What is Quantum Key Distribution?

Shaan Ray
Published in
4 min readMay 12, 2020


We have long sought secure ways to exchange data. Some current methods include cryptography, hashing and requiring the solution of math problems that demand enormous computing power. Quantum computing could render some of our current methods insecure and obsolete, while enabling new methods.


Cryptography uses codes to protect information and communications. Data is encrypted using a secret key. The message as well as the secret key is given to the recipient. The recipient then uses the key to decrypt the message. The problem with cryptography is that if the key is compromised, anyone with the key can decipher the secret message.


Hashing is a protocol used in cryptography. It reveals the integrity of data by creating a ‘digital fingerprint’ of the original data. This is useful for verification (for example, to sign into your e-mail). The e-mail provider compares a hash of your data to a hash of your original data to verify that it is the same. So, the e-mail provider can verify that the person signing in knows the password, without storing your actual password (it stores only a hash of it). To learn more about cryptographic hashing, see my article here. Also see my article on merkle trees, which use hashing extensively.

Conventional key distribution

Current ciphers used to distribute keys use math problems to protect themselves from attackers. The problems are fairly simple to solve but need a large amount of processing power to complete. It’s easy to find the product of two prime numbers, but difficult to factor the product and derive the two prime numbers.

Such conventional key distribution methods will be rendered obsolete by advances in CPU power and the rise of quantum computers.

Quantum Threats to Encryption

Quantum computing will likely be able to break any encryption and hashing algorithms easily. It could also impact the integrity of blockchains. Quantum computing will challenge both asymmetric cryptography as well as hashing algorithms.

Enter quantum cryptography, which may enable the secure exchange of information, even in the presence of quantum computers.

Heisenberg Uncertainty Principle

Quantum key distribution proceeds by using light particles exchanged between sender and recipient to establish a key. This protocol uses logic outlined in the Heisenberg uncertainty principle or indeterminacy principle, which states that: “the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory.”

Quantum Key Distribution

Quantum key distribution addresses the challenges of distributing keys using quantum protocols. It is built based on laws of nature that are resistant to increasing computational power. These are physical processes that are not vulnerable to powerful computing systems.

Quantum Key Distribution is an optical technology which automates the delivery of encryption keys between parties that are sharing an optical link. There are two types of quantum key distribution systems:

  1. Discrete-variable quantum key distribution
  2. Continuous-variable quantum key distribution

How Does Quantum Key Distribution Work?

Quantum mechanics has a core characteristic: in quantum systems, the act of measuring the system disturbs the system. Two parties engaged in creating a key exchange protocol will require:

  1. An authenticated communication link between the two parties
  2. A channel to send quantum states of light between the two parties; the channel should be made of free space or fiber
  3. A key exchange protocol which can detect errors or eavesdroppers using quantum properties, while calculating the amount of information which has been lost or intercepted

The transmitter sends the key to the receiver in the form of a stream of light particles, known as photons. These light particles have a spin which can be changed depending on which filter they are passed through. Polarized photons can be polarized in 4 different directions (vertical, horizontal, diagonal, left/right). The recipient measures which direction they are polarized in using two different polarized detectors randomly.

At the end of the process, the recipient will have a key of 1s and 0s. They then get on a call with the sender and compare notes on which detector they used for each photon. They throw out results in which they did not use matching detectors and filters, and keep the ones in which they used matching detectors. They now have a sequence of identically polarized and measured photons — thus forming the final key. Using this key, the recipient can decrypt the message which has been sent to them.

Transmission Losses

Both types of quantum key distribution protocols face the problem of transmission losses over large distances. Transmission losses increase exponentially as the distance increases.


Quantum Key Distribution holds enormous potential for secure data transfer. Researchers are refining both discrete-variable quantum key distribution and continuous-variable quantum key distribution. Once the distance related losses are reduced, this technology will power the next generation of secure, quantum computer-proof data transfer.

Shaan Ray

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