Quantum cryptography, sometimes called quantum encryption, applies quantum mechanics principles to encrypt and decrypt sensitive information securely.

Quantum cryptography has various unique properties that make it the ultimate safeguard against unauthorized eavesdropping. For example, it is impossible to copy data encoded in a quantum state.

Any attempt at reading or replicating the encoded data will result in a quantum state change caused by a wave function collapse (no-cloning theorem). The state change will, in turn, introduce detectable anomalies that reveal the presence of an unauthorized third party.

Quantum computing in general, and quantum cryptography in particular, have recently moved from the theoretical to the practical – so much so, that we’re starting to see many applications of quantum computing and quantum cryptography in use today.

Here, we’ll share a few real-world applications that are being seen, including:

- Quantum key distribution
- Mistrustful quantum cryptography
- Quantum coin flipping
- Quantum commitment
- Bounded- and noisy-quantum-storage model
- Position-based quantum cryptography
- Device-independent quantum cryptography

## Quantum Computing In The Real World

In a series of articles on the development and real-world applications of quantum computers, quantum cryptography and cyber security, Deloitte note that “We are entering a fascinating period in the development of quantum computers. Quantum systems are scaling up in both size and reliability.”

The momentum has really shifted of late. Baidu has released a quantum machine learning toolkit on GitHub. IBM, Google, Alibaba, Microsoft, Amazon and others provide Quantum-as-a-Service (QaaS) cloud computing.

In fact, according to Bob Sutor, IBM’s “chief quantum exponent,” the company now has about 20 quantum computers hooked up to the cloud and is offering free access to about half of them so researchers and the general public can experiment – while its paid, higher-end machines are used by the likes of ExxonMobil, Goldman Sachs, Daimler and Boeing.

The company also announced a partnership where “IBM plans to install its first private-sector, on premises quantum computing system in the U.S. at Cleveland Clinic. Cleveland Clinic also plans to receive first, next-generation IBM 1,000+ qubit quantum system in the coming years.”

Besides for quantum computing in general, real-world applications have been observed in several areas relating to quantum computing.

### Quantum Key Distribution

The best-known example of how our modern society uses quantum cryptography is quantum key distribution (QKD). This protected communication method enables the secure distribution of secret keys known only by the authorized parties.

QKD allows the two communicating users to detect the presence of any third party trying to “look” at the key.

Using quantum superpositions or quantum entanglement and transmitting information in quantum states, a communication system can be implemented that detects any attempts at spying.

Quantum key distribution allows only the production and distribution of a key, not transmitting any message data. This key can then be used in conjunction with an encryption algorithm to encrypt (and decrypt) a message.

Already this technology is starting to be used in practice.

Korean company SK Telecom announced the introduction – in partnership with Samsung – of the Galaxy Quantum2, its second smartphone equipped with quantum cryptography technology. This after the company announced it had successfully applied QKD technology to IP equipment, and completed the development of quantum virtual private network (VPN) technology.

Hyundai shipyard set up quantum cryptography communication to protect its defense technology. The world’s largest shipyard noted that “Quantum cryptography has emerged as an essential security solution for safeguarding critical information in the 5G era. Data encoded in a quantum state is virtually unhackable without quantum keys.”

In the U.S., Verizon recently conducted a trial of quantum key distribution (QKD) in Washington D.C. The company said the successful trial “positioned it as one of the first carriers in the U.S. to pilot the use of QKD”. This follows previous testing of this technology by Telefónica and Huawei.

These are just a few examples of Quantum Cryptography being used in the real world, and the list is growing every day.

What are other applications of the technology?

### Mistrustful Quantum Cryptography

What should you do when you’re unsure of the trustworthiness of another participating party? This is where mistrustful quantum cryptography comes in.

When both parties need reassurance that the opposite side is engaging with good intention, then mistrustful quantum cryptography is an excellent solution.

Let’s look at an example.

Bob and Alice are collaborating, and the project requires personal input from both parties, but neither party has a guarantee that the other won’t cheat.

Examples of tasks in mistrustful cryptography are commitment schemes (which allows one to commit to a chosen value while keeping it hidden to others) and secure computations (which enables parties to jointly compute a function over their inputs while keeping those inputs private).

### Quantum Coin Flipping

Another protocol that two participants who do not trust each other can use is quantum coin flipping. The transmission of qubits allows participants to exchange information and communicate via a quantum channel. Coin flipping aims to reduce the bias of a dishonest player (AKA cheating).

### Quantum Commitment

A commitment scheme (mentioned under mistrustful quantum cryptography) allows a party to commit to a specific value. The sender cannot change the fixed value, and the recipient cannot learn anything about the value until the sender reveals it.

Such commitment schemes are commonly used in cryptographic protocols like quantum coin flipping, zero-knowledge proof, secure two-party computation, and oblivious transfer.

### Bounded- And Noisy-Quantum-Storage Model

In the bounded quantum storage model, or BQSM for short, there is an assumption that some known constant Q limits the amount of quantum data that an adversary can store.

The protocol parties exchange more than Q qubits, and since storage is limited, a large part of the transferred data must either be measured or discarded. Forcing dishonest parties to measure a large part of the data allows the protocol to circumvent the impossibility result.

### Position-Based Quantum Cryptography

Here the geographical location of a player is used as its (only) credential. For instance, a participant sends a message to the receiving party at a known position to guarantee that the recipient can only read the data at the specified location.

Under the name of “quantum tagging”, the first position-based quantum schemes were investigated in 2002.. After that, the U.S. granted a patent in 2006, and the notion of using quantum effects for location verification first appeared in the scientific literature in 2010.

### Measurement-Device-Independent Quantum Cryptography

A unique protocol, often referred to as MDI, removes all attacks from the detection system, the most vulnerable part in many cryptographic implementations. As a result, security is held independently of the quality of the underlying physical devices.

This unique property makes measurement-device-independent quantum cryptography a good candidate for protection against malicious devices.

## The Future Of Applied Quantum Cryptography

While quantum cryptography is still being refined and developed, it is already clear that it is far superior to anything that came before it.

For now, quantum cryptography still feels “new” to a lot of people and businesses. Most users still rely on non-quantum applications, but technology is progressing in leaps and bounds, and the need for more advanced protection grows with it.

As quantum computing becomes mainstream, quantum cryptography is going to become one of the most sought-after technologies by both the public and private sector.