Quantum Cryptography: Advantages and Disadvantages

This page explores the advantages and disadvantages of Quantum Cryptography, covering its benefits, drawbacks, and underlying principles.

What is Quantum Cryptography?

Quantum cryptography involves transmitting information with access restricted to the intended recipient, even if the transmitted message is intercepted by others. In classic cryptography, the sender and recipient share keys of a specific bit length (e.g., 128 bits). This key is used for encryption and decryption. Breaking the system is difficult because of the large number of possible keys. For example, a 128-bit key has 2¹²⁸ possibilities.

In quantum physics, light waves propagate as photons, which are massless and possess properties like energy, momentum, and spin (angular momentum). Spin can carry polarization. Based on the spin of photons, there are two modes of polarization: rectilinear and diagonal. These modes are used to map binary values 0 and 1. Each photon carries one qubit of information. Polarization can represent 0 or 1; in quantum computation, this is called a qubit. Unlike a classical bit that must be in a state of either 0 or 1, a qubit can be in both states 0 and 1 simultaneously.

To determine a photon’s polarization, the recipient must measure it by passing the photon through a filter. Like other cryptography systems, key distribution is a challenge in quantum cryptography.

Figure 1 illustrates a quantum cryptography system. Quantum keys (in the form of photons) are transmitted over a quantum channel, while encrypted messages are transmitted over a public channel.

How is the Key Shared?

A user (or sender) can suggest a key by sending a stream of randomly polarized photons. If someone tries to intercept the key, the recipient becomes aware of it, discards the received key, and asks the sender to retransmit a new stream of randomly polarized photons. Figure 2 depicts the quantum key distribution process.

Both the sender and the recipient have two polarizers each: one with a 0-90 degree basis (+) and one with a 45-135 degree basis (x). The sender uses polarizers to transmit random photons to the recipient in one of the four possible polarizations: 0, 45, 90, and 135 degrees. The recipient uses their polarizers to measure each polarization of the received photons.

Quantum cryptography derives its security from the fact that each qubit is carried by a single photon. Each photon will be altered as soon as it is read. Keys are shared using a quantum channel, and encrypted messages are sent using a public channel. Both the sender and recipient can discuss the results and their use of basis (rectilinear or diagonal) over a public channel to make the system more robust.

Benefits or Advantages of Quantum Cryptography

The following are the benefits or advantages of Quantum Cryptography:

• It revolutionizes secure communication by providing security based on the fundamental laws of physics instead of mathematical algorithms or computing technologies used today.
• It is virtually unhackable.
• It is simple to use.
• Fewer resources are needed to maintain it.
• It is used to detect eavesdropping in QKD (Quantum Key Distribution). This is because it is impossible to copy the data encoded in a quantum state. If someone tries to read such encoded data, the quantum state changes.
• The performance of such cryptography systems is continuously improving, resulting in its quick adoption in encrypting the most valuable secrets of governments and industries.

Drawbacks or Disadvantages of Quantum Cryptography

The following are the drawbacks or disadvantages of Quantum Cryptography:

• The worldwide implementation of quantum cryptography could lead to job losses and increased unemployment.
• While traveling through the channel (i.e., optical fiber or air), the polarization of photons can change due to various causes.
• Quantum cryptography lacks many vital features, such as digital signatures and certified mail.
• A dedicated channel is a must between the source and destination, implying high costs. It is impossible to send keys to two or more different locations using a quantum channel because multiplexing goes against quantum principles. This demands separate channels between the source and many destinations, which is a major disadvantage of quantum communication through an optical channel.
• The largest distance supported by QKD is about 250 KM at a speed of 16 bps through a guided medium.