Theorists Make Quantum Communications Breakthrough

KentuckyFC writes "One of the cornerstones of modern physics is Claude Shannon's theory of communication, which he published in 1948. If you've ever made a phone call, watched TV, or used a computer, you've got Shannon to thank for describing how information can be moved from one place in the universe to another using an idea called the channel capacity. But nobody has been able to develop a quantum version of this theory. So physicists have no idea how much quantum information can be sent from one point to another. Now two American physicists have made an important breakthrough by proving that two quantum channels with zero capacity can carry information when used together. That's interesting because it indicates that physicists may have been barking up the wrong tree with this problem: it implies that the quantum capacity of a channel does not uniquely specify its ability for transmitting quantum information (abstract). And that could be the idea that breaks the logjam in this area."

Comments

Re:quantum mechanics

[Colonel Korn]

Conversely if two physicists walk into a bar, how many patrons have lives?

Answer: The same number as there were before they entered.

In my experience physicists are generally rather cool, worldly people who have well developed personal lives.

Re:Non-peer reviewed

[Anonymous Coward]

Most submissions to ArXiv do get submitted to peer-reviewed journals; this one claims to have been submitted in June (although they don't specify where). It's an opportunity for researchers to share their work without the delay of waiting for publication. Usually, papers there do get revised after going through the referee process.

Re:I think you've got it

[The Iso]

In Khashishi's description, the time sequence in the construction of the two random sequences makes one of them seem a priori random, and the other to be a one-time pad encoding of the message, while in the Smith/Yard article you can't tell which is which.

One-time pad ciphertext does appear to be random. Shannon proved that it has perfect secrecy.

Re:Zero plus Zero equals One for large values of Z

[Pinky's Brain]

Not a S/N ratio of zero, their definition of channel capacity is only very tenuously connected to Shannon's channel capacity really. Quantum channels already have 0 capacity at non zero fidelity (the quantum equivalent to S/N). The 0 capacity channel from this paper aren't 0 capacity because of their fidelity though, the channels are 0 capacity for different reasons.

So it's not really directly applicable, "just" interesting math.

Re:I think you've got it

[SpazmodeusG]

The data looking random or not has nothing to do with the information capacity of the channel.
Shannons definition of information capacity is simply the maximum amount of information that can be recovered by the receiver on the channel.
A channel where the receiver can't recover data under any circumstances is a 0 capacity channel, that reason could be interfering noise or the fact that the channel doesn't exist.
Which poses a problem with this theory, it basically says 2 channels that don't exist can transmit information, which is intuitively incorrect.

Re:Original article here:

[oldhack]

So it's like this?

a = -i
b = i

real(a) = 0
real(b) = 0
real(a * b) = 1

where i = imaginary number, guess it may represent the hidden (from us) quantum dimension/domain, like wave function.

Re:So 0+0=1!

[mad_robot]

1=0+0!

There, fixed that for you.