Recently I’ve been thinking about both quantum mechanics and chess, and came up with a chess variant that blends the two. The difference between ordinary chess and quantum chess is the ability to put your pieces in superpositions. Here’s how it works!

Movement

There are five modes of movement you can choose between in quantum chess: Move, Split, Merge, Collapse, and Branchmove.

Modes 1 and 2 are for non-superposed pieces, and modes 3, 4, and 5 are for superposed pieces.

Mode 1: Move

This mode allows you to move just as you would in an ordinary chess game.

Mode 2: Split

In the split mode, you can choose a non-superposed piece and split it between two positions. You can’t choose a position that is occupied, even by a superposed piece, meaning that splitting moves can never be attacks.

One powerful strategy is to castle into a superposition, bringing out both rooks and forcing your opponent to gamble on which side of the board to stage an attack on.

Mode 3: Merge

In mode 3, you can merge the two branches of one of your superposed pieces, recombining them onto a square that’s accessible from both branches.

You can’t merge to a position that’s only accessible from one of the two branches, and you can’t merge onto a square that’s occupied by one of your own superposed pieces, but merge moves can be attacks.

Mode 4: Collapse

Mode 4 is the riskiest mode. In this mode, you choose one of your superposed pieces and collapse it. There are two possible outcomes: First, it might collapse to the position you clicked on. In this case, you now have a choice to either perform an ordinary move…

… or to split it into a new superposition.

But if you get unlucky, then it will collapse to the position you didn’t select. In this case, your turn is over and it goes to your opponent.

Mode 5: Branch Move

Finally, in a branch move, you relocate just one branch of the superposition, without collapsing the wave-function or affecting the other branch.

Piece Interactions

Attacking a Superposed Piece

What happens if you attack a superposed piece? The result is that the superposed piece collapses. If the piece collapses onto the square you attacked, then you capture it.

But if it collapses onto the other branch of the superposition, then it is safe, and your piece moves harmlessly into the square you just attacked.

This means that attacking a superposed piece is risky! It’s possible for your attack to backfire, resulting in the attacker being captured next turn by the same piece it attacked.

It’s also possible for a pawn to move diagonally without taking a piece, in a failed attack.

Line of Sight

Superposed pieces block the lines of sight of both your pieces and your opponent’s pieces. This allows you to defend your pieces or block open files, without fully committing to defensive positions.

Winning the Game

To win quantum chess, you must actually take the opponent’s king. Let’s see why it’s not enough to just get the opponent into a position that would ordinarily be checkmate:

It’s blue’s turn now, and things look pretty lost. But look what blue can do:

Now the red queen has to choose one of the two targets to attack, and there’s a 50% chance that she gets it wrong, in which case the blue king can freely take the red queen, turning a sure loss into a draw!

So how can red get a guaranteed win? It takes patience. Rather than trying to attack one of the two squares, the red queen can hang back and wait for a turn.

Now the blue king has two choices: leave superposition, after which they can be taken wherever they go. Or move one branch of the superposition, but any possible branch move results in a safe shot at the king with the queen. This can be repeated until the king is taken.

And that’s quantum chess! I’ve played several games with friends, and each time have noticed interesting and surprising strategies arising. Let me leave you with a quantum chess puzzle. Here’s a position I found myself in, playing red:

What do you think was my best move here?