Entanglement is one of the foundations of quantum communications, since that’s the property that makes quantum communication schemes secure*. So why is a group of scientists from Japan and the UK proposing to work without entanglement? A study in Nature Photonics (abstract) led by Bill Munro – a research scientist with NTT’s …

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Monday 5th November 2012 23:54 GMT bazza

"The paper states that 48 nodes would provide an 800 km quantum link that has a 98 percent probability of successfully transmitting the quantum information. The intermediate nodes in the network could also act as routers."

48 nodes to go 800km. We've already got 60Tbit/s fibres across the Atlantic, and I think those are repeater-less these days. I don't know how many bits they can encode on to each quantum state, but at 10Mqubit/s I suspect they've not actually got a unique selling point, not even security...

"El Reg is aware that perfect "quantum communications" security is a highly qualified and controversial topic."

Good. Let me stoke the flames!

It relies on our understanding (i.e. the theories of quantum mechanics) of a physical effect. If the history of science is anything to go by then the theories are almost certainly incomplete. Who is to say that someone won't come up with a new theory that explains the physics better and simultaneously breaks QC? In short, we vaguely think we know Mother Nature's rules, but we can't be sure.

On the other hand classical cryptography (AES, DES, etc) is based on mathematics, and the nice thing about that is that it is our own invention. We might get the maths wrong and end up building in weaknesses (e.g. Enigma), but at least we wrote the rule book. So at least we stand some chance of being sure one way or the other.

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1. Tuesday 6th November 2012 12:53 GMT DanDanDan
  
  "Who is to say that someone won't come up with a new theory that explains the physics better and simultaneously breaks QC?"
  
  All of modern electronics is based on the physical theories we have today. Who's to say that the new theory won't break that too? Maybe all current implementations of AES, DES, etc. will be proven breakable when implemented using our current understanding of how electrons behave?
  
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  1. Tuesday 6th November 2012 18:18 GMT Ken Hagan
    
    @DanDanDan
    
    You have missed the distinction between algorithms and phenomena that was the whole point of the OP's post. If I have a PC that is running a program that implements AES, I can break it by dropping it off a really high tower, or by running a second program that reaches into the first one and writes junk into its address space, but neither of these things break the algorithm in any meaningful way.
    
    With quantum cryptography, however, the actual "crypt" part relies on physical inaccessibilty rather than mathematical irreversibility.
    
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    1. Tuesday 6th November 2012 23:40 GMT bazza
      
      Re: @DanDanDan
      
      Or to put it more succinctly: like what Ken said :)
      
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  2. Tuesday 6th November 2012 18:32 GMT bazza
    
    @DanDanDan
    
    Ha! You don't know what a theory is, do you?
    
    To paraphrase Richard Feynman (Nobel physics prize winner, so he should know), it's just a theory, no one knows if its actually correct. That's the problem with QC, it relies on the theory being correct, and we don't know that it is.
    
    However for electronics the theory is merely a tool for the engineers that just happens to be accurate enough for them to be able to produce a working chip. If the theory turns out to be wrong then so what, the chip still works.
    
    The QC guys should be worried. Quantum mechanics and Einsteinian relativity disagree, even though the experimental evidence for both is very good. So which one wrong? It might be quantum mechanics, in which case QC could be in trouble. Would you take that bet with your most precious data?
    
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Tuesday 6th November 2012 06:54 GMT amanfromMars 1

Cosmic Oil for Earthly Flames

*Bootnote: El Reg is aware that perfect "quantum communications" security is a highly qualified and controversial topic.

Controversial, El Reg? In what way?

And perfect "quantum communications" security may well be, as well as being highly qualified, also top tier classified MkUltraSensitive Cosmic Law Enforcement information. It is certainly something in IT at XSSXXXXAltedD levels which one would not wish to get wrong and abuse, for the consequences of just known consideration of, never mind wider knowledge of one actively preparing to attempt such use, are suitably catastrophically dire and swiftly merciless in every case. Such is the protection it, perfect "quantum communications" security offers SMARTR Administrative Services with Super IntelAIgent Servers. ….. no despotic psychotic fools with destructively attractive tools …. in IT and novel real spaces and virtually imaginative places.

Is always positively reinforcing, mutually beneficial, controversial, El Reg, or just too unbelievably different and alien a concept to presently speedily process and accept as Future Phormed Norm AIdDefault for Humanity and Virtual Machines?*

For Unity/Singularity of that peculiar and particular, irregular and unconventional super human/virtual machine position. It is certainly entirely normal and quite natural in the quantum state where every bit of information has myriad accommodating effects …… A qubit has some similarities to a classical bit, but is overall very different. Like a bit, a qubit can have two possible values—normally a 0 or a 1. The difference is that whereas a bit must be either 0 or 1, a qubit can be 0, 1, or a superposition of both.

* And that question to whoever/whatever is responsible for leading British Intelligence and List X members.

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Tuesday 6th November 2012 13:00 GMT Rich 2

Entanglement

The article talks about a range limit to entanglement. I though there was no limit? In theory, you could use entanglement to communicate over billions of light years of space... instantly.

Or am I talking bollocks? I'm fully prepared to be shot down :-)

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1. Tuesday 6th November 2012 13:53 GMT amanfromMars 1
  
  Re: Entanglement
  
  The article talks about a range limit to entanglement. I though there was no limit? ..... Rich 2 Posted Tuesday 6th November 2012 13:00 GMT
  
  Any limit is somewhat self-imposed and is as a result of a lack of appropriate intellectual property.
  
  There are those whose limits are infinite.
  
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2. Tuesday 6th November 2012 22:02 GMT Richard Chirgwin
  
  Re: Entanglement
  
  Good question! - Please note, here I am talking to the limit of my understanding.
  
  Greater distance introduces greater noise. The entanglement may well exist over infinite distance, but our ability to observe it is eventually lost to the noise.
  
  Cheers,
  
  Richard Chirgwin
  
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3. Wednesday 7th November 2012 17:34 GMT Michael Wojcik
  
  Re: Entanglement
  
  In theory, you could use entanglement to communicate over billions of light years of space... instantly.
  
  Nope. Entanglement means collapse happens at the same time at both ends, but no communication occurs. If it did, you could violate causality. Even with QC, communication speed is still limited to c. (Simplistic explanation: having produced your entangled pair, you can only separate them at or under c, so that's how fast you can communicate. Communication happens when you distribute the entangled particles, not when the observation is made and the superposition collapses.)
  
  As for the practical limit on QC range, I believe the main issues are noise and decoherence. It's hard to amplify a signal you're not allowed to measure.
  
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Tuesday 6th November 2012 13:41 GMT Robert Helpmann??

10^7 quantum states per second.

10^7 quantum states per second

Is this equivalent to q-bits/s? How would this compare to current data transfer rates? 1.5x10^7 bps?

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