Why Quantum Computing Requires Quantum Cryptography

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Matt O'Dowd (1:00)

We'd like to thank Audible for supporting PBS. Quantum computing is cool, but you know what would be extra awesome? A quantum internet. In fact, if we want the first, we'll need the latter. And the first step to a quantum internet is quantum cryptography.

Quantum theory may seem like an obscure subject of questionable relevance to the average person. But in fact, much of our technological world depends on our understanding of the quantum properties of the subatomic universe. And soon, perhaps very soon, we'll be interacting with the weirdness of quantum mechanics even more directly, with the coming of quantum computing and the quantum internet.

Today, we're going to talk about the latter, specifically quantum cryptography and quantum key distribution, the foundations of the perspective of quantum internet. We may come back to quantum computers in detail, but for now, just a word on why their advent will demand a quantum internet. The logic gates of a quantum computer exists in the state of quantum superposition, of many simultaneous configurations. This allows the iterations of certain types of calculation to be done in parallel and vastly more quickly than a classical computer. For example, a quantum computer can calculate the prime factors of large numbers extremely quickly. This is bad because prime factoring is a cornerstone of internet cryptography. For the best description of classical internet cryptography on the internet, head to Infinite Series. By comparison, my summary will be pathetic, but here it is.

To send encrypted emails or credit card details, two parties need to share a cryptographic key. This can be as simple as a number that you need in order to unscramble a message. The most secure way to do this would be to meet under a bridge on a rainy night and swap the key in advance, a so-called private key. That's impractical. So we use public keys. The most widespread example is the RSA protocol. Choose two prime numbers, one of which is very large, multiply them together to get an even larger number, and broadcast that as your public key.

Anyone can then send you an encrypted message by scrambling the message with the public key using a special one-way function. It's a mathematical process that can't be undone with the public key, only with its prime factors, which only you have. All of this works as long as the public key can't be easily factorized back into its two primes. This is another type of one-way function. It's much, much harder to factorize large numbers than it is to create them by multiplication in the first place. At least as long as we're limited by...

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This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

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