Quantum Tic-Tac-Toe investigates the concept of quantum entanglement through a simple and fun game. It was created by Allan Goff around 2002. This is an original Flash version. See the rules below the game board. There is no computer player or multiplayer; you play both sides.
As in normal Tic-Tac-Toe, the game is played on a 3-by-3 board, and each of two players takes turns placing pieces, trying to get 3 in a row.
But in Quantum Tic-Tac-Toe, you place two "potential" moves at a time, in separate squares. Eventually, one of these will become a real (or classical) move, and the other will not. Potential moves are marked with the numbers of the turns they were played on. Each pair of potential moves is connected.
Only classical moves count toward a win.
The game continues with each player placing their two potential moves per turn, until a special condition comes about. Eventually, multiple pairs of connected, potential moves will form a closed circuit. This closed circuit represents only two possible sets of classical moves. Depending on which of the last pair of moves becomes "real," all the other squares that are involved in the circuit will necessarily go to player 1 or 2.
The player who closes the circuit chooses which of their last two potential moves becomes real, and all the other potential moves that are part of the circuit are automatically converted into real moves, based on their choice. (Note that it looks like this may be a misinterpretation of the rules on my part, but it's how this version works.)
The game ends when there are one or more lines of three pieces in a row of the same color, or when the board is full. Unlike in regular tic-tac-toe, it's possible for both players to get 3 in a row at once, or for one player to get two of them! There can also be a normal win (one player gets 3 in a row) or no win at all.