What is quantum cryptography? Simple explanation and real examples

Imagine a future where hackers can crack even the strongest encryption in seconds. That’s the kind of challenge quantum computers could bring, and it's pushing scientists to rethink how we protect sensitive data.

One of the most promising solutions? Quantum cryptography. In this quick-start guide, we’ll explain what it is, why it matters, how it’s being used in the real world, the challenges it faces, and what the future might look like in a quantum-powered world.

What is quantum cryptography in simple terms?

Quantum cryptography is a set of data encryption methods that use the principles of quantum mechanics to create a much more secure way to communicate, especially when compared to traditional encryption methods (symmetric and asymmetric encryption).

Traditional encryption relies on solving really complex math problems (like factoring huge numbers) to keep data safe. But the problem is, quantum computers are expected to solve those problems much faster, which could make our current encryption methods vulnerable.

That’s exactly why quantum cryptography is becoming so important.

Note: Quantum cryptography isn’t the same as quantum encryption. Cryptography refers to the broader concept of securing communication, which includes encryption, key exchange, and authentication. Encryption itself is the narrower process of converting data into ciphertext using a key, while decryption reverses it.

How does quantum cryptography work?

Quantum cryptography might sound super complex (and it is), but we can break it down in a simpler way.

Let’s say two people, Alice and Bob, want to exchange messages, files, or other sensitive data securely over the internet (which is a public network).

Alice knows that Bob has two special pairs of glasses (called beam splitters). Each pair lets him see different traits, like colors, in the same particles (photons) that Alice sends.

With that in mind, here are the key principles from quantum physics that make quantum cryptography work:

• Superposition: This means a single particle can exist in multiple states at the same time. So when Alice sends photons, Bob can look at them through his two glasses and get different results depending on which one he uses.
• Quantum entanglement: This allows the linkage of two or more quantum particles in such a way that determining the property of one allows you to obtain the other’s property. For instance, say Alice and Bob had two particles that could either be red or blue. Once Bob uses a pair of glasses that reveals one particle as blue, the other one (with Alice) assumes the red color, and vice versa.
• No-cloning theorem: It’s impossible to create an exact copy of particles in an unknown quantum state. So, if anyone tries to copy the particles that Alice sends to Bob, in a bid to break their encryption, both Alice and Bob will know.
• Observer effect: In quantum physics, simply observing a quantum system will alter its properties. That means if anyone tries to spy on the exchange, Alice and Bob will notice, because the photons will behave differently.

What are the types of quantum cryptography?

There are several types of quantum cryptography, each designed to secure communication in a different way:

Quantum key distribution (QKD)

Quantum key distribution offers a way to securely exchange encryption keys using the principles of quantum mechanics. It’s worth noting that QKD doesn’t generate encryption keys that are quantum-safe by themselves; it just protects the key exchange process.

Here’s a simplified example, continuing with the same assumptions about Alice and Bob:

1. Alice prepares a list of photons that can show multiple colors (polarizations), depending on which “glasses” (beam splitters) Bob uses to look at them.
2. She sends these photons to Bob over a public network without worrying if someone sees them (we’ll get to why that’s okay in a moment).
3. Bob receives the photons and chooses randomly which pair of glasses to use for each one.
4. Then, he tells Alice which glasses he used for each photon.
5. Since Alice knows which glasses would give reliable results for each photon she sent, they can compare notes and discard the ones measured with the “wrong” glasses, leaving only the matching results they both trust.
6. Now, without revealing the resulting colors publicly, Alice and Bob have a set of correlating results.
7. This shared sequence becomes their secret key used to encrypt and decrypt future communications.

Alice is confident sending the photons over a public network thanks to two key quantum principles: the no-cloning theorem and the observer effect. These ensure that if anyone tries to intercept or copy the photons, Alice and Bob will know because the act of observing the photons changes them.

Quantum coin-flipping

Quantum coin-flipping is a cryptographic protocol that lets two parties who don’t fully trust each other agree on a random outcome, like flipping a coin, without either side being able to cheat.

Imagine Alice and Bob want to decide who buys dinner. Normally, if Alice has the coin, Bob has to trust that she reports the result honestly. In the quantum version, they use quantum bits (qubits) instead of a physical coin. Unlike regular bits, which are always 0 or 1, qubits can exist in a superposition (a mix of both) until they’re measured.

This unpredictability ensures that neither Alice nor Bob can know or fix the outcome in advance. And because of the no-cloning theorem and the observer effect (which we saw in QKD), any attempt to copy or tamper with the exchanged qubits would be detectable. The result is both random and verifiable, giving them a fair coin toss even in a low-trust situation.

Other types of quantum cryptography

Besides quantum coin-flipping and QKD, researchers are exploring other quantum cryptography methods, such as:

• Device-independent quantum cryptography: This approach allows any two users to create and share secret communication keys generated on any quantum computer, regardless of whether the computers are considered secure or not, without compromising security.
• Key-Encryption-Key (KEK) protocol: The KEK protocol is designed to protect encryption keys by encrypting them with another key. Think of it like putting the key to your safe inside a locked vault and then locking that vault inside another one.

Why is quantum cryptography important?

Quantum cryptography is crucial for protecting data privacy, securing communications, and ensuring message integrity as we move toward the era of quantum computers.

Even before powerful quantum computers arrive, there’s already a risk from harvest-now, decrypt-later attacks. In this scenario, attackers collect and store encrypted data today (whether it’s intellectual property, financial information, or even state secrets) with the goal of decrypting it in the future once quantum computers are advanced enough to break current encryption.

It also solves long-standing problems in security: how to exchange keys safely over public channels, how to detect eavesdropping attempts immediately, and how to maintain fairness in low-trust settings. Its main advantage over classical cryptography is that it doesn’t rely on math problems staying hard; instead, its strength comes directly from the physical laws of quantum mechanics.

