Ringworld is property of Larry Niven. All characters from the Buffyverse are Property of Joss Whedon. Everything else belongs to me.
It was first discovered in September 2009 on Earth.
Quantum Entanglement had been a mystery for over sixty years in the field of particle physics. If a closed system of particles shared a net characteristic then that net characteristic was preserved no matter the distance or time between the particles. It was as if the particles were eternally connected through a higher special dimension. Changing the state of one particle, say collapsing a waveform, would instantaneously change the value of the other particle even if that particle was moving away at the speed of light.
Quantum computers were based on entanglement. They could in theory process an infinite amount of data in zero time.
In September of 2009 on Earth, scientist at the University of California observed Quantum Entanglement as a naked eye event. They observed the instantaneous change in state between two superconductors separated by space long after an initial microwave pulse had been sent only to one of the superconductors.
The Ringworld had enough superconductor mesh to cover six times ten to the fourteen power square miles or three million times the surface area of the Earth. It was the most powerful computer in the Galaxy. It could store and process the molecular movement of every Human on Earth in a fraction of a second.
In the hands of the Buffy-Protector it found other uses.
Dawn stood on the dais of the Great Machine. The Great Machine was a device build by the Buffy-Protector to pull herself and Dawn through both space and time. The Machine occupied over three thousand square miles and used over several million terawatts of energy. The only reason Dawn consented to being placed in such a machine was that she knew it would work.
She remembered it working when she and Buffy were originally extracted from Earth.
"This sucks!" Dawn muttered. She knew what the Buffy-Protector had to do.
Over the years the Buffy-Protector had traded with travelers from Earth. Centuries ago Humans had invented booster spice which extended human life span. Dawn looked thirty but she was well over two hundred years old.
What the Buffy-Protector was going to do was to use a tiny portion of Dawn's keyness to open up a window into the infinite dimensionality of space time. The Ringworld would serve as a quantum computer sorting the data looking for something that matched the Key frequency exactly.
The Ringworld was capable of processing the dimensional frequency readings from all of the dimensions in existence simultaneously.
Once located, The Buffy-Protector would yank on the quantum threads connecting the now-Dawn to the then-Dawn dropping them at an earlier point in space time.
"Let's do this." Dawn said.
"Already working." The Buffy-Protector replied.
Dawn looked down, and from her navel, a soft green glow began to fill the room. Dawn felt no pain. In fact, it tickled.
"Got it!" The Buffy-Protector said. "Earth, Sunnydale, May 20th 2003." She looked at Dawn wondering if she would remember the significance of that date.
"The day we closed the Hellmouth!" Dawn had remembered. They had both agreed not to extract the earlier Buffy until they knew the world would be safe. The new Slayers would take care of that.
The Buffy-Protector's finger hesitated over the start button for a fraction of a second. She knew this was it. After pushing that button there was no way back. Her arrival to the ringworld would close the temporal loop sealing her fate. Once on the ringworld the earlier Buffy would eventually build a wormhole generator that she would use to call herself to the Ringworld. Along the way she would find a way to contact the Puppeteers and have them save the Ringworld and not incidentally billions of Dawns descendants.
It's just that, she didn't want to be here! It wasn't fair! She was chosen to be a slayer at sixteen without her permission by the powers- that- be and now she was chosen to protect trillions of people she had never met because of the Monks. She hated it. She resented it.
She did her job.
The Buffy-Protector pressed the "start" button and opened the door to herself.
Directly above the Map-Of Mars a single stealth ship appeared. From its nose it fired a quantum singularity directly into the control room where Dawn and the Buffy-Protector worked. A fraction of a second later it tore into the workroom killing Dawn instantly. Moments later the singularity had torn free of the Ringworld and was heading out into deep space.
They were on the last bus out of Sunnydale. The First was toast, Anya was dead, Spike was dead again and the Hellmouth was closed.
Buffy walked up behind Willow and tapped her on the shoulder. As Willow turned, Buffy softly hugged her and said:
"It's time."
Willow had been dreading this moment. Two years ago Buffy had come under a terrible compulsion to go to the alien place known as the Ringworld. They had managed to hack into the SLAC national accelerator laboratory operated by Stanford and modulate the particle accelerator in such a way as to produce packets of dark matter with the embedded message: "Save the Ringworld." The message was complete with directions to find the Ringworld. Shortly afterwards, they modified the Tabula Rasa spell to make Buffy forget about the Ringworld until she knew beyond a shadow of a doubt that she could leave the Earth safely behind.
That moment was now. Buffy could feel her memories of the Ringworld returning. Buffy held the soft hug a few moments longer then stood and grasped Dawn's hand.
"Something's coming…" was all Buffy managed to say.
Willow watched as the view behind Buffy began to melt into what appeared to be a laboratory. The original background of the bus snapped back into existence and then something weird happened.
Both Buffy and Dawn split in two. The originals remained on the bus, the dim copies merged with the laboratory view. Things returned to normal. Willow was awed.
"That was it?" She said. "Looks like someone just made a copy of you."
"I hope they got what they needed." Buffy said wishfully.
She had no time for shock.
The Buffy-Protector ran to the tracking station and located the quantum black hole that had just wrecked her laboratory. The automatics sealed the lab against air loss as she brought the Ringworld QE generator online. QE was short for quantum entanglement.
Quantum black holes were notoriously unstable. Their event horizon, the place where escape velocity exceeded the speed of light, was so sharp they were able to cut the bonds that held virtual partials together as a result they grew hotter.
Quantum black holes were warm and if you fed them enough they exploded. It was called Hawking radiation.
The black hole was still embedded in the solar wind that blew outward from the Ringworld star. She hoped it would be enough. The Buffy-Protector aimed carefully. Every particle of matter near the singularity suddenly turned into a virtual particle pair.
Three minutes later the black hole exploded in a Gamma-ray burst returning mass, energy and information back to the universe. The information released back into the universe was so encrypted that it bared no relationship to the information it consumed.
It took the ringworld almost three hundred milliseconds to reconstruct Dawn.
The Buffy-Protector turned to Dawn who was sitting on the floor of the laboratory grinning from ear to ear. Smoke was rolling off of her body.
"That was fun!" She said.
The Ringworld had created a perfect quantum copy.
Hey, I don't make this stuff up:
.com/wiredscience/2009/09/quantum-entanglement/