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From: Francois Grieu on 18 May 2010 19:30
On 18/05/2010 16:23, Mike Amling wrote:
> There is much I have never seen explained about quantum crypto. E.g. if
> the system involves Alice sending single photons to Bob, how does Alice
> know when her device has emitted a photon?

As far as I understand, the archetipal method is that when Alice wants a photon, she fires a short (that's an understatement) laser pulse, and atenuates the signal, in a way such that often a single photon survives the process. Sometime that's none, sometime that's more than one.

> Photon emission is
> probabilistic AFAIK, not like pulling a trigger. And how does she know
> her device has not emitted two photons, one of which could be
> intercepted without her or Bob realizing it?

She does not, and my understanding is that some early security proofs do no apply in reality for that reason.


Francois Grieu
From: Mok-Kong Shen on 19 May 2010 03:24
Francois Grieu wrote:

> the feasibility of an attack on a quantum key distribution system used
> in a commercial quantum crypto product has been demonstrated
> experimentally. Or something on that tune.

There is BTW a section "Hacking Attacks" on such issue in
http://en.wikipedia.org/wiki/Quantum_cryptography

M. K. Shen
From: unruh on 19 May 2010 03:29
On 2010-05-18, Maaartin <grajcar1(a)seznam.cz> wrote:
> On May 18, 9:31?pm, unruh <un...(a)wormhole.physics.ubc.ca> wrote:
>> On 2010-05-18, Maaartin <grajc...(a)seznam.cz> wrote:
>>
>> > Isn't the whole quantum cryptography simply too impractical, at least
>> > at the moment? It gives us some proven security based on physical
>> > theories which are believed to be right. Conventional cryptography
>> > gives us some proven security based on cryptographic theories which
>>
>> No, it does NOT give us proven security. That is its problem. It is
>> secure as long as certain things are believed to be too hard to do, but
>> it is known that all of the used cyphers are "weak" and not secure
>> against an opponent with arbitrary resources.
>
> Sure, but there's provably no opponent with arbitrary resources in a
> limited universe. We may assume opponents with extreme computing
> power, but it's possible to find an upper limit and IMHO it's possible
> to make the problem hard enough for anybody given the state of the art
> of attacks. I agree that it's impossible under the assumption of an
> opponent using yet unknown attacks.
>
>> Quantum crypto is secure
>> against an opponent with arbitrary resources.
>
> OK.
>
>> > Or am I talking nonsense? I do not argue against the research, I only
>> > think that the state of the art in quantum cryptography is not
>> > advanced enough.
>>
>> For what?
>
> For practical and economical use - I know it's been already used, but
> weren't there alternatives? Was it a pure commercial application or
> was it (partly) a scientific experiment? My knowledge here is very
> limited, so bear with me.

No, it is not developed enough for "economical " use. Ie, you cannot (
nore will you ever) use quantum crypto for anything. However the
military, or big banks might. the current situation is still
development.

From: unruh on 19 May 2010 03:32
On 2010-05-18, Francois Grieu <fgrieu(a)gmail.com> wrote:
> On 18/05/2010 16:23, Mike Amling wrote:
>> There is much I have never seen explained about quantum crypto. E.g. if
>> the system involves Alice sending single photons to Bob, how does Alice
>> know when her device has emitted a photon?
>
> As far as I understand, the archetipal method is that when Alice wants a photon, she fires a short (that's an understatement) laser pulse, and atenuates the signal, in a way such that often a single photon survives the process. Sometime that's none, sometime that's more than one.
>
>> Photon emission is
>> probabilistic AFAIK, not like pulling a trigger. And how does she know
>> her device has not emitted two photons, one of which could be
>> intercepted without her or Bob realizing it?
>
> She does not, and my understanding is that some early security proofs do no apply in reality for that reason.

Sorry, but she can know, if that single photon is part of a two photon
process. She sends the one and detects the other and thus knows that
the first was sent.
But, for the usual process, she does not care if she knows it was sent.
Bob can tell here if he received one.


>
>
> Francois Grieu
From: Mok-Kong Shen on 19 May 2010 16:36
Francois Grieu wrote:
> According to this article
> http://www.technologyreview.com/blog/arxiv/25189/
> and online paper
> http://arxiv.org/abs/1005.2376
>
> the feasibility of an attack on a quantum key distribution system used
> in a commercial quantum crypto product has been demonstrated
> experimentally. Or something on that tune.

The following URL seems also to be relevant:

http://www.iet.ntnu.no/groups/optics/qcr/

M. K. Shen
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