Tell-Your-Boss Version: The “Key” Idea

Quantum cryptography is a topic where it’s easy to lose the plot with all the exchange of information taking place to transmit a single message. So before getting into the nitty-gritty details, you’ll look at the key insight that undergirds all of quantum cryptography. The intent is to get you to appreciate the quantum effects at play rather than making sure that all the auxiliary logic is properly connected up.

The central problem in cryptography is making sure that the sender and receiver have the necessary encryption and decryption keys before any message is communicated. With the keys in place, the sender encrypts the message and transmits it over a public channel, confident that only the receiver can decrypt it. Thus, we’ll focus on getting the keys across to the receiver: the Quantum Key Distribution (QKD) problem.

In classical cryptography, information is encrypted with a key and then sent along a public channel to the receiver. In quantum cryptography, information is also encrypted with a key, but what’s sent is a mega-qubit. To put it another way, instead of sending a single encrypted string, the mega-qubit contains, in effect, several encrypted strings. The following figure shows a mega-qubit carrying two such strings:

Should the mega-qubit fall into the wrong hands, the thieves wouldn’t count themselves among the lucky few. Despite the mega-qubit carrying the encrypted strings, it’s virtually impossible to extract them. Because the strings are in a superposition in the mega-qubit, the only way to get anything out of a mega-qubit is to collapse its qubits. But this is easier said than done. In the mega-qubit shown in the previous figure, the first qubit can collapse to or , the second to , the third to or , and so on. Thus, for this example, the mega-qubit could collapse to , or even or or any of the other combinations. As the number of bits in the binary string gets larger in any real-life transaction, the chances that a random collapsing of the mega-qubit results in the actual encryption key is virtually nil. In other words, the quantum nature of the mega-qubit means that the secret encryption key is safe from any attempt to pry it out.

Yet, the receiver needs to pull out the correct encryption key from the mega-qubit. Quantum cryptographic algorithms are a way for the sender and receiver to legitimately get around the inherent tamper-proof seal of the mega-qubit. They’re a carefully choreographed sequence of actions taken by both the sender and receiver so that the latter can recover the correct key, despite eavesdroppers on the public channels over which the sender and receiver communicate. In the next section, we’ll describe one such mechanism.