Quantum communication is the task of sending quantum information, typically represented by strings of qubits that can realized as entangled photons, from one point to another. Quantum information can be shared via quantum channels, realized for example by coherence-preserving optical fibres or transmission through free space, or even through classical channels via quantum teleportation or its close relative, entanglement swapping. Quantum communication is valuable for applications such as information-theoretically secure quantum cryptography employing quantum key distribution or sharing entanglement in a quantum network such as quantum secret sharing, secure quantum computing on the cloud , as a resource for teleportation in quantum computing and quantum-enhanced metrology.
Quantum communication over short distances such as within a laboratory is well developed, but quantum communication over long distance is a challenge. At the single-photon level, such as for single-photon sources or weak coherent states, the reach of quantum communication is finite due to losses and dark counts, and exceeding that bound requires extra technology such as quantum relays or repeaters. A quantum relay is a device that enables entanglement-swapping , and a quantum repeater exploits quantum information processing and quantum memory; the former case delivers unbounded range for quantum communication but at a photon-number cost that is super-exponential in the distance whereas the latter case delivers unbounded range at a photon-number cost that is only polynomial in the distance.
We are working on a project titled "Long distance quantum communications: Repeaters and Relay technologies" under the Quantum Enabled Science and Technology network programme funded by the Department of Science and Technology, Govt of India. Our aim is to make long-distance quantum communication feasible with existing and near-future technology. Our approach is multi-pronged: improve existing technology via better techniques such as equipment, alignment and electronics; combine accurate numerical modelling with experimental testing to optimize parameter choices; and improve the technology such as quantum memory and parametric down-conversion sources. We will be developing cutting edge technologies for Quantum Teleportation in the current project with future goals in entanglement swapping and quantum relay devices towards the national objective of quantum communication based technologies.
Suppose, Alice wants to send to Bob an unknown quantum bit. The resources they have with them are only a classical communication channel, and a pair of entangled qubits. One way to do this for Alice, would be to measure the qubit, guess the state based on results of the measurement and communicate it to Bob over the classical channel. However, the fidelity of this transfer will be very poor with this method. Further it is impossible to describe an unknown qubit by classical means, as it would then become cloneable, which would further violate the No-cloning theorem.
Quantum teleportation refers to the method of moving the quantum state from one place to another (without copying it). It has received its out-of-this-world name from the fact that the state is here transmitted (teleported) by setting up an entangled state-space of three qubits and then separating two qubits from the entanglement (through measurement). While doing these measurements, since the information of the source qubit is kept-safe, the state ends up in the third, destination qubit. This happens, however, without the source (first) and destination (third) qubit ever directly interacting with each other. Entanglement remains the medium of this interaction.