Scientists provide a new angle on quantum cryptography

Aug 06, 2010

An ultra-secure form of coded communication could be given a boost, thanks to scientists from the Universities of Glasgow, Strathclyde and Rochester.

In a paper published in the journal Science, quantum physicists have demonstrated the randomness of also applies to the measurement of a particle’s angle and rotation.

The discovery creates new opportunities for increasing bandwidth in quantum cryptography - an ultra-secure way of communicating secretly which can intrinsically detect third-party interception.

Quantum cryptography relies on the strange rules of quantum mechanics, as exhibited by ‘’ where the condition of one is instantaneously reflected in the other even if they are separated by a great distance - a phenomenon called ‘nonlocality’. Thus, by measuring one particle, you can know the condition of the other.

can occur, for example, when a light particle - called a photon - is split into two separate but related which shoot off in opposite directions. The apparent instantaneous communication between the two violates Einstein’s law of relativity which says that nothing can travel faster than the speed of light.

Einstein called this ‘spooky action at a distance’ and thought that this showed quantum mechanics theory was incomplete. He believed some yet-to-be-discovered local ‘hidden variables’ accounted for this seemingly paradoxical behaviour, but nonlocality has been shown to exist.

The outcome of a measurement on either particle seems random, but the measurements of both always agree with each other and it is this agreement that can be used to transmit a secret message - making quantum cryptography the only guaranteed form of secure communication.

Einstein did not like this randomness and famously said, ‘God does not play dice with nature’.

However, modern physics disagrees and now the research by the Glasgow scientists and their colleagues shows that the randomness of quantum mechanics also applies to the angles.

Professor Miles Padgett, Professor of Physics at the University of Glasgow, pictured above, said: “God does play dice. It has been difficult to show this is the case with angle because angles are problematic: no-one can agree on how to measure them. If I say an angle is 10 degrees, you might say its 350 degrees. It depends on your starting point.

“The fact that an angle can take many different values means that each particle of light, a photon, can be encoded to carry the whole alphabet, potentially increasing the data rate of - the only guaranteed form of secure communication.

“It is guaranteed because you can detect when someone is attempting to read the message because the very act of an unauthorised measurement of an entangled photon destroys the message.”

Explore further: Physicists provide new insights into the world of quantum materials

More information: The research, entitled 'Quantum Correlations in optical angle-orbital angular momentum variables' is published in the latest edition of Science.

Provided by University of Glasgow

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Bob_B
not rated yet Aug 06, 2010
The article says: "unauthorised measurement of an entangled photon destroys the message", does this imply the "unauthorised measurement" was able to be read? And when the message has another measurement applied, it is not readable because it has been destroyed?
PhilosophersSage
not rated yet Aug 06, 2010
Wow the author can quote Wikipedia! No new information except one line "The discovery creates new opportunities for increasing bandwidth in quantum cryptography" How about useful information about how they increased the bandwidth.

And Bob B check this out for a simple understanding.
http://en.wikiped...tography
gwrede
1 / 5 (1) Aug 06, 2010
Einstein did not like this randomness and famously said, ‘God does not play dice with nature’.

However, modern physics disagrees and now the research by the Glasgow scientists and their colleagues shows that the randomness of quantum mechanics also applies to the angles.

Professor Miles Padgett, Professor of Physics at the University of Glasgow, pictured above, said: “God does play dice."
I'm not comfortable with this idea.

Everyone knows that nonlocality implies information transport faster than light, and, that the only other alternative is hidden state.

What I really have a hard time understanding is, why doesn't the Scientific Community take the hidden state alternative as the default, and the non-locality-thing as the contender?

It would be less arbitrary, less "science-as-a-religion", and less hard to understand. (Please, "mr Occam", help me out here!)

Would it be too embarrassing to acknowledge that we JUST MAY not have recognized ALL the possible photon states yet?
eachus
1 / 5 (1) Aug 06, 2010
What I really have a hard time understanding is, why doesn't the Scientific Community take the hidden state alternative as the default, and the non-locality-thing as the contender?

Because hidden variable theories have this nasty property. They don't agree with the results of experiments. In fact, there are results which falsify the entire class of hidden variable theories, so we don't have to worry about someone coming up with a clever new HV theory that works.

When I was in grad school, the 'joke' was causality, relativity, quantum mechanics. Pick any two.

Today most physicists are willing to accept a version of relativity which includes causal loops and is still consistant. To vastly simplify, imagine we have a path near a black hole where a photon turns into an electron and positron, then back into a photon. And when you look at the geometry of the warped space, the 'new' photon has the energy, momentum and location of the original.
eachus
1 / 5 (1) Aug 06, 2010
You might think you can reach in and block the light without getting your arm ripped off. (Very extreme gravity gradients) But that doesn't cause any problem with the theory. You changed the conditions and got different results. The results sound more spectacular when applied to someone travelling backwards in time, but as long as you don't cheat, you can have a causal loop consistant with current theory. It is just that some effects have causes which are in their future light cone.

Incidentally a lot of current work in this area is looking at closed timelike loops (CTLs). If you do get to the point where interesting physics occurs, CTLs can act as cosmic erasers. If you have to cross a CTL to go back in time, it can contain any number of photons at any energy. Even though the actual values won't be infinite, a CTL is likely to vaporize any matter crossing its path. So the trick now is to design time machines without CTLs in the wrong place.