galactic network for Type 2, type 3, and type 4 civilizations.

When a colony ship leaves earth, it has scores of transmitter/reciever pairs which are entangled with a hub on the earth. In addition. the ship itself has lots of hubs and backups which are entangled with backups on earth, as well as entangled with hubs on other colony ships.

in theory, entanglement allows this network to perform instantaneous communication anywhere in the universe with no signal long as both the sender and receiver are entangled directly, or at least entangled with other receivers in the network.

Encryption is irrelevant because the signal does not pass through space at all. It simply jumps to a single specific target hub by design.

Jul 06, 2010
Some ideas just float around. Backups may be irrelevant if entangling really mean that waveforms become one. Then if you have one entangled particle you can entangle additional particles with it and they also become entangled with all partners the initial particle has. So that even if the initial one become lost you still had channel and you can repair and extend it continuously.

Jul 06, 2010
My synopsis: Yes.

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