Over the weekend, a video of the boyishly handsome Canadian prime minister, Justin Trudeau, briefly discussing quantum computing at a news conference made the rounds. “A bit of genius,” rhapsodized New York magazine, while Gizmodo’s headline said, “Everyone Should Be Able to Explain Quantum Computing Like Justin Trudeau.” Choose a popular science or tech publication (or The Washington Post) and you’ll find a version of the same breathless headline. Even the Guardian’s science blog, while arguing that “we should raise our expectations,” called his explanation “quite a good one.”
Except that Trudeau’s explanation of quantum computing was actually not quite right. (Never mind that the question was the punchline to a joke that Trudeau had set up earlier.) “Normal computers work, either there’s power going through a wire or not,” he said. This much is correct, as was Trudeau’s explanation of classical bits: “Regular computer bit is either a one or a zero, on or off.” But Trudeau’s succinct attempt at glossing quantum bits was wrong: “What quantum states allow for is much more complex information to be encoded into a single bit.”
This isn’t the place for a full primer on quantum computing, though Seth Lloyd’s “Programming the Universe” is a good non-technical introduction. A single qubit, or quantum bit, is neither a 0 nor a 1 until it is measured. This doesn’t mean it has “more complex information” encoded into it, though. Once you measure it — which you have to do if you are to do any computing — you can get only a 0 or a 1. There’s no additional complexity there. As this quantum computing textbook explains, you can gain only one bit of information about whatever question you were originally asking by measuring one qubit.
Why, then, are quantum computers powerful?
A quantum computer measures a qubit (or qubits) at the end of a computation or series of computations. Because intermediate steps have been taken while each qubit is neither a 0 nor a 1 but in a mixed state that has some probability of being one or the other, a quantum computer can do some types of computations much faster than a normal computer could. The power of quantum computers grows when you entangle many qubits together. The capability of quantum computers, in a sense, grows exponentially — very, very fast — with the number of qubits you can successfully entangle together. Keeping qubits entangled together until you want to measure the output — or “coherent,” in the parlance — has been very difficult, which is why progress in quantum computing has been slow.
At present, a quantum computer isn’t typically smaller than a conventional one, as Trudeau said. Quantum computers have only recently been able to perform computations such as figuring out that 15=3×5. (Some earlier quantum computers were able to make this calculation, but only by what one expert calls “cheating,” in a technical rather than moral sense.) The laser trap needed to figure this sort of thing out isn’t huge — it fits on a table top — but it is much bigger than microchips that can figure out the prime factors of 15 in the conventional manner.
Exactly what “measurement” consists of is a thorny question that physicists (and philosophers of physics) have still not fully resolved — it is at the heart of the famous paradox about Schrödinger’s cat. One of the most exciting things about quantum computing is that it turns the question of measurement from one of primarily philosophical interest to one of vital engineering importance — in so doing, quantum computing has the potential to not only perform some computations (like factoring numbers) faster, but to help scientists learn about the very nature of reality.
Does it matter that Trudeau was wrong? As Scott Aaronson, a noted quantum computing expert, says, “The widespread praise for this reply surely says more about how low the usual standards for politicians are, and about Trudeau’s fine comic delivery, than about anything intrinsic to what he said.” The experts polled by Motherboard can’t really be taken at face value, as who is going to go out on a limb to criticize a photogenic politician who has just brought your discipline into the limelight?
Trudeau’s mischaracterization matters only insofar as we care about getting it right for the sake of it. No physics student is going to be led astray by his remarks, and people who don’t actually want to learn about quantum computing are happy to be wooed by his starry eyes. The 50 million in Canadian dollars that Trudeau announced was to be given to the Perimeter Institute for Theoretical Physics is a very good thing. He created a new cabinet-level science post and has renewed attention on the pressing issue of climate change in Canada.
That’s all great news for scientists. It just doesn’t make him one.