The Unbelievable Power of Quantum Key Distribution
By: Eesha Santosh
Tags: Encryption, Quantum Physics, Technology, Quantum Computing, Science

Image from GNC.com. No copyright infringement intended.

In a world where almost everything is stored in the cloud and top-secret messages are sent across the Internet, society faces a growing necessity for stronger encryption methods. It seems like hackers and cybercriminals can break into virtually any system nowadays, no matter how many precautions are taken to secure the information. But, finally, engineers are starting to explore a powerful new technology that might just be unhackable- quantum key distribution.

First, let’s review how information is normally sent across the Internet. Suppose you want to send an email to someone. This email is broken up into packets, with each packet containing up to 1500 bits. This packet also has a wrapper on it, which tells the computer what information the packet contains. If sensitive information is being sent, the information will be encrypted using an encryption key (a random algorithm-generated pattern to scramble the information). The packets are then sent across the Internet, and once they reach the reciever of the message, the packets are put together like a puzzle and decrypted for the reciever to read.

However, a hacker can easily intercept the message, and read and copy the bits in transit. That’s where quantum communication comes into play. Qubits, or quantum bits, harness the quantum power of superposition, meaning that they can exist as 0 and 1 simultaneously. But when these fragile qubits are observed, they collapse into one state, either 0 or 1. This means that if information were sent in qubits, a hacker would not be able to intercept the message without the qubits collapsing into one state, leaving a telltale mark behind. This unique property has enabled companies to create guarded quantum networks, using a process called quantum key distribution(QKD). In essence, QKD involves transmitting information as classical bits, but uses an encryption key in the form of qubits to secure the connection.

Now, let’s dive into this a little further by using the classic example of Alice and Bob. To create the key, Alice and Bob will have to run through a process called key sifting. Alice will send photons through a fiber-optic cable with each quantum particle having one of the following polarization states: vertical, horizontal, diagonal left, or diagonal right. Now Bob will measure the states of these photons, guessing at random at which detector to use. Keep in mind that, when measured, the photons will collapse into qubits with states of either 0 or 1. So he might measure 0 for the first photon, 1 for the next photon, and so on. Once this is done, Bob will tell Alice which detectors he used, and she will tell him if he has measured the states of the photons correctly based on which filters she used to polarize the photons. Now, they will eliminate all of the photons which Bob measured incorrectly, leaving them with the photons that he measured with the appropriate detector. This sequence of leftover photons is the key.

After key sifting, the quantum key should ideally be free of errors. Of course, the state of some of the qubits will collapse due to the fragility of these particles. Bob and Alice will now need to run through a procedure called key distillation to check if the error rate is high enough to suggest that someone is eavesdropping on their conversation. Firstly, they use an error correction protocol to correct all of the errors in the key. The protocol will also allow them to estimate the actual error rate. Then, they will execute privacy amplification, which involves compressing the key to limit the amount of information the eavesdropper has. The higher the error rate, the more they will need to compress the key.

And that’s pretty much it! QKD opens up a realm of possibilities in terms of sending and receiving critical information. Encryption is one field in which integrating quantum computing has a clear advantage over integrating the power of supercomputers. And as engineers across the world begin to apply quantum mechanics to their various fields, society begins to make the great leap towards a utopic future.