Quantum key distribution

Quantum key distribution (QKD) is a special way to send secret messages that uses the rules of tiny particles called quantum mechanics. It helps two people create a secret code that only they know, which they can use to hide their messages from others. This secret code is very safe because, in the world of quantum mechanics, trying to look at it changes it. This means if someone tries to sneak a peek at the secret code, the people sending the message will know, and they can stop the communication to keep things safe.

Unlike normal ways of hiding messages that depend on hard math problems, QKD’s safety comes from the basic rules of how tiny particles work. This makes it very strong and proven to be safe. However, QKD needs a regular, safe way to talk to each other before it can start, which is something already used in normal message hiding.

QKD doesn’t send the actual message; it only helps create the secret code needed to hide the message. This code can be used with many different hiding methods, making sure that only the people who know the code can read the message when it travels through normal communication channels.

Quantum key exchange

Quantum communication uses special particles called qubits, instead of regular bits used in normal communication. Often, these qubits are made using particles called photons. Quantum key distribution uses special properties of these qubits to keep information safe.

There are two main ways to do this. In one way, two people, named Alice and Bob, send special signals through a channel. Alice sends these signals, and Bob receives them. Later, they check if the signals match. If they do, it means no one tried to peek. If they don’t, they know someone might be trying to listen in.

In another way, Alice and Bob share pairs of connected particles. These particles are linked, so if someone tries to look at one, it changes both. This helps Alice and Bob know if their secret is safe. Both methods help create a secret code that only they know, keeping their messages private.

Information reconciliation and privacy amplification

Quantum key distribution helps two people, Alice and Bob, create a secret code that only they know. Sometimes, small mistakes happen, and their codes don’t match perfectly. These mistakes might be from someone trying to listen in or from problems with the equipment used to send the code.

To fix this, Alice and Bob use two special steps. The first step, called information reconciliation, helps them correct mistakes so their codes match. They compare parts of their codes over a public channel, finding and fixing differences without giving away too much information.

The second step, called privacy amplification, makes sure that even if someone tried to learn part of the code, they now know almost nothing about the final secret. They use a special method to create a shorter, new code that keeps the secret safe.

Implementations

Experimental

In 1991, researchers showed a special way to share secret codes using light. In 2008, they made this work very fast over long distances using special light pulses.

Since then, scientists have kept testing this idea in many places. They have sent secret codes over very long distances, even between islands and from satellites in space. In 2024, they sent secret codes through the air between South Africa and China, a distance of over 12,000 kilometers, using a tiny satellite.

Commercial

Many companies now sell this special way to share secret codes. In 2004, the first bank used it to send money. Since then, more places have started using it to keep information safe.

Quantum key distribution networks

DARPA

A network in the United States ran for four years, keeping information safe with this special method.

SECOQC

In 2008, a network in Vienna, Austria, used special light to keep information safe over a big area.

SwissQuantum

A network in Geneva, Switzerland, tested this method for almost two years to see how well it works.

Chinese networks

In China, they made a big network that connects many places. In 2016, they used a satellite to talk securely between China and Austria.

Tokyo QKD Network

A network in Tokyo, Japan, brought together companies and researchers from many countries to test this safe way to share information.

Los Alamos National Laboratory

A network in the United States uses a central hub to keep messages safe. Only the hub needs special equipment to receive the secret codes.

Singapore's National Quantum-Safe Network Plus (NQSN+)

In 2023, Singapore started a new network to help businesses keep their information safe using this special method.

Eagle-1

In late 2026 or early 2027, a satellite called Eagle-1 will be launched to test this method from space.

Attacks and security proofs

One simple way someone might try to interfere is by measuring the special signals sent between two friends, Alice and Bob, and then sending new signals to trick them. This can cause mistakes in their shared secret code. However, Alice and Bob can find these mistakes by checking some of their code together.

Another way someone might try to interfere is by pretending to be Alice or Bob to trick the other. To prevent this, Alice and Bob need to have a secret way to verify each other's identity before they start sharing their code.

There are also tricks where an attacker might use extra signals to learn more about the secret code. Even with these tricks, Alice and Bob can still create a safe secret code if they use special methods to check for interference.

