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I am doing some research for an undergrad project and have hit a conceptual stumbling block. I have recently been looking at some of the achievements in practical QKD using trusted node quantum networks. The achieved secret key rates are usually measured in kbps (SECOQC, DARPA, Tokyo QKD etc.)

I know that there is an upper-bound for the secret key rate based for any repeaterless QKD protocol, the PLOB limit. In the paper (Pirandola et al., 2017), they plot the secret key rate against distance. But here the secret key rate is measured in bits per channel use and it is several orders of magnitude lower than the secret key rate mentioned in the practical papers mentioned above.

Sadly, I don't have time to delve into this, but would like to understand how I can compare these numbers. In a practical QKD protocol between two parties, as far as I know, there is usually a single channel (a fibre cable perhaps). So, would I be right in thinking that, depending on the protocol, the channel will be used many times a second in order to share information, hence, the secret key rate (in kbps) when looked at per channel use would actually be a much lower value?

All I need is enough information, perhaps a paper which I could look at, which would help me understand the PLOB limits use of secret key rate (as measured in bits per channel use) versus the practical QKD usage which measures the secret key rate in kbps.

Thanks in advance to anyone who can help

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Indeed, the PLOB bound is an ultimate upper bound for repeaterless quantum communications, and is thus derived by averaging over $n\rightarrow \infty$ uses of the communications channel - Hence why the capacity is given as bits per channel use.

If you wish to more closely compare the PLOB bound to other well known protocols, look at Supplementary Note 6 in the original paper which provides key rates of these other well known CV and DV protocols. If you want a closer look at the derivation of the upper bound see this paper also .

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