A week after Ålice’s and Bøb’s discussion, a mysterious parcel arrives in the far north of Norway. Around the same time in Finland, Carolinaa is already unpacking a similar package from Ålice. They have not dared to talk much to one another other since last year and after reading the first lines of the note Ålice enclosed, her face lights up and Carolinaa starts to follow the instructions at once. A bit later, everything is set up and Caro exchanges her first quantum secure messages with Ålice.
Caro: “Hey Ålice, thanks for taking care of our communication problem! I read your note, but I still do not fully understand how it works. Can you explain it again?”
Ålice: “Hi Caro! Last week I tested our new entanglement source with Bøb, but we soon realised, that we all have to talk to one another, not just the two of us. I came up with this idea: We use a second PolyBoard, which enables us to share pairs of entangled photons between any two of us. For that, we need different colours of light and each colour represents one connection. I borrowed the idea from a similar technique used in classical communication, but made it work for entangled photons as well! I call this a PolyBoard qROADM.”
Caro: “Fabulous! This year we will not have such a mess. So, not even Dag or Bøb can read this?”
Ålice: “Correct, no one except us.”
Caro: “Uh, that is cool. Let me try it out with Dag.”
Caro adjusts her qROADM and can now talk with Dag in private.
Dag: “Hello Carolinaa! Yes, I am also quite impressed with Ålice’s solution. I wonder though, if she really cannot follow what we are talking about.”
Caro: “No no, I checked it. If the quantum states we share are maximally entangled, there is no way, she could eavesdrop on us! Anyway, you remember I have been the one to deliver all the gifts last year AND the year before? It’s not fair and – just between us – I think Ålice has been fiddling with the machine, so she is chosen less often than the rest of us.”
Dag: “Well, you might be right, but can you prove it? Or do you even have an alternative?”
Caro: “Maybe I do have an idea…”
To be continued…
Quantum reconfigurable optical add-drop multiplexer (qROADM) for the distribution of private keys
across more than two participants. The PolyBoard is based on arrayed waveguides gratings
with a waveguide routing and an optical switch in order to connect Ålice, Bøb, Carolina and Dag.