For centuries, governments, military officers, and spies have used secret codes to protect confidential messages. This practice, known as cryptography, uses mathematical techniques to secure information. These early codes, or ciphers, could be as simple as shifting each letter of the alphabet by a few positions, so ‘A’ becomes ‘D’ and ‘B’ becomes ‘E’. But cryptography has evolved significantly since then.

Today, computers use complex mathematical operations to scramble information. Modern cryptography also includes authentication, which verifies the identities of both the sender and receiver. Whether it’s sending secure data or even something as simple as using a service like fax for free, encryption ensures your information stays protected.

What is quantum cryptography?

Quantum cryptography uses the principles of quantum mechanics to encrypt, transmit, and decode information securely. It relies on quantum devices, like sensors that can record single particles of light (photons), to protect data from attacks. Although technically challenging, quantum cryptography offers potential advantages over classical methods, such as better detection of eavesdroppers.

One early example is quantum key distribution (QKD), which uses a string of characters (an encryption key) shared between two parties to scramble and unscramble data. While the key itself isn’t quantum, it’s transmitted using photons. In 2004, Austrian scientists used QKD to secure a bank transfer to Vienna City Hall.

However, QKD systems have technological and theoretical loopholes that could allow eavesdroppers to intercept messages. Due to these limitations, the National Security Agency (NSA) does not currently recommend using QKD for national security systems.

How does quantum physics protect information?

The key quantum mechanics principles that safeguard data are superposition and entanglement.

Superposition means that quantum objects like atoms, electrons, and photons can exist in multiple states simultaneously. A famous hypothetical example is Schrödinger’s cat, which is both dead and alive until observed.

For a real-world example, consider a photon passing through polarizing film, like that in sunglasses. Before hitting the film, the photon can be in a superposition of both horizontal and vertical polarizations. Once it strikes the film, the superposition collapses, and the photon takes on a definite polarization.

Researchers use this phenomenon to encode information. A horizontally polarized photon can represent a 0, and a vertically polarized one can represent a 1. This is a quantum bit, or “qubit.” While qubits can be in a superposition of 0 and 1, they are fragile. Measuring them destroys their superposition, forcing them into a definite state of either 0 or 1.

This collapse has a security benefit. Because measuring a qubit alters it, it’s theoretically impossible to copy the information it encodes without being detected. This “no-cloning rule” means any attempt to intercept a code based on qubits will destroy it, alerting the sender and receiver to a breach. In contrast, traditional bits can be copied perfectly without detection.

Entanglement takes superposition a step further. Two or more particles can be linked so that measuring a property of one instantly determines the property of the other, no matter how far apart they are. Imagine two gloves, each in a superposition of being black and white. When you observe one and it becomes black, the other, even if miles away, instantly becomes white.

What are the challenges of quantum cryptography?

Quantum cryptography requires highly sensitive, low-noise devices that can reliably send and record individual particles to transmit a message. However, no instrument is perfect, and these physical limitations create potential security flaws that an eavesdropper could exploit.

For instance, many systems use photons to encode information, but it’s difficult to create detectors that register every photon or light sources that emit only one photon at a time. Other challenges include minimizing false detections and building hardware resistant to tampering.

In theory, scientists can address many of these loopholes with more sophisticated, “device-independent” protocols whose security doesn’t depend on the physical properties of the devices. Building such devices is difficult, but NIST is developing key components like highly sensitive single-photon detectors.

These detectors operate near absolute zero to minimize noise, enabling them to record individual photons with high accuracy and promising applications far beyond quantum cryptography.