Researchers have already found a way to factor a 2048-bit RSA key in merely 8 hours (rather than trillions of years) by using a quantum computer. This makes it clear why we need to prepare now. Quantum cryptography provides one of the most promising ways forward.

What are the advantages and disadvantages of quantum cryptography?

Quantum cryptography isn’t a magic bullet; it comes with strengths and weaknesses. On one hand, it offers future-proof protection against quantum threats and makes eavesdropping detectable. On the other, it faces technical hurdles, cost issues, and scaling challenges that limit how quickly it can be adopted in the real world.

Key advantages

• Future-proof protection: Quantum cryptography promises secure online exchanges and interactions that are safe from quantum computer hacks.
• Prevents undetected eavesdropping: Both sides of a conversation can know when another entity starts eavesdropping on their conversation.
• Secure key distribution: Two sides of a conversation can securely share encryption keys without worrying about attackers breaking them using quantum computers.
• Faster performance: Photons, on which quantum cryptography relies, are extremely fast since they can move at the speed of light.

Main disadvantages

• Implementation cost: Quantum cryptography requires specialized expertise, hardware, and software, all of which can drive up the implementation cost.
• Complexity: Quantum cryptography is inherently complex, as the principles of physics governing it are still being extensively studied.
• Distance limitation: Maintaining quantum entanglement over great distances is currently limited, making it impractical for long-range communications.
• Eavesdropping loopholes: A third party could isolate some photons from a quantum beam of light and measure them without detection. Instead, the change in state from this photon-harvesting attack would be chalked down to noise, which could typically occur when light travels a distance between the communicating parties.

Is quantum cryptography used today?

Quantum cryptography is yet to gain widespread adoption, but it’s already been introduced in some systems today, including:

• Samsung Galaxy Quantum: Samsung introduced quantum cryptography into its Galaxy Quantum line of devices via a Quantum Random Number Generator (QRNG), which aids secure authentication and encryption key generation.
• Hyundai Heavy Industries: HHI implemented quantum cryptography to reinforce the security level of the defense technology.
• Verizon: The company has begun testing quantum key distribution, claiming it successfully sent encrypted streaming video from a 5G Lab to two of its East Coast offices.

That said, organizations like the National Security Agency (NSA) don’t support using quantum cryptography until its limitations are properly understood and mitigated, leaving room for future research. This caution applies only to physics-based quantum cryptography, not to post-quantum cryptography, which relies on standardized mathematical algorithms and is already being adopted in real-world systems.

What is the future of quantum cryptography?

Quantum cryptography holds huge promise, as it’s theoretically far more secure than classical cryptographic algorithms. But before it can become widely adopted, researchers still need to overcome several hurdles:

• Extending distance limits: Quantum entanglement currently works only across relatively short ranges. To make quantum cryptography practical for global communication, it needs to support long-distance exchanges.
• Reducing hardware dependency: Right now, most implementations require specialized (and expensive) hardware. Researchers are exploring ways to make quantum cryptography more compatible with existing network infrastructure to cut costs.
• Better authentication methods: While QKD secures key exchange, new methods are still needed to confirm that the keys truly come from the right source.
• Software integration: Progress depends on making quantum cryptography work seamlessly with software systems, not just hardware setups.
• Minimizing hardware errors: Current quantum devices introduce errors that can reduce both security and reliability, so more robust systems are still being developed.

What is post-quantum cryptography?

Post-quantum cryptography (PQC) refers to encryption algorithms designed to withstand attacks from both classical and quantum computers. Unlike quantum cryptography, which uses the physics of quantum mechanics (like photons and entanglement), PQC is built on complex mathematical problems that are believed to be hard even for quantum machines to solve.

Governments and organizations worldwide, led by efforts like the NIST Post-Quantum Cryptography Standardization project, are already standardizing algorithms such as ML-KEM (Kyber) for encryption and ML-DSA (Dilithium) for digital signatures. These tools are intended to replace or complement today’s public-key cryptography (like RSA and ECC), ensuring that data remains secure against quantum attacks while still running on classical computers.

Learn more: Read more details about post-quantum cryptography.

How does quantum cryptography compare to quantum computing?

Quantum computing and quantum cryptography are related but serve very different purposes.

Quantum computing is about building powerful new computers that use qubits to solve problems far faster than classical machines. This includes breaking many of today’s encryption algorithms, which rely on mathematical problems that quantum computers can crack efficiently with algorithms like Shor’s (for factoring) or Grover’s (for searching).

Quantum cryptography, on the other hand, uses the same principles of quantum physics not to compute faster but to secure communication. Techniques like quantum key distribution make it possible to share encryption keys in a way that even a quantum computer can’t break.

Who invented quantum cryptography?

In the early 1970s, Stephen Weisner proposed quantum cryptography. This proposal was first rejected by IEEE Information Theory before it was published by SIGACT News, a quarterly newsletter for theoretical computing, in 1983.

At this time, other scientists and researchers had started to take interest in quantum cryptography, and the first recorded quantum encryption protocol, the BB84 protocol, was published just a year later (in 1984).

One of the major milestones in quantum cryptography has been extending the distance and practicality of entanglement-based communication. In 2004, researchers in Austria demonstrated the world’s first quantum-encrypted financial transaction, transferring funds between Vienna City Hall and Bank Austria Creditanstalt 500 meters apart using entangled photons sent through fiber.

More than a decade later, in 2017, scientists working with the Micius satellite distributed entangled photons over 1200 kilometers, proving that quantum entanglement can remain viable across intercontinental distances. Together, these achievements highlight how far the field has progressed toward scalable, real-world quantum networks

Learn more: Read about the history of encryption: from ancient codes to digital security.