What on Earth Is Quantum Chess?

You won't believe you eyes when you play quantum chess, literally.
Christopher McFadden

Have you ever heard of Quantum Chess? If not, we are confident you are in for a real treat. 

Read on to find out more about this interesting take on a very ancient strategy game. But brace yourself, things are about to get a little "spooky". 


What is Quantum Chess?

Quantum Chess is a variant of the classical strategy game. It incorporates the principles of quantum physics. For example, unlike traditional chess, the pieces can be placed into a superposition of two locations, meaning that a piece can occupy more than one square.

Unlike chess pieces in the conventional game where, for example, a pawn is always a pawn, a quantum chess piece is a superposition of "states", with each state representing a different conventional piece.

Conventional chess is a very complex game, although it is possible for computer algorithms to beat the world's greatest chess players by accurately determining the moves necessary to win the game at any point. 

The main rationale behind the creation of Quantum Chess is to introduce an element of unpredictability into the game, and thereby place the computer and the human on a more equal footing. The game can also help "level the playing field" somewhat between human players of widely different skills and experience with chess.

“It’s like you’re playing in a multiverse but the different boards [in different universes] are connected to each other,” said Caltech physicist Spiros Michalakis during a Livestream of a recent Quantum Chess tournament. “It makes 3D chess from Star Trek look silly.”

But don't let the term intimidate you. New players to the game don't need to be experts in quantum physics — a basic understanding of chess is more important actually. 

While it might sound like something of a gimmick, Quantum Chess is an interesting and entertaining spin on the classic game that many find enjoyable. Unless, of course, you cannot live without knowing for sure what and where each piece is at any given time. 

If that is the case, you might find this one of the most frustrating games ever created!

quantum chess image
Source: murat efe/iStock

What is the difference between Quantum Chess and normal chess? 

Quantum Chess, as you have probably already worked out, is not like any game of classical chess you have ever played. But, it is important to note that there are also several variants of Quantum Chess.

The best known is probably the one created by Chris Cantwell when he was a graduate student at the University of Southern California. This variant differs from other examples by the fact that it is more "truly quantum" than others. 

“My initial goal was to create a version of quantum chess that was truly quantum in nature, so you get to play with the phenomenon,” Cantwell said in an interview with Gizmodo back in 2016.

“I didn’t want it to just be a game that taught people, quantum mechanics.” The idea is that by playing the game, a player will slowly develop an intuitive sense of the rules governing the quantum realm. In fact, “I feel like I’ve come to more intuitively understand quantum phenomena myself, just by making the game,” he added.

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In Cantwell's version of Quantum Chess, this superposition of pieces is indicated by a ring that details the probability that the piece can actually be found in a given square. Not only that, but when moving a piece, each action can also be governed by probability.

You can think of the pieces of the game existing on multiple boards in which their numbers are also not fixed. The board you see is a kind of overview of all of these other boards and a single move acts on other boards at the same time. 

Whenever a piece moves, many calculations are made behind the scenes to determine the actual outcome, which could be completely unexpected.

That being said, moves do follow the basic rules of traditional chess, including things like castling and en passant. However, there are a few important differences: 

  • Unlike regular chess, the concept of check and checkmate does not exist in Quantum Chess. A king, for example, can move into, or castle through what might traditionally be considered check in regular chess.
  • A Quantum Chess piece occupying a square does not block standard moves. For example, this means that a knight can move to a square that is occupied by another piece that is under superposition. A queen can also slide "through" another piece that is in superposition. 

Pieces in this version of Quantum Chess can make a series of either "quantum moves" (except for pawns) or regular chess moves. In this sense, the pieces can occupy more than one square on the multiverse of boards simultaneously. 

These moves also come in a variety of "flavors". 

The first is a move called a "split move". This can be performed by all non-pawn pieces and allows a piece to actually occupy two different target squares that it could traditionally reach in normal chess.

quantum chess computer game
Source: Ruocaled/Flickr

But, this can only be done if the target square is unoccupied or is occupied by pieces of the same color and type. A white knight, for example, could use this kind of move to occupy the space of another white knight.

Such a move cannot; however, be used to capture an opponent's piece. 

Another interesting move is called a "merge move". This can be performed by all pieces except pawns and, like a split move, can only be performed on an unoccupied square or one occupied by a piece of the same type and color. 

Using our previous example of a white knight, this would mean that two white knights could merge together on the same square. Again, this move cannot be used to capture enemy pieces. 

So how do you take pieces in Quantum Chess?

Well, when two pieces of different colors meet on the same square the game makes a series of measurements. These measurements are designed to answer a specific yes or no question. 

quantum chess
Source: Chess Stackexchange

For example, the game's mechanics will look at certain squares to determine if they are occupied or not. The outcome of this can be to cause a piece's "superposition" state to "collapse". 

If the superposition state collapses, then the desired move will be performed. If not, the move is not made and the player's turn ends. 

Capturing is also very different in a game of Quantum Chess. When a player attempts to do this, the game will make calculations for the square where the piece is situated and for its target square, as well as any other squares in its path, to answer the question, "is the attacking piece present and can it reach the target?".

If the answer is no, it is important to note that this doesn't necessarily mean the attacking piece is not present. Nor does it mean that its path is blocked.

