Most crosswords have a single correct solution. A quantum crossword has several.
As a teenager I used to solve crosswords with my grandmother. She was very canny to the tricks and games that cruciverbalists (crossword puzzle creators) use. To the best of my recollection she never encountered a quantum crossword, but I know that she would’ve gotten a kick out of it.
The name “quantum crossword” alludes to quantum physics. Specifically, it refers to the idea that something can exist in two different states simultaneously – like the famous Schrödinger’s cat. I’m skipping over some rather dense mathematics of quantum superpositions here, for two reasons: 1. I understand the details poorly. 2. This is supposed to be about crosswords!
Traditional crosswords have a set of clues and a grid to fill. There’s one correct answer to each clue, and so just one way to fill in the grid. Some crosswords, though, violate that rule. Consider the following clue: “Black Halloween animal (3 letters).” The answer could equally be “BAT” or “CAT.” And in one very famous crossword, the answer was both.
November 5, 1996. The United States presidential election saw Bill Clinton facing off against Bob Dole for leadership of the country. The New York Times crossword puzzle included one seemingly ridiculous clue: “The headline of tomorrow’s newspaper (!)” The answer to that clue accurately predicted the winner of the election… before that election had even been decided.
How did the puzzle creator (Jeremiah Farrell) achieve this absurd feat of cruciverbal augury? Well, he carefully crafted the clues so that no matter who won, the clue would predict the election correctly. The answer would either be “CLINTON ELECTED” or “BOBDOLE ELECTED.” Each clue intersecting with that answer made sense with either possibility. So the down clue for the first letter was “black Halloween animal.” If you answered “BAT” then that “B” filled in the first letter of “BOBDOLE.” If you answered “CAT” then the “C” became the first letter of “CLINTON.”
Other ambiguous clues: “French 101 word” (“OUI” or “LUI”); “trumpet” (“BOAST” or “BLAST”), and my personal favourite, “provider of support, for short” (“IRA” or “BRA”). Both candidates’ names fit perfectly in the same grid. Beautiful. You can see the full puzzle in the third link below.