Charles H. Bennett and Gilles Brassard, winners of this year’s Turing Award, spent their lives touting the advantages of the quantum world
) next to an exhibit depicting their BB84 protocol for quantum cryptography. Bennett and Brassard are the recipients of the 2026 A. M. Turing Award, an annual prize given by the Association for Computing Machinery.
Most people find quantum mechanics complicated and difficult to grasp. Add information theory—the math behind computing—into the mix, and it’s a real headache. But information theorists Charles H. Bennett and Gilles Brassard argue that quantum information is something we should all be getting used to. It’s simple and beautiful, they contend, and it won’t stay relegated to the remote world of the subatomic for long. Soon, for instance, it could disappear all the money in our bank accounts if we don’t act fast. That’s because quantum computers based on the theory could one day break the cryptography that secures our Internet and our financial system. Bennett and Brassard recently received the A. M. Turing Award, which is bestowed annually by the Association for Computing Machinery. Named after the father of computing, Alan Turing, the award is often called the “Nobel Prize of Computing.” This year’s prize recognizes how the duo’s discoveries made quantum information relevant and inescapable.. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. Before their work, even experts considered the quantum world separate from our own. Quantum theory’s math worked, but its logic was different, they thought. When it came to computing, the fact that the microscopic world is quantum was a troublesome headache, something that needed to be sorted out. Everything would work better, scientists believed, if they could safely ignore its twisty rules. But rather than avoiding strange quantum phenomena such as superposition and entanglement, Bennett and Brassard embraced them. They found ways of inscribing uncrackable codes and transmitting microscopic states across huge distances that would be impossible with the classical computers Turing had envisioned.spoke with the computing pioneers about their achievements, their forewarnings and why we should all get comfortable with quantum.BRASSARD: The first time I heard about Charlie Bennett was by reading the. A column printed one of Charlie’s manuscripts word for word. I read it on the plane to San Juan, Puerto Rico. The next day I was swimming, minding my own business, when this complete stranger comes up to me and starts telling me about physicist Stephen Wiesner and how to make bank notes that are impossible to counterfeit.BENNETT: People try to make money hard to counterfeit. But if you’re really good at it and work very hard and just sort of dissect it under a microscope, there’s no physical barrier to duplicating it. Wiesner realized you could take advantage of quantum mechanics to make money that’s physically impossible to counterfeit. Based on Wiesner’s ideas, Gilles and I realized it was possible for two people to establish an encryption key without anyone else being able to eavesdrop.BENNETT: So if “Alice” wants to send a secret key to “Bob,” she produces a train of photons, and the key is the polarizations of these photons. But anyone who wants to measure one of the photon’s polarizations has to pick one of two ways of measuring it—called rectilinear and diagonal. And if they choose the wrong one, they’ll get a random answer and spoil the photon’s original polarization.BENNETT: Indeed. Bob doesn’t know which polarizations Alice chose, so he guesses, randomly performing a rectilinear or diagonal measurement on each photon as it arrives. BRASSARD: He spoils half of the states in the process—and he doesn’t even know which ones he spoiled. But then he tells Alice which measurements he chose—without telling her what results he got. And then Alice tells him which choices were correct. They each throw away half of these bits because Bob measured them along the wrong axis. And the ones they keep form a key that they share and no one else knows. So after Bob receives the photons and measures them, it’s crucial that he and Alice can communicate about which measurements he did. BRASSARD: And that communication can be in the open, on a public channel susceptible to eavesdropping. But it needs to be authenticated—Alice must be sure that the message she receives is from Bob and has not been altered in transit, and vice versa. BENNETT: We modestly called this protocol “BB84,” after our two last initials and the year we discovered it. At first, no one paid attention. The thing that really did it was when Peter Shor discovered his quantum factoring algorithm in 1994, which meant that classical cryptography, as it was then practiced, would become totally insecure at some future date.BENNETT: Well, that and quantum teleportation, which is the reason that quantum entanglement suffuses this entire field to this day.Star TrekBENNETT: No! Our late