Qubit By Qubit

An Introduction To Quantum Key Distribution (QKD)

Pawel Czerwinski on Unsplash.com

Quantum physics upended classical physics. It is now upending the race to technological superiority. In a recent MIT Technology Review article, Mark Harris-the author wrote about Google enlisting NASA’s help for quantum supremacy. All superpowers today are competing at the edge of the technology frontier. That’s because the next battlefield is a technological one. It is also a bit like Mixed Martial Arts (MMA) because real strength lies at the intersection of many multidisciplinary technologies i.e. quantum physics and cryptography, gene editing and eugenics etc. While this article doesn’t attempt to explain the complete battlefield, it tries to simplify and break down one of the key technologies reshaping it- Quantum Key Distribution (QKD).

Quantum computing will upend the internet as we know it. The web, in its initial avatar i.e. Web 1.0 consisted of static pages. Web 2.0, powered by the telecom revolution (4G, LTE), resulted in rich dynamic web pages with multimedia content. Web 3.0 will create an Internet of Everything — a combination of the Internet of Value (where crypto currencies and anything of value can be transmitted in its virtual form, using Augmented Reality, over the Blockchain) and the Internet of Things (with zettabytes of data generated by millions of sensors monitoring our every waking moment). Life, literally, will be on the internet. Data will be as ubiquitous as thin air. Such a future needs a future proof encryption method which many believe to be quantum encryption.

The Problem With Encryption Today

There are basically two types of encryption ie symmetric and asymmetric.

Symmetric key encryption uses the same “secret” key for both encryption and decryption. For a very simpliatic example, let’s say we want to use numbers to denote alphabets. Now, let’s say Alice wants to send Bob the word “yes” it would be encrypted as “25 5 19". Alice and Bob should know the secret key. So, Bob can decyrpt Alice’s message easily. Of course, symmetric key encryption isn’t that simple. As the number of users in a network increases, the number of keys have to increase and at a point, it becomes hard to manage those keys. Asymmetric or “public key” encryption uses two different sets of keys to encrypt and decrypt data. Every user in asymmetric encryption will have two sets of keys each I.e. a “public” key and a “private key”.

Modern day encryption uses public key algorithms. As per wikipedia, ‘These algorithms are fundamental security ingredients in cryptosystems, applications and protocols. They underpin various Internet standards, such as Transport Layer Security (TLS), S/MIME, PGP, and GPG. Some public key algorithms provide key distribution and secrecy (e.g., Diffie–Hellman key exchange), some provide digital signatures (e.g., Digital Signature Algorithm), and some provide both (e.g., RSA)’


Modern cryptographic algorithms are based on finding ‘factors’ of larger prime numbers. For example, if you multiply 87*13, you get 1131. It is easy to multiply two prime numbers but the reverse case i.e. decomposing 1131 into its prime factors 87*13 is not that easy.You would have to try all of the primes that are less than 1131 until you found which prime numbers that when multiplied together come to 1131. This is just a simple example but for large prime numbers, it is incredibly complex for today’s computers to factorize.

Shor’s algorithm is a quantum (requires quantum computers) algorithm developed to find the prime factors of a large prime number N. It was developed in 1994 but with the advent of quantum computing, the algorithm is likely to become a reality. What this means is that all the existing encryption will be vulnerable to attack and privacy of data will mean nothing.

The Building Blocks

Fundamentally, quantum physics is used to describe matter and energy at the sub-atomic level whereas classical physics explains the observable physical universe including the sun and the moon.

The word quantum comes from the Latin word ‘quantus’ meaning ‘how great’ or ‘how much’. In physics, quantum means the minimum amount of phyisical energy in any interaction. For example, the elementary particle ‘photon’ is the minimum quantum of light. The hypothesis of quantization means that any physical property can take on only discrete (not continuous) integer values in multiples of of one quantum. As per wikipedia, ‘Quantization of energy and its influence on how energy and matter interact (quantum electrodynamics) is part of the fundamental framework for understanding and describing nature’. Quantum physics evolved over a long period of time and draws upon the works of many eminent scientists i.e. Ludwig Boltzmann, Herbert Heinrich, Niels Bohr, Albert Einstein, Max Planck, Erwin Schrodinger etc. Classical physics was unable to explain the Black Body spectrum of radiation over all frequencies. In 1901, Max Planck came up with a formula that helped crack the puzzle. As per Planck, the energy (E) of a photon is denoted by E=vh where E denotes the energy of a photon and v its velocity. The letter h denotes a constant number called ‘Plancks Constant’.

