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A Quick Overview of Quantum Cryptography for Beginners

What is Quantum Cryptography?

Cryptography is the procedure of encrypting and safeguarding a set of data with the aim of only permitting the individual possessing the correct secret key to decrypt it. In simple terms, you could equate cryptography to the process of converting plain text into scrambled ones so that no one besides the intended recipient will be able to decipher and thereby access the message.

Quantum cryptography (also known as quantum encryption or quantum key distribution), by extension, is a different type of cryptographic system that employs quantum mechanics principles for data encryption. It differs from conventional cryptographic systems as physics-related principles form the basis of its security models instead of mathematical ones. To elaborate, the fundamental tenets playing a role in how quantum mechanics function are as follows:

  • Particles making up the universe are inherently uncertain and could exist simultaneously in more than one state and/or place.
  • Photos are created randomly in either one of the two quantum states.
  • A quantum property cannot be measured without it being disturbed or changed.
  • Specific quantum properties of a particle, but not the entire particle, can be replicated.

All in all, quantum cryptography takes advantage of a particle’s multiple states alongside its “no change theory” (meaning that it can never be unknowingly interrupted). With these principles, quantum cryptography becomes intruder-proof because no one can view or copy the quantum state-encoded data without the system automatically alerting the sender or receiver. Nonetheless, performing quantum cryptography requires immense computing power to enable the encryption and decryption of data. Thus, a quantum computer is a necessity for performing these tasks.

How Does Quantum Cryptography Work?

Quantum cryptography utilizes a series of individual light particles termed photons to transmit data from the sender to the recipient via a fiber optic cable. The key reason for that is that photons’ behavior has been well-studied. Meanwhile, cables do not have to be secured as photons possess a randomized quantum state. As a result, they have all the qualities required to serve as the optimal information carrier through fiber optic wires.

 In the case of quantum cryptography, these photons essentially represent binary bits. By comparing measurements of the properties of a fraction of these photons, both endpoints can identify the key used and determine if it is secured for decryption. Let’s break the process down into steps to help you better visualize the technique:

  1. Photons will be transmitted from the sender via a polariser (i.e., a filter) that will randomly assign one of four types of polarisations alongside a bit designation, namely horizontal (zero bit), vertical (one bit), 45-degree right (one bit), or 45-degree left (zero bit).
  2. The photons will make their way to the receiver, which employs two beam splitters (vertical/horizontal and diagonal) to analyze the polarisation of each photon. Since the reader would not know the specific beam splitter to be used for each photon, they would have to make a guess on which to use.
  3. Once the photons have been transmitted, the receiver will inform the sender of the beam splitter used for each photon based on the sequence in which the stream of photons was sent. Consequently, the sender can then use that piece of information to be compared with the arrangement of polarisers used to transmit the key. Henceforth, the photons that were read with the incorrect beam splitter would be discarded, and the resulting sequence of photons will serve as the quantum key.

Should the photon be read or duplicated in any way by an unintended recipient or eavesdropper, the polarisation of the photons will be altered. As a result, errors will be incorporated into the quantum key, and both endpoints will notice the changes immediately. With that, both parties will be well-informed of the message’s compromised security.

On that note, if the progress of quantum cryptography as a whole continues to fascinate you, do consider checking out the works by The Quantum Insider to stay updated on the happenings in this space!

Why Does Quantum Cryptography Matter So Much?

Given that the necessity for an unbreakable encryption method becomes increasingly prominent and severe, quantum cryptography will serve as the solution for safeguarding sensitive data from malicious actors in the near future. This is due to its ability to permit much more secure communication between users than traditional cryptography.

As a matter of fact, several corporates and governments across the globe are already in the quantum arms race (i.e., the race to engineer the first-ever usable quantum computer) with the hopes of devising a novel technology solving all variants of computing issues in a simplified manner instead of adhering to the approach and solutions provided via classical computers. In particular, some of the most significant technological breakthroughs recorded in this field thus far would include the replacement of special pipes with existing fibre networks for quantum cryptography practices.

Nevertheless, the technology is still in its infancy stages. As a result, practising quantum cryptography would necessitate the use of expensive equipment to transmit as well as receive a series of photons. As a matter of fact, each customer will be required to purchase a transmitter alongside a receiver, which costs approximately US$100,000.  

However, the good news is that the price is expected to decline over time as a rising number of firms are predicted to eventually provide the hardware needed to conduct quantum cryptography. In particular, there has been a number of case studies showcasing the successful implementation of quantum cryptography, particularly amongst prominent institutions:

  • The first-ever quantum network in the United States was launched by an organisation called Quantum Xchange, which established a 1000-kilometre-long fibre optic line for its quantum network. Other corporations, including Quintessence Labs, MagiQ Technologies, and ID Quantique, have followed suit since then in developing in-house commercial quantum cryptography systems.
  • Harvard University, Boston University, and IBM Research developed and operated a 10-node quantum cryptography network known as the Defense Advanced Research Projects Agency Quantum Network between 2002 and 2007.
  • Toshiba Corporation and the University of Cambridge developed a high-bit quantum cryptography system that employs the BB84 quantum cryptography protocol.
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