Our contemporary understanding of the cosmos largely rests on deploying state-of-the-art technology to probe deep, wide and sharply into the sky. Among the myriad discoveries are the heftiest things we know, namely giant black holes that inhabit the centers of galaxies. The early evidence that they are real rested on the Sherlock Holmes argument. More recently, however, the sharpest eyes that technology can make have given us the most direct evidence possible for black holes. Some of the strands in this fascinating story will be discussed in the talk.

We briefly review some of the manifestations of spin-orbit and orbit-orbit interactions in optical systems in general. We focus on optical systems with off-axis beams incident with small tilt angles. We present explicit results for diffraction of an off-axis vector beam incident on a tilted circular aperture. We extend the results to a tilted tweezer system resulting in enhanced transverse spin and Belinfante momentum.

The $SU(N)$ Yang-Mills matrix model admits self-dual and anti-self-dual instantons. When coupled to $N_f$ flavors of massless quarks, the Euclidean Dirac equation in an instanton background has $n_+$ positive and $n_-$ negative chirality zero modes. We show that the index $(n_+ - n_-)$ is equal to a suitably defined instanton charge. Further, we show that the path integral measure is not invariant under a chiral rotation, and relate the non-invariance of the measure to the index of the Dirac operator. Axial symmetry is broken anomalously, with the residual symmetry being a finite group. For $N_f$ fundamental fermions, this residual symmetry is $\mathbb{Z}_{2N_f}$, whereas for adjoint quarks it is $\mathbb{Z}_{4N_f}$. Finally, we remark on the importance of the chiral anomaly for the QCD $SU(3)$ gauge matrix model.

I will describe our recent theoretical work, which initiates contact between two frontier disciplines of physics, namely, atomic superfluid rotation and cavity optomechanics. It considers an annular Bose-Einstein condensate, which exhibits dissipationless flow and is a paradigm of rotational quantum physics, inside a cavity excited by optical fields carrying orbital angular momentum. It provides the first platform that can sense ring Bose-Einstein condensate rotation with minimal destruction, in situ and in real time, unlike demonstrated techniques, all of which involve fully destructive measurement. It also shows how light can actively manipulate rotating matter waves by optomechanically entangling persistent currents. Our work opens up a novel and useful direction in the sensing and manipulation of atomic superflow.

The works of J.D. Bekenstein and S. Hawking led to thermodynamic features of a black hole. In this talk the precise argument for why do a black hole behaves as a thermal system, will be addressed. Moreover, a possible road to understand the microscopic description of entropy and temperature of the horizon will also be paved. Finally, I shall mention the important open questions and related challenges.

The fledgling field of quantum information and computation boasts several quantum-enabled devices that outperform their classical counterparts, as also quantum protocols and phenomena without classical analogs. We will discuss some such situations when the corresponding system is inflicted with impurities and noise in different forms. In particular, we will discuss the effect of impurities on the behavior of population inversion and entanglement sudden death in single and double Jaynes Cummings models. We will also talk about localization-like effects due to disorder insertion in quantum random walks. Time permitting, we can also examine the effect of disorder on the behavior of entanglement in certain quantum many-body systems.

Composite fermions are topological bound states of electrons and quantized vortices. I will begin with an introduction to composite fermions and illustrate how the prominent phenomenology of the fractional quantum Hall effect, one of the most remarkable strongly correlated states of electrons realized in nature, is explained as a manifestation of composite fermions. I will then present a generalization, called the parton construction, that captures certain other delicate fractional quantum Hall states. The talk is meant to be understandable to a first year physics graduate student.

The Indian multi-wavelength space observatory AstroSat has just completed five years in orbit. The observatory is designed to study the sky simultaneously at multiple electromagnetic wavebands ranging from optical to gamma rays. Excellent imaging resolution at ultraviolet wavelengths and high time resolution in X-Ray bands, along with the broad spectral coverage has enabled a wide variety of science to be carried out with this mission. The observatory is still functioning well and is open for observing proposals from users. This talk will discuss the science areas being explored and present some highlights of the results so far.

