The Cosmic Bridge

Mar 2024

Sergio Juárez

In a time where technology and nature are becoming more intertwined as science progresses, live Alice, a brilliant physicist with a curiosity as vast as the universe, and Bobbix, a post-quantum cryptographer with a fervent belief in privacy as a human right. Together, they are designing a communication machine so secure that not even the cleverest and most powerful eavesdropper could intercept their messages. Alice’s dream was to make Nature the guardian of their secrets, utilising the wonderful rules of quantum mechanics.

After months of tinkering and experimenting in Alice’s cosy (but a bit chaotic) lab, filled with quantum physics books, scribbled papers, and a couple of burnt lasers, Alice and Bobbix succeeded in creating a quantum communication system. This system relied on encoding messages into the quantum states of light, utilising its various degrees of freedom (like polarisation, phase, and frequency) a quantum alphabet in which they could encode. They decided to call it the BB84, and now it was time for a real-world experiment!

However, as it often happens in science, they encountered a problem as soon as they moved out of the lab. They noticed that the distance between their labs was bigger than they could handle; their newest invention was too weak, and trying to communicate across a bustling city didn’t help either. Buildings, electronic noise, and the sheer distance caused significant photon loss, making their quantum communication nearly impossible. The very fabric of their city seemed to conspire against them, swallowing their quantum messages into an entropic void where not even the laws of physics could help them much.

Determined not to let their dream vanish, Alice delved deeper into the physics behind quantum communication, where she discovered what she would call the Twin-Field protocol (or in cool science jargon, TF-QKD). “Bobbix, this could be our answer!” Alice exclaimed, her eyes shining with excitement. “Instead of sending our quantum keys directly to each other, risking them getting lost or degraded, we use a third location as a quantum relay.”

Bobbix leaned in, intrigued by the simplicity and elegance of the idea, and checked Alice’s schematics. “Okay, I get this part. We both send a weak laser signal to this midway station, where we don’t measure the signals directly but interfere with them. But wouldn’t interfering with them destroy them? And what is this Twins part?” At this moment, Alice’s eyes started shining with excitement; this was exactly the exciting point of her new idea. “If we manage to send the same signal, they will interfere constructively, and we know we had a correlation, and even better, nobody will ever know, except for us, what we sent! This is how we get our encoding key!” Bobbix could not believe it. He blurted out, “Unbelievable, the third location acts like a cosmic bridge. The city itself is going to help us!”

With renewed spirits, Alice and Bobbix set to work, upgrading their system to incorporate Twin-Field QKD. They chose the abandoned Charly Observatory as their interference station. Only stargazing young adults would gather there; no one would mind them doing weird stuff. Night after night, they calibrated their lasers and fine-tuned their superconducting detectors until finally, it was time for another test.

Bobbix was looking in the direction of Alice’s lab when his phone chimed. He read to himself in a whisper, “Nature is now our secret keeper,” with a wide grin spreading across his face.