Classics gone quantum: Back to the future

Challenge: After finding himself in 1955, Marty McFly must return to 1985 while causing as little disruption to history as possible.

Life on Earth unfolded normally from 1955 to 1985...The first box in the algorithm (Uf) represents a transformation equivalent to all the logical gates required to model the world’s evolution during this period. The movie “Back to the Future” begins in 1985. That’s why the DeLorean’s flames appear right after the first box. The movie begins when Marty McFly accidentally finds himself transported back to 1955. He must find a way back to 1985 while causing minimal disruption. To travel back in time in a quantum way, we use gates that invert those applied in the quantum algorithm. To go back in time —to 1955— we apply the opposite transformation (marked by the dagger). The box with the guitar represents all the disruptions Marty McFly causes during his visit to 1955. Finally, the box representing the evolution of the world (Uf) from 1955 to 1985 is applied one last time, bringing Marty back to 1985.