Quantum Chess

Quantum Chess is chess with quantum mechanics: pieces can split across two squares, become entangled, and resolve through measurement. You play to capture the king in all possible universes; there is no check or checkmate. Most classical rules remain, including castling, en passant and promotion.

Quantum moves

Drag a piece between two squares it can reach to split it between them. Each half can move, threaten and block, and rings show how much of the piece occupies each square. You can merge the halves back together.

When a capture depends on a piece’s location, the board measures to determine the outcome. Moves through a square occupied by a split piece can also depend on the measurement: until it happens, both outcomes remain possible. Entangled pieces resolve together when one is measured.

Game modes

Ranked and casual games include live play and correspondence. You can practice with pass-and-play without an account, or use the Quantum Lab sandbox to set up positions, play against the AI or yourself, undo moves and branch into different lines. The tutorial teaches one move at a time on a live board; in-app guides cover the rules, modes and overlays with playable boards. The game also includes puzzles, AI opponents up to Aqaqaq, its quantum engine, and Steam achievements for badges you earn.

The Alchemy League

The Alchemy League changes its rules each season. Season 1 runs from October 1 to January 1, removes merges and adds a phase dial beside the board. Turning the dial lets you split a piece back onto one of its own halves; the two paths interfere, and the dial determines where the piece ends up. Rated games earn extra league badges, and participation can earn the Alchemy board theme. The league is free to play and has its own season ladder, with live, correspondence and pass-and-play options.

Playing with others

Ranked and casual games support cross-play with the browser version through one account. While the queue searches, you can play a bot near your rating; if a human opponent appears, you have 15 seconds to join, and the bot game waits. An open-games board lists posted live and correspondence games, and you can post your own or send a friend a private link.

A status indicator shows when a search, open table, opponent or move is waiting for you. Notices can arrive in the app, by push notification, Discord message or Steam alert. Correspondence games give you days to move, with a reminder before the deadline, and let you step back through finished games. You can request takebacks in casual, ranked and league games, watch live games with the clocks running, and play or run Swiss, round-robin, knockout and arena tournaments, live or by correspondence. Anyone signed in can run a tournament.

The Quantum Lab

Premium players can use Universal QC in the lab, a ruleset with two additional controls. Its phase dial can turn to any angle rather than only quarter turns, and its strength slider controls how a split divides a piece—for example, a knight can stand 79% on one square and 21% on another. Merging at the same strength reverses the split. Premium players can also use the rules of any current or past Alchemy League season in the lab.

Cloud Chamber theme

The remastered game includes the Cloud Chamber board theme for owners of Quantum Chess on Steam. The board sits inside a particle detector, moves leave vapor trails, and measurement turns the branch that did not happen into vapor. The theme follows your Quantum Chess account to the web and phone versions.