Quantum Chess

Quantum Chess is the game from Caltech's "Anyone Can Quantum", where Paul Rudd beat Stephen Hawking. It is chess with new moves. A piece can split and stand on two squares at once. Two pieces can become entangled, so one outcome settles both. And when a capture depends on where a piece really is, the board measures. Almost every classical rule is still underneath: castling, en passant, promotion. There's no check or checkmate, you must capture the king, in all possible universes.

This is the Remaster, rebuilt from scratch. It is a free update for everyone who owns the original, and buying it on Steam adds six months of Premium to your account.

Owning Quantum Chess on Steam unlocks the exclusive Cloud Chamber theme. You can't get this anywhere else. The board sits inside a particle detector. Moves leave vapor trails, and when you measure, the branch that didn't happen breaks up into vapor. In the clip, a knight splits to d4 and g5, and its g5 half takes on f7. The board measures, the knight was on g5, and the d4 branch drifts away.

Cloud Chamber follows your Quantum Chess account to the web and your phone. Pick it in Settings, under Theme. It needs your own copy; a Family Sharing loan doesn't unlock it.

Select a piece, then drag between two squares it can reach. It now stands on both, half on each, and the ring around each half shows how much of it is there. Either half can move, threaten and block. Bring the halves back together with a merge.

A knight that is half on f3 takes on e5. Was it there to make the capture? The board measures and finds out. Plan for both answers, because your opponent will.

The queen slides to h5 through f3, where a split knight might be standing. If the knight is there, the queen didn't move. If it isn't, she went through. Until something measures, both stories are true, and one measurement settles the queen and the knight together.

The Alchemy League changes the rules every season. Season 1 runs from October 1 to January 1. It takes merges away and puts a phase dial beside the board. Turn the dial, split a piece back onto one of its own halves, and the two paths interfere: the dial decides where the piece ends up. In the clip, a rook split across a2 and a3 meets itself on a2 at 100%.

• Free to play, on its own season ladder
• Live games or correspondence
• Practice with pass-and-play, no account needed
• Rated games earn extra league badges, and participation can earn you the Alchemy board theme

The Quantum Lab is the sandbox: set up any position, play it out against the AI or yourself, undo and branch. Premium players can switch it to Universal QC, a ruleset with two more controls. The phase dial turns to any angle, not just quarter turns. The strength slider sets how a split divides a piece, so a knight can stand 79% on one square and 21% on the other, and a merge at the same strength undoes it. Premium also gets access to any Alchemy League season's rules, current or past, in the lab.

Ranked and casual games, cross-play with the browser version on one account.

• Play a bot near your rating while the queue searches. When a real opponent turns up you get 15 seconds to join, and the bot game waits for you.
• The open games board lists posted live and correspondence games. Post your own, or send a friend a private link.
• A status pill on every screen shows what is waiting on you: a search, an open table, an opponent who sat down, or your move.
• Notices reach you where you choose: in the app, by push, Discord message or Steam alert.
• Correspondence give you days to make a move. You get a reminder before a deadline, and you can step back through any finished game.
• Ask for a takeback in casual, ranked and league games.
• Watch live games with the clocks running.
• Tournaments: Swiss, round robin, knockout and arena, live or by correspondence. Anyone signed in can run one.

The tutorial teaches one move at a time on a live board. The in-app guides cover every rule, mode and overlay with boards you can play on. Then there are puzzles, AI opponents up to Aqaqaq, our quantum engine, and Steam achievements for the badges you earn.

For returning players

• The Remaster is a free update in the same library entry.
• Your copy unlocks Cloud Chamber and comes with six months of Premium. Link your Steam account on the account page and press Claim.
• Ratings and stats started fresh, so the new ladder began with clean standings.
• The original Unity version stays on the "legacy" beta branch: right-click Quantum Chess in your library, Properties, Betas.

If you reviewed the original, we'd love a second look.

Development happens in the open on our Discord. Come talk quantum chess, report bugs or test builds early.