By Dr Balachandar Settu, Head, Research and Development, and Ramgopal Yadavalli, Executive Director, Aksh Optifibre

Light-wave communication plays a vital role in broadband and data centre networks, and has enabled fast and reliable exchange of information. Currently, in India, optical fibre communication (OFC) systems support several data transmission applications including voice, text, real-time video, internet, navigation for businesses and transportation.

Further, OFC systems can be stretched to meet numerous emerging services and applications, including control-operation of industries, medical diagnosis and treatment, smart homes, education, traffic safety and guidance, internet of things, artificial intelligence, fibre-to-the-home and blockchain. Several technologies are under development to ensure high security for data storage and transmit massive amounts of data over long distances at high speed.

This article briefly discusses one of the em­erging secured technologies, called qu­an­­­tum communication (QC), and the comparison between binary bit transmission and quantum bit transmission.

QC is the transfer of a quantum state from one location to another. It holds the promise of providing unbreakably secure communication. Quantum cryptography was discovered independently in the US and Europe. While the American approa­ch was based on coding in non-commuting observables, the European approach was based on correlations due to quantum entanglement.

In conventional OFC, information is encoded in the form of binary digital pulses with the possible states being either state 0 or state 1, and transferred from one point to another through optical fibre. The basic approach to secure communication bet­ween A and B is to establish a shared random binary key that only they know. The problem of secure communication is then reduced to the secure distribution of the shared key. The quantum app­roach to this distribution is referred to as quantum key distribution (QKD).

In simple terms, QKD can be done in a completely secure fashion because quantum information is so fragile that one cannot eavesdrop on a quantum channel without perturbing the quantum information that is being transmitted.

To be more precise, classical information relies on the concept of a binary bit, which can be in state 0 or state 1 and the possible permutation for N bits is 2N. In the QC system, information is transferred in the form of a quantum bit called Qubit. Qubit is a key ingredient in QC governed by the laws of quantum mechanics such as superposition and entanglement.

While classical information relies on the concept of a binary bit, quantum bits can be in both states (0 and 1) at the same time, a phenomenon referred to as a quantum superposition. A measurement of a Qubit can still only be 0 or 1 (with the probability of each outcome determined by the superposition, in which the measured Qubit is prepared). Moreover, the measurement destroys the underlying superposition by projecting it on the result of the measurement. Therefore, by measuring the transmitted state, an eavesdropper destroys the underlying state and, at the same time, does not acquire enough information to fully reconstruct it.

QC faces many challenges in developing source, quantum teleportation, detectors, multi-partite entanglement and QKD. Besides the relatively simple case of point-to–point QKD, all QC task require sources of entangled photons. Today, such sources are based on spontaneous parame­tric down-conversation and hence, are probabilities. The grand challenge is to develop handy and deterministic sources of entangled photons.

  • Quantum teleportation: This process allows one to transfer quantum states or quantum information using pre-established entanglement as a channel. In addition to the distribution of entanglement, teleportation requires joint measurement, which allows one to measure relative properties of a system. The correlation between the properties of two systems get “quantum correlated”, that is, entangled. The major challenge in quantum teleportation is to understand the degree of freedom of quantum state.
  • Detectors: QC requires excellent single-photon detectors. Recently, single-photon detectors have made huge progress with an improvement in efficiencies from 20 per cent to 90 per cent.
  • Multi-partite entanglement: Future co­m­­­plex quantum networks will routinely produce a multi-partite entangled state. So quantum correlation in complex networks remains to be explored.
  • QKD: QKD is the most advanced application of QC. It is already commercialised and offers guaranteed randomness and secrecy.

Going forward, quantum networks will look similar to the conventional internet system. One will be able to simply buy components and plug them together via optical fibres, all driven and synchronised by higher-order control software. Ran­dom­ness and secrecy will come for free. Entanglement, non-locality and teleportation will be common, and applications unthinkable today will proliferate.