Because the special signals need a direct path to travel safely, someone could try to block that path to stop communication. This is why creating networks with many paths can help keep the communication going even if one path is blocked.

Researchers have studied many ways attackers might try to learn the secret code, and they have proven that with the right safeguards, Alice and Bob can keep their code safe no matter what the attacker tries.

Quantum hacking

People try to find ways to trick quantum key distribution systems. They look for weak spots in how the system works or in the tools used to build it. For example, if someone can change the tools, they might make the keys less safe.

One common trick is called a Trojan horse attack. This does not need someone to touch the tools directly. Instead, a person sends extra light toward one of the users. This light can show information about how the user’s tools are set up. But there are ways to notice this trick, like checking for extra light coming into the system.

Researchers have found many other tricks, like faked-state attacks and time-shift attacks. Some of these tricks have even been tried on real, sold quantum key distribution systems. These discoveries help scientists find better ways to keep quantum communications safe.

Main article: Random number generator attack
Main article: Trojan horse
Main article: Norwegian University of Science and Technology
Main article: Max Planck Institute for the Science of Light
Main article: Avalanche photodiodes

Counterfactual quantum key distribution

See also: Counterfactual quantum computation

Scientists have found a way to share secret messages safely even when the particles carrying the message never actually travel between the two people. This method was created by Tae-Gon Noh. In this method, one person, named Alice, creates a tiny particle of light, called a photon, that exists in two places at once. One path stays with Alice, and the other goes to another person, named Bob. By looking at which particles Bob does not receive, Alice and Bob can create a secret code that others cannot discover. This works because, in the world of tiny particles, just the possibility that something might happen can still have an effect, even if it does not actually happen. This idea is also used in experiments where scientists can find out if a bomb will go off without actually lighting it.

Main article: Interaction-free measurement
Main articles: Bomb testing problem, Counterfactual

History

Quantum cryptography was first suggested by Stephen Wiesner at Columbia University in New York during the early 1970s. He wrote a paper called "Conjugate Coding," which was not accepted at first but was published in 1983. In this paper, he explained how to send two hidden messages using light properties, like straight and curved waves, showing how this could create special bank notes that cannot be copied.

Ten years later, Charles H. Bennett from IBM and Gilles Brassard from the University of Montreal built on Wiesner's ideas to create a way for two people to share a secret code safely. Then in 1990, Artur Ekert, a student at Wolfson College, University of Oxford, came up with another method using a special connection between particles called quantum entanglement.

Stephen Wiesner, Charles H. Bennett, Thomas J. Watson Research Center, Gilles Brassard, University of Montreal, Wolfson College, University of Oxford

Challenges

Even though quantum key distribution (QKD) has made progress, there are still some problems to solve before it can be used widely. One big problem is that making, sending, and measuring quantum states is not perfect. Because of this, it can be hard to tell if someone is trying to listen in or if the mistakes are just normal errors.

These problems affect how fast a secret key can be made, how far it can be sent, how much it costs, and how safe it really is. Even though new ways to do QKD have been suggested, using it on a large scale is still hard because of signal loss and other technical issues. Because of these challenges, some governments do not recommend using QKD for important things like military or business communications. To make QKD useful for everyday secure messages, scientists need to find better ways to solve these problems.

Future

Current quantum key distribution systems are mainly used by governments and big companies that need very strong security. These systems can tell if someone tries to spy on the secret keys they create, unlike older methods that can't prove if the keys were seen by others.

However, quantum key distribution has some challenges. It is expensive, needs special equipment, and isn't easy to use widely. Some important groups, like the USA National Security Agency and the European Union Agency for Cybersecurity, suggest using other methods called "post-quantum cryptography" instead. These methods are cheaper and can work better with existing technology. While quantum key distribution is interesting, it may not be the best choice for everyone right now.

Related physical layer technologies

Quantum Key Distribution (QKD) helps create secure keys, but other technologies focus on protecting the actual data being sent. Some new methods, like optical chaos or optical steganography, hide the data by mixing it with noise, making it hard to catch.

These methods make the data look like normal static, so attackers cannot record or save it. Tests have shown this can work at very fast speeds over long distances in regular networks, keeping data safe without needing special setup.