colleague Asher Peres, someone with little respect for spiritualism, once met someone of the opposite persuasion. And they asked him, “If you teleported a person, would it teleport just the body or also the soul?” And Asher gleefully answered, “Only the soul!” You transfer the quantum information—the state of the system—not the physical system itself. And you do it by entangling Alice’s quantum system with Bob’s. BRASSARD: This could be used to shuttle quantum information inside a quantum computer and someday could be used to implement a “quantum Internet” to transfer quantum information across the world, the way the Internet does with classical information. Our paper just showed that teleportation was theoretically possible. There was no doubt in our mind that it could be done. Of course, we never dreamed of doing the experiment ourselves. But you did demonstrate BB84 encryption over a distance of 30 centimeters. Now both ideas have been performed over thousands of kilometers. Are you surprised by this progress? BRASSARD: I don’t think we expected any of that. It was not our day job yet. We were just having a lot of fun tossing these crazy ideas around. But as years went by, especially after Shor’s algorithm came out, we realized that it was more and more serious. Superposition and entanglement are aspects of quantum mechanics that most of us find weird and counterintuitive. How did you realize they could be an asset, and why hadn’t anyone else thought that until then? BENNETT: There were very smart people involved in the early days of information theory and computer science—Turing and Shannon and von Neumann—and they all knew about quantum mechanics. But they viewed it as sort of a nuisance for information processing because it made communication noisier and measurements less reliable. Fortunately, because of the success of their work, we realized this nuisance could be put to positive use. Wiesner was the first person to say, what can we use this nuisance for? And then our work showed that entanglement is a resource and that it can be used to do things that you can’t do with classical information. It’s simple and beautiful.BENNETT: But it is! You just have to accept that a perfectly definite whole can have random parts. It’s completely counterintuitive, but once you accept that, then much of the weirdness goes away. BRASSARD: I was so successful at brainwashing myself that I consider quantum information and the laws of quantum theory totally natural. It’s so ingrained in my thoughts. The only thing I do not understand is that I’m not able to go through walls!BENNETT: Yes. I want it to become part of the knowledge of any educated person. Relativity is also counterintuitive—space and time are intertwined—yet most educated people know that light can’t escape a black hole. BRASSARD: We’re using our new fame to promote laypeople’s awareness of quantum information. But I’m also using it to promote awareness of the disaster that’s upon us once quantum computers completely undermine the current encryption methods. BENNETT: And you find your bank has closed because it can’t protect the assets that you deposited in it. BRASSARD: I’ve been going around the world telling people how bad it’s going to be. And until fairly recently, I felt like Cassandra of Greek mythology—cursed to see the future only for no one to believe her. But now people are finally listening.BRASSARD: Well, any information that’s ever been transmitted on the Internet is a lost cause—because even if it was encrypted, an opponent could have downloaded and saved it for a future where quantum computers can decrypt any of the old methods. So we must accept the fact that the past is lost. We can only hope to save the future. And for that, there’s so-called postquantum cryptography: cryptography that hasn’t been proven to be breakable by a quantum computer. But that doesn’t mean it can’t be—for all we know, it could even be broken by a classical computer. Then there’s quantum cryptography, which is secure if implemented correctly. But if you use both—establish independent keys with both techniques and combine them, then use that key to send a message—then, to break it, an adversary would need to break both systems, which would be much, much harder.Scientific American has served as an advocate for science and industry for 180 years, and right now may be the most critical moment in that two-century history.subscriber since I was 12 years old, and it helped shape the way I look at the world.always educates and delights me, and inspires a sense of awe for our vast, beautiful universe. I hope it does that for you, too., you help ensure that our coverage is centered on meaningful research and discovery; that we have the resources to report on the decisions that threaten labs across the U.S.; and that we support both budding and working scientists at a time when the value of science itself too often goes unrecognized.
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