# Quantum Battleships: The first multiplayer game for a quantum computer

## Like normal Battleships, but simpler and more complex at the same time.

If you want to play a multiplayer computer game, there are many options to choose from. But f you want a multiplayer game *that runs on a quantum computer*, there is but one option. Unfortunately, it is not Mario Kart Q. It’s Quantum Battleships.

The game is a simple one. One player places a battleship in one of six possible locations. The other then has three bombs to send the ship into oblivion. The winner is determined by whether the ship survives or not.

The game is played on IBM’s cloud controlled processor. They are real life quantum bits, and you can use them to play a game! They can be little noisy, but that’s just the weather buffeting the ship and bombs. Both players will have to learn how best to deal with it.

The quantum nature of the bits allows us to have some weird stuff going on. At the end of the game you’ll be told how intact the ship is. It will never be 100% intact: it’s an old ship that’s seen better days. You can expect an unbombed ship to be around 70% intact (depending on the weather).

After a single bomb you’d probably expect it to be less intact. But you’d be wrong: It will still be around 70% intact. If you think that means the bombs are useless, you’d be wrong again: After two bombs, it will be down to -70%. Or, to get rid of the minus sign, we can say that it is 70% broken. Either way, the ship has been smashed to smithereens!

This is not just weirdness I put in by hand. It is a quantum effect. In fact, it is one of the most important and celebrated quantum effects of them all. It is a result of entanglement: a kind of correlation that is unique to quantum systems.

Entanglement has been called ‘spooky action at a distance’. Objects with these correlations can seem to communicate with each other faster than light. That’s something that unsettled Einstein so much that he thought quantum mechanics must be wrong.

For all the details on the science behind this, you can check out another article I wrote. Or you can look up Bell’s inequalities. The internet is full of people trying to explain it to you.

For how it applies to the game, I should tell you more about my quantum battleships. They are actually pairs of entangled quantum bits. If we measure the bits, we’ll get an answer of 0 or 1. The correlation means that they’ll agree more than they disagree.

We use this to define the measure of intactness. For every game we make 1024 of these pairs. For each, we see if the results for the two half agree or disagree. If they agree all the same, we say that the ship is 100% intact. If they disagree all the time the ship is said to be 100% broken. All other possibilities lie somewhere between these two extremes.

Bombs are a kind of operation we can do to the quantum bits. They twist the bits around, pushing a 0 towards 1 and a 1 towards 0. This can affect the degree to which they’ll agree with each other. But it doesn’t always. It all depends on how they are entangled.

Our ability to bomb effectively gives us two ways to measure the quantum bits: measurement without bombing first, and measurement with bombing first. This ability to choose measurements is fundamental to proving the non-local nature of entanglement. After the ship is created, the two halves never actually talk to each other. But quantum mechanics predicts that they’ll nevertheless be able to act as if they know whether the other has been bombed. So is quantum mechanics right?

This question can be answered by maths. We need to look at all four possible cases (no bombs, a bomb on one side, a bomb on the other side, and bombs on both sides) and find out how intact they are. Then we add up the intactness for the first three cases and the brokenness of the fourth.

If the two halves of the ship don’t interact at all during the bombing, and if their correlations are the normal kind that we see in real life, this sum cannot ever come out higher than 200%. But if something weird and quantum is going on, it can come out higher.

Check it out for yourself. Play some games. Get some numbers and add them up. See the result beats the 200% bound. If it does, you’ll have proven Einstein wrong about the nature of reality. And you’ll have done it by playing Battleships! Who said games are a waste of time?

*The game can be downloaded **here**. It is played using **ProjectQ** to control the **IBM Quantum Experience**. You’ll first need to install ProjectQ and get a Quantum Experience account.*

*This is the world’s second computer game to run on an actual quantum computer. Check out the first below.*