## What is Quantum Cryptography?

Transmitting information with access restricted to desired recipient even if transmitted message is intercepted by others. In classic cryptography, both sender and recipient share keys of few bits length, for example 128 bits long. This key is used for encryption and decryption process. Breaking the system is difficult due to large number of possible keys, for example for 128 bit long there are 2128 possibilities of the key used.

In quantum physics, light waves are propagated in the form of photons. The photons are massless and have properties such as energy, momentum and spin ("angular momentum"). Spin can carry polarization. Based on spin of photons there are two modes of polarization viz. rectilinear and diagonal. These modes are used to map binary values 0 and 1. Each photon carries one qubit of information. As mentioned polarization can be used to represent 0 or 1. In quantum computation it is called as qubit. But unlike a classical bit that must be in a state either 0 or 1, a qubit can be in both state 0 and state 1 at the same time. To determine photon's polarization, recipient must measure the polarization by passing it through the filter. Like other cryptography systems, key distribution is a challenge in quantum cryptography too. The figure-1 depicts quantum cryptography system. As shown, quantum keys (in the form of photons) are transmitted over quantum channel where as encrypted messages are transmitted over public channel.

How key is shared? A user (or sender) can suggest a key by sending stream of randomly polarized photons. If someone tries to intercept the key, the receipient comes to know and it will discard the received key and asks sender to retransmit new stream of randomly polarized photons.

The figure-2 depicts quantum key distribution process. Both sender and receipient have two polarizers each; one with 0-90 degree basis (+) and one with 45-135 degree basis (x). Sender uses polarizers to transmit randomly photons to the receipient in one of the four possible polarizations viz. 0, 45, 90, 135 degrees. The receipient uses his polarizers to measure each polarization of received photons.

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

## Benefits or advantages of Quantum Cryptography

Following are the benefits or advantages of Quantum Cryptography:
➨It revolutionizes secure communication by providing security based on fundamental laws of physics instead of mathematical algorithms or computing technologies used today.
➨It is virtually unhackable.
➨It is simple to use.
➨Less resources are needed in order to maintain it.
➨It is used to detect eavesdropping in QKD (Quantum Key Distribution). This is due to the fact that it is not possible to copy the data encoded in quantum state. If someone tries to read such encoded data then quantum state changes the existing state.
➨The performance of such cryptography systems is continuously improved. This results into its quick adoption in encrypting most valuable secrets of the government and industries.

## Drawbacks or disadvantages of Quantum Cryptography

Following are the drawbacks or disadvantages of Quantum Cryptography:
➨The world wide implementation of this can take up lots of jobs and hence unemployment will increase.
➨While traveling through the channel (i.e. optical fiber or air), there is possibility of change in polarization of photon due to various causes.
➨Quantum cryptography lacks many vital features such as digital signature, certified mail etc.
➨ Need of dedicated channel is must between source and destinations which implies high cost. It is impossible to send keys to two or more different locations using a quantum channel as multiplexing is against quantum's principles. This demands separate channels between source and many destinations. This is major disadvantage of quantum communication through optical channel.
➨The largest distance supported by QKD is about 250 KM at a speed of 16 bps through guided medium.