First, classical physics dealt with atoms and molecules. Quantum physics deals with sub-atomic particles i.e. life at a microscopic level. When we go to the sub-atomic level, classical physics no longer provides reliable answers. Sub-atomic particles are of two types i.e. elementary particles and composite particles. Elementary particles cannot be broken down into sub-particles and include the fundamental fermions (quarks, leptons, antiquarks, and antileptons), which generally are “matter particles” and “antimatter particles”, as well as the fundamental bosons (gauge bosons and the Higgs boson), which generally are “force particles” that mediate interactions among fermions. Composite particles are composed of elementary particles e.g. protons, neutrons, baryons etc.

Second, sub-atomic particles like photons and electrons behave both like a particle and a wave. In the quantum world, the trajectory of a particle like a photon is uncertain. Also, mere observation of its trajectory will change its properties. A famous experiment by Thomas Young called ‘the double slit experiment’ explained both the duality of nature and the observer phenomenon.

Thirdly, quantum indeterminacy can be quantitatively characterized by a probability distribution on the set of outcomes of measurements of an observable. In other words, quantum physics relies on probabilities instead of determinism.

Enter Quantum Key Distribution

Quantum computing uses all of the aforementioned concepts of quantum physics to store data in mini transistors called ‘qubits’ which are used in supercooled quantum computers. Instead of bits which can take a value of ‘0’ and ‘1’ , qubits can take the value of ‘0’ , ‘1’ or any superposition of the two. There are two fundamental qualities of particles that are used in quantum computing i.e. quantum superposition and quantum entanglement explained in the video below.


The words “Key Disribution” refer to the challenge of how do we distribute keys to the people who need them. The technique of data transmission using photons in order to generate a secure key at quantum level is usually referred as Quantum Key Distribution process. Sometimes QKD is also wrongly referenced as Quantum Cryptography. QKD is only a part of Quantum Cryptography. Quantum Key Distribution (QKD) is a method of generating and distributing keys using quantum physics. It is not used to transmit data. Thus, QKD can be used in combination with traditional encryption to encrypt and decrypt data.


In a QKD system, Alice sends a message (bits) encoded using streams of photons which use the polarization (horizontal or vertical) or spin (up, diagonal or down) of photons to encode the bits into a wrapper of photons. Polarizing a photon means passing the photon through a photon filter. The result is that the filter allows only one type of polarization. On the other end, Bob uses a detector to detect the photon values which can be converted to bits using polarization. The result may or may not be right because of probabilistic nature of quantum mechanics. Subsequently, Alice tells Bob which bits to keep. If an eavesdropper observes the stream of photons, it will change their state and the result will be a higher error rate thus alerting the parties of a hacking attempt.

Batelle On Youtube

There are different types of QKD protocols such as BB84 protocol, HDQKD E91 protocol etc which use similar or different types of properties (e.g. superposition or entanglement)

Is The Future Secure?

Quantum Key Distribution (QKD) is a very dense topic. Quantum Cryptography even more so. I have tried to create a collage that will hopefully help my readers understand the basics of QKD systems. However, as long as data is on the grid, the answer to the question: is it secure? is ‘no’. People can hack things through various means. The most vulnerable being the employees of a company. Even in today’s age, people have to be reminded to not leave their computers unlocked or not to share their passwords. Perhaps, there is no fool proof system.

Quantum key distribution is considered to be future proof (because many see the future of computing to be quantum computers) but not hack proof. MIT Technology Review reported hacking of quantum cryptography in 2010 titled ‘Experimental Demonstration Of Phase-Remapping Attack In A Practical Quantum Key Distribution System’. Early stage Venture Capital (VC), public and private companies are racing to build a quantum future. The website ‘Quantum Computing Report’ keeps a track of many entities involved in this technology race. When the stakes are that high, criminals are not far behind. The reason I wrote this article is not to show the incredibly complex technologies at play today and in the future. Rather, it is to help people like me who believe re-education is the only way to survive. Once we stop learning, we are obsolete. Obsolescence is also quantum in the sense that it is measured in nanoseconds. We have to constantly keep updating our knowledge lest we go the way of the dinosaurs.

The other important reason I wrote this article is to remind my readers that it is time to give up Game of Thrones Season 7 (honestly, I have never watched the show) and start learning something new. It is not wrong to unwind and watch a little bit of entertainment to destress. The issue is too much entertainment.

Lastly, the future is being created every second today perhaps by a kid with a super computer. If you believe in the ‘butterfly effect’ outlined in the chaos theory, that kid could literally be erasing a job somewhere else on the planet. My urge to everyone is to become a voracious reader to read more about kids that are reshaping the lives of millions of adults. If not, we might have the rug pulled out underneath our feet.

Writer @ The Intersection of Finance, Tech & Humanity. Stories of a Global Language: “Money”. Contributor @ Startup Grind, HackerNoon, HBR. Twitter@akothari_mba

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