Websites and messaging apps use cryptography, or encryption, to keep information private. Each time you visit a website with a secure connection (i.e., a site that shows a padlock icon next to the URL in your web browser) to make a purchase or log into your bank account, the website uses encryption to transfer data in a way that ensures that no one else can access your private information.

Quantum information science, which harnesses the properties of quantum mechanics to create new technologies, has the potential to change how we think about encryption in two main ways.

Post-quantum cryptography, also known as quantum-proof cryptography, aims to create encryption methods that cannot be broken by algorithms, or calculations, that run on future quantum computers. Today's encryption methods will not necessarily remain secure if and when quantum computers become a reality.

Take RSA cryptography: RSA is a widely used secure data-transmission system on which things like internet browsers and digital signature software are built. It creates sets of public and private codes, or keys. The process happens in the background when you use an internet browser or sign a document using a digital signature, for example. In RSA, the private key, which is kept secret, consists of two large prime numbers generated by an algorithm. The product of those two numbers then is used, along with an exponent, to create the public key, also using an algorithm. Anyone can encrypt information using the public key, but once they have, the information can only be decrypted using the private key.

The encryption system relies on the fact that it is prohibitively time consuming and computationally intensive to factor the large integer in the public key to determine the two prime numbers that make up the private key. However, Shor's algorithm, published in 1994 by mathematician and Caltech alumnus Peter Shor (BS '81), describes how, in theory, quantum computers could factor incredibly large numbers efficiently. This means that Shor's algorithm could be the downfall of RSA cryptography.

As a result, "most likely, people will switch to new public key cryptography systems based on problems that we don't think quantum computers can solve efficiently," says John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter. Identifying such problems is an active area of research in mathematics and cryptography.

Quantum cryptography uses the laws of quantum physics to transmit private information in a way that makes undetected eavesdropping impossible. Quantum key distribution (QKD), the most widely studied and viable method of quantum cryptography, uses a series of photons to transmit a secret, random sequence, known as the key. By comparing measurements taken at either end of the transmission, users will know if the key has been compromised. If someone wiretapped a phone, they could intercept a secret code without the callers knowing. In contrast, there is no way to "listen in" on or observe a quantum encrypted key without disturbing the photons and changing the outcomes of the measurements at each end. This is due to a law in quantum mechanics called the uncertainty principle, which says that the act of measuring a property of a quantum system may alter some of the other properties of the quantum object (in this case, a photon).

"Everlasting Security"

According to Thomas Vidick, a Caltech professor of computing and mathematical sciences who teaches courses on quantum cryptography, QKD only makes sense to use for data that needs to stay private far into the future.

"If you encrypt your data today using standard techniques, it will likely be kept private for a decade. It's hard to know what the status of current cryptosystems will be beyond that time," says Vidick. "Today's cryptography is based on math that is hard to solve today, but in 50 years, maybe it won't be so hard to solve. For credit card transactions, that's fine. For medical records or government information that is meant to stay secret for longer, it may not be."

Is quantum cryptography used today?

Scientists have demonstrated that QKD works, but it is not widely used due to significant technological limitations. To send a quantum key, a single-photon laser beams a signal, one photon at a time, via a fiber optic cable. This method is slower than current telecommunication technologies and requires a dedicated fiber optic cable between the two parties. For example, Amazon could not secure customer transactions using quantum encryption because it would require cables between its servers and individual devices that make purchases. Distance is also a factor. When fiber optic cables are used to transmit data, as in your home internet and cable systems, they use repeaters to send the data over longer distances. However, those repeaters disturb the delicate quantum state that is crucial to QKD.

Researchers in China have demonstrated QKD over long distances using a combination of fiber optic cables with "trusted relay nodes" as repeaters and a satellite that transmits photons through the air. However, more research is needed to create a system that transmits keys reliably and efficiently.

In theory, quantum cryptography is unhackable, because eavesdropping would always be detected, but its practical uses are limited. "If you build a house, it's only going to be as strong as the weakest pillar," says Vidick. "To have a truly usable system, you may need to combine quantum cryptography with elements that are not quantum, and those other elements could be vulnerable to attacks that theorists have not envisioned."