Satellite-based quantum communications including quantum key distribution (QKD) represent one of the most promising approaches toward global-scale quantum communications. Quantum technology uses cryptographic keys that are highly secure and impregnable, allowing only the sender and receiver to read information.
Quantum technology, according to scientists, involves a very small quantity of electromagnetic energy. To determine the feasibility of transmitting quantum signals through the atmosphere, it is essential to conduct atmospheric simulations for both uplink and downlink quantum communications and practicality of potential locations for the same need to be determined.
Scientists at the Raman Research Institute (RRI) analysed existing open-source data available on three of India’s most sophisticated observatory sites and found that the Indian Astronomical Observatory (IAO) in Hanle, located at an altitude of 14,760 feet in south-eastern Ladakh as the prime candidate for this revolutionary technology.
This site is a dry and a cold desert, with temperatures in winter plummeting to minus 25 to 30°C and as characterised by low atmospheric water vapour levels and oxygen concentrations.
“Hanle offers all required natural settings suitable for setting up a ground-station and undertaking quantum communication over long distances,” said Professor Urbasi Sinha, head, Quantum Information and Computing Laboratory lab at RRI, said. “India offers a vast variety of geographical terrains and this diversity could potentially make this work as a universal template that could be applied anywhere in India or across the globe for future quantum satellite projects worldwide,” he added.
Satellites for establishing secure satellite-based quantum communications orbiting in the Low Earth Orbit (LEO), where the maximum altitude from earth is 500 kms, are considered in this work.
For establishing quantum communication, initially beacon signal from the ground station of a particular site is sent every time the designated satellite hovers close to the location. Once the beacon signal is detected by the satellite, another beacon signal is sent by the satellite to the ground station to lock it. It is then ready to facilitate quantum signal transmission. Scientists said that the main challenge is to identify a site that will allow them to send quantum signals through the multi-layered and complex Earth’s atmosphere and yet continue to travel to the receiver satellite.
“In order to transmit the beam to a distance across 500 km, the beam width has to be magnified and its divergence has to be minimal. Hence, a telescope is used for this purpose and ideally small telescopes are best suitable. In the same manner, the receiver side of the telescope is used to collect and de-magnify the beam for detection purposes,” Satya Ranjan Behera, the study’s lead author, said. Apart from Hanle, two other sites in India, Mount Abu and Nainital, that also have space research establishments, were studied for their optimisation.
