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Guide: Quantum Key Distribution (QKD) and How it Works

The guide explains the concept of QKD in detail along with its workings and applications.

Quantum computing and supermacy may still be a race, but quantum cryptography is a reality and is here to stay

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The research on quantum mechanics was path-breaking a century ago. The applications of quantum mechanics are disruptive technologies of this century. One of the most anticipated and probably the most talked about applications is quantum computing. Quantum computing uses fundamental principles of quantum mechanics to perform complex processing.

The potential benefits of quantum computing, from advances in cancer research, and gene studies to unlocking the mysteries of the universe, are limitless. But that same computing power can be used to unlock secrets - from one’s finances, and health records, to corporate research projects and classified government intelligence.

We live in a highly digitised global society and data is the backbone of the digital economy. Data has virtually become the new oil. The value of confidential data is tempting enough for malicious hackers to harvest this data today. Even if they decrypt the data at some point in the future, the benefits reaped will surpass the efforts of harvesting.

The need for safeguarding data seeks immediate attention and governments, corporations, healthcare providers, and financial institutions are actively looking for solutions.

Even with heavy investments in cybersecurity, avoiding breaches has become difficult. The number of breaches is increasing every passing year and the economic impact of these breaches has exploded.

The Looming Threat

The greatest impact of the quantum revolution will be felt in cryptography. A sufficiently large quantum computer running the existing Shor’s algorithm can crack RSA or Diffie Hellman system of encoding in minutes as opposed to millions of years by brute force method today.

Symmetric key systems such as AES, 3DES, etc. which are used for end-to-end bulk encryptors are more resilient, however, even these can be cracked in relatively quicker time frames by running Grover’s algorithm on a quantum computer. Even these systems use Diffie Hellman for key exchange which will become completely insecure.


The threat of a quantum attack on our existing communication channels looms large. Quantum cryptography is the only possible solution to this problem. Quantum cryptography is a process of encryption using properties of quantum mechanics to transmit data securely, making it hack-proof.

Why is Quantum Cryptography Important?

Experts agree it would take a 2,000-qubit fully fault-tolerant system to be theoretically capable of breaking some public-key cryptography algorithms, such as the 1,024-bit RSA or 256-bit ECC.

That might be a step down into the future. But, the impact of quantum computing is not restricted to public encryption alone. A company’s encryption system is layered. Various layers above the public infrastructure will play a role in overall data security. It can be best explained with a pyramid structure.

quantum pyramid structure

Conventional encryption methods will not contribute to building quantum-safe cryptography. Quantum physics equips us not only with the required principles to battle with future quantum computers but also equips us to make the whole crypto-system unbreakable.

Quantum physics by its very nature is very different from classical physics. Using these new laws, the keys in cryptography attain perfection with unconditional security.

What is QKD?

QKD works on the quantum principle of entanglement. The two quantum particles are correlated such that the state of one particle affects the state of the other particle, even when they are separated by a large distance. In QKD, the two parties, Alice and Bob, use a pair of entangled particles to establish a secure communication channel.

Photos are the quantum particles used in the shared key. Any attempt to intercept or measure the photons during the transmission would disturb their state, and the disturbance would be detected by Alice and Bob, alerting them to the presence of an eavesdropper.


In the whole communication, the key is never transacted which makes the probability of the eavesdropper accessing the key equal Zero. The symmetric key always remains as a shared secret between the QKD at both ends.

The key is securely transferred to the appliances/applications that need the key for secure communication. QKD does not provide encryption or key management. The key is generated in real time and this mechanism does not use a stored key. The key can be used directly for encryption or as an input to seed which will generate a key.

How Does QKD Work?

The QKD mechanism allows organisations to generate and manage their quantum keys. In their base model, this allows an organisation to implement an architecture to communicate and transmit secure keys over traditional channels. It enables organisations to secure the keys in use while still leveraging the standard communication channels. In this way even if the standard channel is compromised the keys are still secure.

The concept of QKD is similar to that of symmetric key cryptography except for the fact that the key is shared over a quantum channel and the data is shared over a public channel. One of the challenges for QKD is the distance over which the photons can travel, which is typically around 100 KMS.

Several organisations globally have circumvented this restriction and shared keys for up to 500 KMS using newer protocols such as Sending-or not-Sending Twin Field (SNS-TF).

Applications of QKD

QKD provides a secure communication channel that is immune to cyber-attacks. Some of the industries in which QKD applies are:

How can QNu Labs Help?

Armos is a state-of-the-art QKD device engineered using the principles of quantum mechanics by the experienced team at QNu Labs. It is a highly sophisticated engineering innovation that involves the power of complex deep-tech technologies (such as semiconductors, optoelectronics, high-precision electronics, complex software algorithms, and quantum physics) that work together to create unconditional security.

Armos uses a series of randomly generated photons in a quantum state to exchange quantum information. They produce the same key at both ends of the cable. An attempt to read the quantum state of a photon causes the keys to collapse. Subsequent key generation stops and the admin is notified of a possible attack on the channel. The simultaneous generation of keys at both ends makes eavesdropping a futile exercise. Armos fully encapsulates quantum principles and provides a robust technology you can trust.

QNu has the Quantum Umbrella ready!