After the completion of the Standard Model (SM) through the Higgs discovery particle physicists are waiting for the discovery of new particles either directly with the help of the Large Hadron Collider (LHC) or indirectly through quantum fluctuations causing certain rare processes with a change of quark flavour to occur at different rates than predicted by the SM. While the latter route is very challenging, requiring very precise theory and experiment, it allows a much higher resolution of short distance scales than it is possible with the help of the LHC. In fact, in the coming flavour precision era, in which the accuracy of the measurements of rare processes and of the relevant theory calculations will be significantly increased, there is a good chance that we may get an insight into the scales as short as 10^{-21} m (Zeptouniverse) corresponding to energy scale of 200 TeV or even shorter distance scales. The main strategies for reaching this goal will be explained in simple terms. We will summarize the present status of deviations from SM predictions for a number of flavour observables and list prime candidates for new particles responsible for these so-called anomalies. A short outlook for coming years will be given.

We will begin by introducing the quantum walk, its most significant behaviour and some applications. Then we will move on to discuss some recent results where the quantum walker is allowed to take steps of variable lengths at each discrete time step randomly. Finally we discuss how a persistent quantum random walker behaves using two different schemes. The studies lead to some surprising results, which is quite characteristic of a quantum walk!

Physicists have attempted to construct minimalistic laws to describe varied phenomena of nature. In history of science, there have been successes in this direction-—Newton's universal law of gravitation, Maxwell’s unification of EM laws, Unification of forces, etc. Symmetries play an important role in such description. But, can we explain all the natural phenomena starting from fundamental laws? In this talk, I will present different perspective.

It turns out that many interesting phenomena such as time asymmetry, origin of dissipation (including quantum), symmetry breaking, etc. can be demonstrated quite easily under the framework of multiscale description. One important example of multiscale system is turbulence where the energy fed at large scale cascades to small scales, where it is dissipated by microscopic interactions.

In this talk, we are going to provide a pedagogical introduction to the physics of a chain of ultracold Rydberg atoms. This will be followed by a discussion of their dynamical properties and the role played by a special class of states, quantum scars, in shaping their details. Finally, we shall discuss such systems in the presence of a periodic drive and show that the drive frequency can be used to tune their ergodicity property. In addition, the driven systems show several phenomena such as dynamical freezing and sub-thermal steady states. We shall discuss this and chart out experiments which can test these theoretical predictions.

In this talk I will first review a long-standing discrepancy between the neutron lifetime as measured in beam and in bottle experiments. If this discrepancy is not due to a systematic error, it may be due to novel mechanisms for neutron transmutation into new, as yet unknown elementary particles. These particles would be electrically neutral, or so-called “dark”. We will explain several scenarios for the possibility of neutron transmutation into dark particles. For example, in one interesting scenario the products of the neutron transmutation include a monochromatic photon with energy in the range 0.782 MeV–1.664 MeV and this is predicted to occur in 1% of all neutron decays. We will describe recent theoretical developments as well as ongoing and planned experiments looking directly to establish or rule out the “dark decay” hypothesis.

Through much of history, novel materials have been discovered either by accident or through a process of trial and error. Worldwide, efforts are now underway to replace this by a program of rational materials design. In this endeavor, considerable time and effort can be saved by developing “descriptors” that, though possibly approximate, are quick to compute. Using descriptors, one can rapidly identify candidate materials that are likely to possess a target property, saving time when compared to experiments or first principles calculations. I will briefly review the field, and present work in my group on formulating descriptors for the structure of self assembled monolayers of organic molecules on surfaces and descriptors for the morphology and activity of Nanocatalysts.

The last decade has seen the most spectacular rise of a class of materials, known as the hybrid halide perovskites. Their photovoltaic and light emissive properties have reached superlative levels of performance within this exceptionally short span of time and taken the world by surprise.

Along with the intense effort to further improve the efficiency, stability and other technological aspects, there is a considerable effort in understanding the origin of such exceptional attributes. Curiously enough, there does not appear to be any universally accepted understanding of even the most basic properties of these materials. For example, an intensely debated issue concerns the ability of permanent dipoles on organic moieties to give rise to polar fields, either in the normal state (as in any ferroelectric material) or in the photo-excited state, contributing to its spectacular photovoltaic properties. Even estimates of the excitonic binding energy in these materials have proven to be controversial with various estimates differing by more than an order of magnitude. In a similar vein, while the stability issue has been addressed to a large extent by the substitutional chemistry at the A-site of the perovskite, its implications for various fundamental properties have not yet been clearly elucidated.