Another interesting concept of Quantum Chess is called "exclusion". If a moving target is occupied and is in superposition by a piece that cannot be captured by the move, it is called an exclusion move. 

quantum chess moving
Just because you move a piece in quantum chess doesn't mean the outcome will be what you'd expect! Source: Eigenberg Fotografie/Flickr

Again, calculations are made for the target square and any squares in the path of an allowed move by a piece in superposition. This is done to answer the same question as capturing, with similar outcomes.

Castling is also very different in Quantum Chess. This move always involves two targets, and the same measurements are made for both targets. Castling cannot be used to capture, and will always be an exclusion move.

So, you might be wondering how you actually win a game of Quantum Chess?

Just like traditional chess, the aim of the game is to capture the opponent's king. However, unlike in traditional chess, the concept of checkmate does not exist. 

To win, the enemy king must no longer actually exist on the board. As any piece, including the king, exist in a state of superposition, they can either be captured or not which further complicates the issue.

quantum chess board
The board and pieces are the same in quantum chess, but that is where the similarity ends. Source: Edith Soto/Flickr

The game, therefore, continues until it is known, with certainty, that a particular player has no king left. For this reason, it is possible for both players to lose their king at the same time and the game would then be considered a draw. 

Another important thing to note is that each player has a set amount of time for the game. For this reason, you can also win by running an opponent's time out. 

How do you play Quantum Chess?

How you play Quantum Chess depends on the variant of the game you are playing. We have already covered the rules of one variant above, and that game can be played through Quantum Realm Games. But another version created by Alice Wismath at the School of Computing at Queen's University in California has some slightly different rules.

You can try that game for yourself here

In her version, each player has sixteen pieces. These pieces are in a quantum state of superposition of two types: a primary and a secondary type.

They are also in an unknown (quantum) type or a known (classical) type. When a piece is "touched" it collapses into its classical state and has an equal probability of becoming either a primary or secondary type. The king, however, is an exception, and is always in a classical state.

Each player has one king and its position is always known. 

quantum chess example
Alive Wismath's version of Quantum Chess. Source: Queen's University, California

All other pieces are assigned the following primary piece types: left rook, left bishop, left knight, queen, right knight, right bishop, right rook, and pawns one through eight. Secondary piece types are then randomly assigned from this same list of piece types so that each type occurs exactly twice in the player's pieces.

Each piece is created at the start of each game and superpositions are not changed throughout the game. Pieces also start as they would in regular chess, on the first two rows, according to their primary piece type with all, except the king, in a state of superposition.

Once a quantum state piece is touched (i.e. chosen to move), it collapses into one of its two predetermined states, and this state is suddenly revealed to both players.

This can mean that a pawn in the front row can suddenly become a white knight once the piece has been "touched". You won't know until the piece's quantum state collapses. 

Quantum Chess boards are the same as regular chess boards except that when a piece lands on a white square it remains in its classical state. When pieces land on black squares, however, they undergo a quantum transformation and regain, if lost, their quantum superposition. 

quantum chess change
An example of a piece suddenly changing from an unknown to a known state. In this case, a white pawn has "collapsed" into a rook. Source: Queen's University, California

This means that a previously "revealed" pawn can also suddenly transform into a queen if that was one of its predetermined primary or secondary types. A very interesting concept indeed. 

To play the game, each player chooses a piece to move and must move it. If the quantum piece collapses into a piece type with no possible moves, then the player's move is over.

Pieces in classical states with no possible moves cannot be chosen. All pieces move as they would in classical chess with some of the following exceptions: 

  • The en passant rule for pawn capturing is left out.
  • Castling is not allowed.
  • The king may be placed or left in check.

Pieces can also be captured as normal, and quantum pieces collapse from their superposition state and are removed from play. 

If a player touches a quantum piece that collapses into a state that puts the opponent's king in check, their move is over. The opponent, however, is not required to get out of check in such circumstances.

Pawns that reach the opposite side of the board can be promoted to a queen, bishop, rook, or knight, regardless of the number of pieces of that type already in the game. Also, if a piece in the quantum state on the far row is touched and revealed to be a pawn, it is promoted, but the promotion takes up the turn. The superimposed piece type is not affected.

To win the game, each player must capture the enemy's king, as a checkmate does not happen in Quantum Chess. For this reason, kings can actually move into a position that would normally be considered check. 

Games are considered a draw if both opponents are left with only their king in play or 100 consecutive moves have been made with no captures or pawn movements by either player. 

Who is the best Quantum Chess player?

It was recently announced that the world's first Quantum Chess tournament had been won by Aleksander Kubica, a postdoctoral fellow at Canada's Perimeter Institute for Theoretical Physics and Institute for Quantum Computing. The tournament was held on the 9th of December 2020 at the Q2B 2020 conference.

The tournament games are timed, and Kubica managed to beat his opponent, Google's Doug Strain, by letting him run out of time. This currently makes Kubica officially the best Quantum Chess player in the world. 

Not a bad way to see out one of the worst years in living memory. 

And that, ladies and gentlemen, is a wrap.

If you like the sound of playing Quantum Chess, why not check out either of the versions we have discussed above in this article. Who knows, you might get proficient enough to challenge Kubica for the title in the not too distant future?

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