While quantum computing researchers today vigorously chase a demonstration of a quantum advantage – an application which when run on a quantum computer provides sufficient advantage to warrant switching from a classical computer – a group of MIT and Caltech researchers has found hints of a QA application that seems to fit the bill. Blackjack.

OK, this may fall loosely under the heading of ‘fascinating-but-don’t-quit-your-day-job’ summer reading but the researchers report finding a way to better the odds for blackjack players against the house using quantum communications and entanglement. Their paper, Quantum blackjack: Advantages offered by quantum strategies in communication-limited games, was published last week in APS Physical Review.

“We examine the advantages that quantum strategies afford in communication-limited games. Inspired by the card game blackjack, we focus on cooperative, two-party sequential games in which a single classical bit of communication is allowed from the player who moves first to the player who moves second. Within this setting, we explore the usage of quantum entanglement between the players and find analytic and numerical conditions for quantum advantage over classical strategies. Using these conditions, we study a family of blackjack-type games with varying numbers of card types, and find a range of parameters where quantum advantage is achieved,” write the researchers in their abstract.

If that’s a little too abstract, MIT has posted a more accessible and fun article on its website. You may recall a MIT rocked the blackjack world decades ago with a successful card-counting scheme.

“This calculating strategy, known as card-counting, was made famous by the MIT Blackjack Team, a group of students from MIT, Harvard University, and Caltech, who for several decades starting in 1979, optimized card-counting and other techniques to successfully beat casinos at blackjack around the world — a story that later inspired the book “Bringing Down the House,” wrote Jennifer Chu in the MIT article posted today. The latest work was spearheaded by MIT professor Joseph Formaggio, along with Aram Harrow, and Anand Natarajan of Caltech.

This excerpt is from Chu’s article:

After casually entertaining the idea during a regular poker night with friends, Formaggio decided to explore the possibility of quantum blackjack more formally with his MIT colleagues. “I was grateful to them for not laughing and closing the door on me when I brought up the idea,” Formaggio recalls.”

In blackjack, the dealer deals herself and each player a face-up card that is public to all, and a face-down card. With this information, each player decides whether to “hit,” and be dealt another card, or “stand,” and stay with the cards they have. The goal after one round is to have a hand with a total that is closer to 21, without going over, than the dealer and the other players at the table.

In their paper, the researchers simulated a simple blackjack setup involving two players, Alice and Bob, playing cooperatively against the dealer. They programmed Alice to consistently bet low, with the main objective of helping Bob, who could hit or stand based on any information he gained from Alice.

You’ll have to read the paper to get the full gist. The researchers lay out a theoretical scenario in which two players, playing cooperatively against the dealer, can better coordinate their strategies using a quantumly entangled pair of systems. Chu notes, “Such systems exist now in the laboratory, although not in forms convenient for any practical use in casinos. In their study, the authors nevertheless explore the theoretical possibilities for how a quantum system might influence outcomes in blackjack.”

It also turns out the advantage you can gain using ‘quantum card counting’ is very slight. “It would require a very large investor, and my guess is, carrying a quantum computer in your backpack will probably tip the house,” Formaggio says. “We think casinos are safe right now from this particular threat.”

Link to MIT article: http://news.mit.edu/2020/quantum-blackjack-0803

Link to APS paper: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.102.012425