Game State
Quantum Chess uses all of the same pieces, in the same starting positions, as classical chess. Quantum Chess allows all pieces to exist in superposition, where they can occupy more than one square at a time. This is shown by a ring indicating the probability that the piece will be found in a given square by a measurement.
Movement
All movement in the game is done according to the rules of quantum mechanics. This means there is a lot of math under the hood, and sometimes the outcome of a move might be counterintuitive. The goal has been to design the game so that players don’t need to understand any of the math in order to learn how to play. We hope that through play, the counterintuitive will become intuitive.
Standard Move
Quantum Chess pieces follow all of the same movement patterns as those in standard chess, including moves like castling and en passant, with the following caveats:
- There is no concept of check, so the king can move into, or castle through, what would be considered check in classical chess.
- A square occupied by a piece in superposition does not block standard moves. This means that, for example, a knight can move to a square that is occupied by a piece of the same color in superposition, or a queen can slide “through” a piece in superposition.
Split Move
All non-pawns can perform a split move. This move allows the piece to split and occupy two different target squares that it can reach. Only squares that are unoccupied, or are occupied by a piece of the same color and type, are considered legal targets. This means a white knight may split to a square occupied by another white knight. It also means a split move cannot be used to capture opponent pieces.
Merge Move
All non-pawns can perform a merge move. This move is designed to undo a split move. Only squares that are unoccupied, or are occupied by a piece of the same color and type, are considered legal targets. This means two white knights may merge to a square occupied by another white knight. It also means a merge move cannot be used to capture opponent pieces.
Measurement
Any time a move targets a square that is occupied by a piece of a different type and color, the game will perform some measurement. This means it will look at certain squares on the board to determine if they are occupied or empty. This could cause some of the superposition to “collapse” so that pieces are no longer in superposition. Measurements are designed to answer a specific yes or no question. If the answer is yes, the move is then performed. If the answer is no, no further action is taken, and play passes to the next player. Note that in this case, the state of the board will still be changed through superposition collapse. There are two different measurement scenarios to consider.
Capture
During an attempt to capture, the game will measure the square occupied by the piece that is being moved and, for sliding pieces, any square in the path between source and target that is occupied by a piece in superposition. The measurement answers the question:
“Is the attacking piece present and can it reach the target?”
Note, if the answer to that question is no, it doesn’t necessarily mean that the attacking piece is not present. Nor does it mean the path is blocked. It could be no for either reason, so you may see the probabilities change to reflect that both are more likely.
Exclusion
If a move target is occupied, in superposition, by a piece that cannot be captured by the move being performed, we call it an exclusion move. In this case the game will measure the target square and, for sliding pieces, any square in the path between source and target that is occupied by a piece in superposition. The measurement answers the question:
“Is the target square empty and can the moving piece reach the target?”
Note, if the answer to that question is no, it doesn’t necessarily mean that the target square is occupied. Nor does it mean the path is blocked. It could be no for either reason, so you may see the probabilities change to reflect that both are more likely.
Castling
A special note for castling. Castling always involves 2 targets, and so a measurement will occur for both targets. Castling cannot be used to capture, so it will always be an exclusion measurement.
Winning
The game is won by capturing your opponent’s king. This means your opponent has no king left anywhere on the board. It is possible for any piece, king included, to exist in a superposition of being captured and not captured. The game will continue until you can say, with certainty, that one player has no king.
Note, it is possible to find the game in a state where both players lose their king on the same turn. In this case the game is considered a draw.