Flux capacitor

A flux capacitor is a theoretical design of a passive, on-chip microwave circulator based on a ring of superconducting tunnel junctions. However, the term "flux capacitor" is better known as the fictional component of the Delorean responsible for time travel in the 1985 film Back to the Future.

The theoretical design of a flux capacitor was proposed in a 2018 paper titled "Passive On-Chip Superconducting Circulator Using a Ring of Tunnel Junctions" by researchers at two Australian Research Council Centres of Excellence, the Centre for Engineered Quantum Systems (EQUS) and the Centre for Future Low-Energy Electronics Technologies (FLEET) and collaborators at the Institute for Theoretical Physics, ETH Zürich. The electronic device can control the directional movement of microwave signals using quantum tunneling of magnetic flux around a capacitor, breaking time-reversal symmetry, meaning signals circulate around the circuit in a single direction. The device does not facilitate time travel, but it does demonstrate the potential for high isolation, robustness against fabrication imperfections and bias fluctuations, and a bandwidth in excess of 500 MHz for realistic device parameters.

The device could have applications isolating parts of apparatus from each other important for sensitive quantum systems. The researchers behind the paper stated in the long term, the device could become important for quantum computing, or in the shorter term, it could help develop better electronics for mobile phones and wifi antennas as well as improve radar.

The paper investigates two physical realizations of flux capacitor designs based on Josephson junctions (JJs) or quantum phase slip elements (QPS), with microwave ports coupled either capacitively (JJ) or inductively (QPS) to the ring structure. One of the designs resembles the three-pointed star design of the flux capacitor from Back to the Future.

The Back to the Future film series references the flux capacitor as a device responsible for time travel in the Delorean time machine and the Jules Verne Time Train from Back to the Future Part III. In the Back to the Future universe, Dr. Emmet Brown came up with the idea for the flux capacitor on November 5, 1955, after slipping and bumping his head while standing on his toilet to hang a clock. After being knocked out, the idea came to him in a vision, and he drew the schematic of an inverted Y-shape with wires along with calculations. The flux capacitor consisted of a box with three small, flashing incandescent lamps arranged in a Y-shape above and behind the passenger seat of the Delorean time machine.

An image of the flux capacitor in the 1985 film Back to the Future.

As the Delorean nears 88 miles per hour, the light of the flux capacitor pulses faster until it produces a steady stream of light that users are not supposed to look at. The coils seen across the front and along the rear sides are referenced as the temporal demodulation coils.