Annual Report_Abridged - Second Version - FINAL

15 computation, communication, sensing, and metrology. Subhadeep De and his team at the Precision & Quantum Measurement Laboratory (PQM lab), IUCAA is developing a state-of-the-art facility dedicated to explore the fundamental aspects of science using optical atomic clocks as a quantum sensor. The lab's research interests involve developing quantum phenomena-based t e c h n o l o g i e s f o r m e t r o l o g y - g r a d e measurements and accurate sensing. The heart of the experimental setup is a trapped ytterbium- ion-based quantum clock. For this, we shall probe the highly forbidden electric octupole (E3) transition at 467 nm wavelength of a single trapped and laser-cooled ytterbium-ion. To excite that clock transition, an ultra-stable sub-Hz line- width laser will be produced by referencing the laser to an indigenously developed ultra-stable Fabry-Pérot cavity. Upon development, the change in the tick rates of such clocks is altered by unimaginably tiny perturbations of the energy states associated with the clock transition. The resulting shift in tick rates of the clock could be caused by variations of the fundamental constants, breaking of fundamental symmetries, gravitational red-shifts at the submillimeter scale, gravitational waves, cosmic microwave background, and so on. For such scientific explorations, the lab-based clocks must be part of a geographically distributed “quantum clock network.” To pursue this, the reference clock photons must be disseminated from one node to another within the clock network using “phase stabilized optical fibers”; the PQM lab has already developed the required technology. X-ray Observational Studies of Active Galactic Nuclei The enigmas of Black Holes are not easy to study due to the lack of any light information coming directly from them. However, black holes in X-ray binary systems and at the centres of active galaxies accrete material that results in copious amounts of radiation primarily in the X-ray and Ultraviolet bands. India's first multi-wavelength space observatory AstroSat has been vital and has allowed Indian astronomers to study the immediate environments near stellar mass black holes in X-ray binaries and super-massive black holes in active galaxies. Gulab C. Dewangan has played a major role in AstroSat, India's first dedicatedmulti-wavelength space observatory which has special instruments particularly sensitive to UV and different energies of X-rays. IUCAA researchers are members of instrument teams, run the Astrosat science support cell (ASSC), and host the payload operation centre for the CZTI instrument aboard the satellite. Some of the software for AstroSat was written by members of ASSC. The ASSC also runs training workshops, schools, and instrument calibration meetings which are extremely useful, enabling researchers all over India to use AstroSat, in writing proposals for scientific observations and analysing its data. Using AstroSat, IUCAA researchers are also involved in studying complex processes in the innermost regions close to supermassive black holes (SMBH) in active galaxies (AGN) and stellar-mass black holes in X-ray binaries. They have found accretion disks that do not extend to the innermost stable orbit as predicted by Einstein's general theory of relativity. They have also found the first evidence for a state transition in a changing-look active galaxy where the standard