Currently Symmetric and Asymmetric keys are the most popular tools in the digital world either to protect information or to transfer confidential information securely over commonly used storage media like USB or hard disks or through the network. However, a distinguished phenomenon in physics that is related to the sub atomic particles called Quantum entanglement, has paved way for secure communications using the quantum states of particles.
What is Quantum Entanglement?
In simple words, any subatomic particles will be spinning in all the states all the time. However when, a laser beam is fired through a certain type of crystal then individual sub atomic particles like photons form into a pair and exhibit quantum entanglement i.e. they spin relatively to each other. In this phenomenon, even if the particles are separated to greater distances even hundreds of miles away, both the particles have effect on each other. That is if particle A and B are entangled, and particle B is separated 100 miles away from particle A, then both of them are connected in their quantum state. In this state, if particle A is physically affected somehow then the same action will be seen at particle B that is 100 miles away from Particle A. This effect on the particles in a pair is called Quantum Entanglement, which has numerous advantages along with Cryptography.
How Quantum Entanglement can be used in Cryptography?
The science of using the Quantum properties of particles in cryptography is called Quantum Cryptography. The well-known method of cryptography or encryption in digital storage or data transfer is by using a pair of keys, called the public and private keys. Any data encrypted using private key can be decrypted using public key and vice versa. Hence, public keys are generally available publicly, however whoever has the private key alone will be able to decrypt the data encrypted using a public key. This is analogous to an email address and its password; email address can be publicly available, however only the password will unlock an email account to view its contents. The process of exchange and transfer of public keys over internet or any network is called Public key infrastructure, which helps us securely communicate over the unreliable network of the internet.
In a similar way, quantum key distribution defines the process of key exchange securely using quantum properties of particles. In this process, encryption keys are coded in the states of photons and transmitted to two different parties using the process of entanglement. Therefore, both the parties receive same encryption information in the form of spin states of the photon. The security provided by this mechanism is the two particles need not be transferred using any media or through the internet and if someone tries to tamper the state of one particle the pair, the other particles will immediately change its state and make the encryption key unusable.
To illustrate with a live example, in June 2017, Chinese scientists were able to transfer entangled particles from Satellite to Earth stations, which are separated by more than 1200 kilometers. The same process can be applied to transfer cryptographic keys encoded in particle states. Though this is an ambiguous topic, I have tried to explain as simple as possible. If anyone is more interested in Quantum cryptography can refer to the following links: