1. Cover the LED so it cannot be hit by light 2. Charge it in reverse bias to write a 1 or do nothing to write a 0
inverting the polarity and reading the + you detect a voltage for up to 3-5 seconds depending on the diode if a 1 was written in earlier :)
https://en.wikipedia.org/wiki/Williams_tube
> The Williams tube works by displaying a grid of dots on a cathode ray tube (CRT). Due to the way CRTs work, this creates a small charge of static electricity over each dot. The charge at the location of each of the dots is read by a thin metal sheet just in front of the display. Since the display faded over time, it was periodically refreshed. It cycles faster than earlier acoustic delay line memory, at the speed of the electrons inside the vacuum tube, rather than at the speed of sound. However, the system was adversely affected by any nearby electrical fields, and required constant alignment to keep operational. Williams–Kilburn tubes were used primarily on high-speed computer designs.