## How to win at Bridge using Quantum Physics

Wired posted this fun story on using quantum physics to win at bridge.

Contract bridge is the chess of card games. You might know it as some stuffy old game your grandparents play, but it requires major brainpower, and preferably an obsession with rules and strategy. So how to make it even geekier? Throw in some quantum mechanics to try to gain a competitive advantage.

The idea here is to use the quantum magic of entangled photons–which are essentially twins, sharing every property–to transmit two bits of information to your bridge partner for the price of one. Understanding how to do this is not an easy task, but it will help elucidate some basic building blocks of quantum information theory. It’s also kind of fun to consider whether or not such tactics could ever be allowed in professional sports…

…Let’s say that two physicists named Alice and Bob decide to enter a bridge tournament. With them, they bring a laser and a special crystal that produces pairs of entangled photons when hit with the laser. Entanglement is a bizarre quantum mechanical property where two particles are perfectly identical. If you measure the characteristics of one of the pair, you immediately know that the other one is exactly the same.

Alice and Bob place their laser-crystal apparatus on the table, and each holds a device capable of measuring different aspects of photons. They fire the laser on the crystal and each take one of the entangled photons. They have agreed beforehand on a convention to pass information to one another using these implements. In bridge, no team is able to have secrets and so the two physicists have to tell everybody what they’re doing (whether or not their opponents understand quantum mechanics is their own problem).

The cards are dealt and the bidding starts. Bob has strong cards and thinks he and Alice can set the highest possible bid and win all the hands during the gameplay round. But he needs to know if Alice’s cards are good enough to support him in the places where his cards are weak. So he uses an agreed-upon convention to ask Alice indirectly about the strength of her cards.

Alice wants to tell Bob about two things: She has the queen in the suit that Bob is strongest in, and she has one ace in another suit. In normal bridge, conveying these two pieces of information would eat up two rounds of bidding. Because each bid must always be higher than the one before, Alice would also drive up the final contract sending these two signals. But then she and Bob might not have strong enough cards, and would end up bidding too high and losing the round and some points. Usually, Alice would just decide to tell Bob about the ace, because it is more powerful.

Read the full story here.

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