I shall provide a detailed account of this field to underline the reasons for unprecedented excitement with these materials before introducing some of the open issues. I shall follow this up by discussing some of our efforts to resolve these puzzles.

This work is a result of collaborations with B Bhattacharyya, M Bokdam, C De, C Franchini, S Ghara, TN Guru Row, A Hossain, BP Kore, G Kresse, A Kumar, J Lahnsteiner, P Mahale, A Mohanty, S Mukherjee, R. Mukhopadhyay, S Pal, A Pandey, MS Pavan, S Picozzi, T Sander, Sharada G, V. K. Sharma, A Stroppa, A Sundaresan, and D Swain.

Relevant references:
1. M Bokdam et al., Sci. Rep. 6, 28618 (2016).
2. J Lahnsteiner et al., Phys. Rev. B 94, 214114 (2016).
3. Sharada G et al., J. Phys. Chem. Lett. 7, 2412 (2016).
4. Sharada G et al., J. Phys. Chem. Lett. 8, 4113 (2017).
5. Sharada G et al., J. Phys. Chem. C 122, 13758 (2018).
6. A Mohanty et al., ACS Energy Lett. 4, 2045 (2019).
7. V. K. Sharma et al., J. Phys. Chem. Lett. 2020 (DOI: 10.1021/acs.jpclett.0c02688)
8. Unpublished results from the group.

There are three open questions in physics which seem unrelated: Why is there only matter around us? How neutrinos acquire their tiny masses? Why all particles in Nature have integer electric charges? It turns out that these open questions are related. In this talk, I will explain these open questions, the connection between them, and describe the on-going theoretical and experimental efforts in understanding them.

Van der Waals heterostructures represent a new paradigm of material design, where two atomic or molecular planes of different chemical origin are brought together within the sub-nanometer van der Waals distance. When two atomic layers are placed so close their electronic states may hybridize, and the physical properties are modified by the rules of momentum conservation and structural commensurability. In this talk I shall present several new physical phenomena, in multiple domains ranging from electronic, opto-electronic to thermoelectric properties, that emerge as a result of van der Waals heterostructuring of two-dimensional (2D) materials. Apart from achieving high carrier mobility and ultra-low noise in electrical transport, encapsulating graphene by boron nitride leads to manifestation of edge transport and trigonal warping at low energies. Optoelectronic properties are strongly enhanced on graphene and transition metal dichalcogenide heterostructures, that can be extended to single photon detection. I shall also show new phenomena in thermoelectric transport in twisted bilayer graphene, where the Seebeck coefficient is strongly determined by the angular misorientation between the graphene layers in the van der Waals stack.

Skyrmions are topological textures which were first proposed in field theory as a model to understand nucleons and their stability. In recent years, skyrmions have been discovered as spin textures in magnetic solids, with potential applications in the form of `topological memories'. In this colloquium I will discuss the physics of single skyrmions, their crystalline orders, and how quantum melting of skyrmions may induce unusual quantum magnetic liquids.

In this talk, we will discuss two seemingly disconnected but well studied topics and show how they can be connected towards observing a new physical effect. The first topic is a physical effect called synchronization, widely observable in nature everywhere and all the time. The second topic is related to the quantum world, where things behave very differently than what we see in our surrounding life. Recently, such quantum world behaviour is also being actively used towards building quantum computers and quantum communication devices. This has led to a new field of quantum technologies.

After introducing these two topics, I will discuss what it means to observe such synchronization in the quantum world and what are the consequences of such quantum synchronization of spins and atoms in today's cutting-edge quantum technologies. I will end the seminar with a discussion of experimental results from our laboratory at IIT-Kanpur, that led to the very first observation of quantum synchronization.

References:
1. Arif Warsi Laskar, Niharika Singh, Arunabh Mukherjee and Saikat Ghosh, New J. Phys. 18, 053022 (2016).
2. Arif Warsi Laskar, Niharika Singh, Pratik Adhikary, Arunabh Mukherjee and Saikat Ghosh Optica, 5, 1462 (2018).
3. Arif Warsi Laskar, Pratik Adhikary, Suprodip Mondal, Parag Katiyar, Sai Vinjanampathy and Saikat Ghosh, Phys. Rev. Lett. 125, 013601 (2020).