Light-driven micro-nanomotors

Micro and -nanomotors that can swim in various fluid mediums autonomously under the control of external light signals have gained a considerable amount of interest. Considering their precise swimming and controlled activities, they have been proposed to be a potential candidate for important applications like targeted drug delivery, bacteria capturing and sensing, cargo transport, and the formation of reconfigurable colloidal structures, etc. In this talk, I will discuss things like what is the physical principle of such tiny motors, how we can design them, and what are the possible ways to control their motion. A few interesting applications of the light-driven micromotors will also be discussed

Periodically Refreshed Baths: a numerical technique and a thermodynamic cycle

In this talk, I will introduce the Periodically Refreshed Baths (PReB) scheme. Simulating non-Markovian dynamics of interacting quantum many-body systems strongly coupled to multiple baths at different temperatures and chemical potentials is a challenging and experimentally relevant problem. The PReB scheme allows for efficient tensor network-based numerical simulation of such situations much beyond what is possible in most other techniques. Going further, one can extend the PReB scheme beyond just a numerical technique, to a dynamical process that can be potentially realized in experiments. We then explore the thermodynamics of the steady state of the PReB process. This leads to novel Periodically Refreshed Quantum Thermal Machines, which can interpolate between autonomous quantum thermal machines and those based on collisional or repeated interactions with single-site environments. These quantum thermal machines work on a single-stroke thermodynamic cycle where the working material is always out-of-equilibrium. They can have many counter-intuitive properties, which I will demonstrate via a simple example.
Refs: Phys. Rev. B 104, 045417 (2021), Quantum 6, 801 (2022).

Quantum measurement fueled engines and the Maxwell demon's arrow of time

In this talk, I will present some recent developments in the field of characterizing the quantum measurement process from a thermodynamic view point. I will briefly summarize how to associate an arrow of time for individual realizations of the quantum measurement process, which obey's a fluctuation theorem and a stronger version of the second law. The arrow of time can be associated to information acquisition in the measurement process, and therefore also finds applications in quantifying the relations between work extracted, and entropy generated, to the acquired quantum information in feedback controlled quantum measurement engines. I will conclude by presenting an example of a single qubit based quantum measurement engine where these connections are explored.

References:
[1] Manikandan, Sreenath K., Cyril Elouard, and Andrew N. Jordan. "Fluctuation theorems for continuous quantum measurements and absolute irreversibility." Physical Review A 99.2 (2019): 022117.
[2] Jayaseelan, M., K Manikandan, S., Jordan, A. N., & Bigelow, N. P. (2021). Quantum measurement arrow of time and fluctuation relations for measuring spin of ultracold atoms. Nature communications, 12(1), 1-7.
[3] Yanik, K., Bhandari, B., Manikandan, S. K., & Jordan, A. N. (2022). Thermodynamics of quantum measurement and Maxwell's demon's arrow of time. Physical Review A, 106(4), 042221.

FriQuant Seminar: Superconductivity at very low carrier density: Bismuth

Superconductivity is an intriguing phenomenon pursued under extreme conditions. It could be either temperature or pressure. With a brief introduction to Prof. Srinivasan Ramakrishnan’s earlier works, the seminar will cover in detail about the superconductivity of ultrapure Bismuth at 0.00053 K with a critical field of 0.000005 Tesla (one-tenth of earth’s magnetic field) which has been discovered at TIFR. This was a surprising discovery as superconductivity of Bismuth at ambient pressure has the lowest carrier density (one conduction electron shared by 100,000 atoms) and it was presumed that Bi would not show SC. The seminar will also cover the search for new superconductors.

Emerging 2D Lateral Heterostructures for Optoelectronics

Atomically thin layered materials such as graphene and transition metal dichalcogenides (TMDs) have opened a new and rich field with exotic physical properties and exciting potential applications in the “flatland”[1-9]. There are enormous possibilities in combining diverse 2D materials for the unique design of ultra-smart and flexible optoelectronic devices, including transistors, light-emitting diodes, photovoltaics, photodetectors, and quantum emitters. Considerable efforts have been devoted to the van der Waals hetero-integration of different 2D layered materials to form vertical superlattices via transfer of their exfoliated or as-grown flakes. On the other hand, lateral heterostructure is possible only via direct growth, which can offer exciting opportunities for engineering the formation, confinement, and transport of electrons, hole, exciton, phonon, and polariton. Unlike vertical heterostructures, lateral heterostructures can be fabricated only via direct growth. Furthermore, the performance of most 2D heterostructure-based devices falls far below the predicted values owing to several intrinsic and extrinsic factors. These significant issues will be discussed.

We reported the direct fabrication of seamless, high-quality TMDs lateral heterostructures and superlattices in the chemical-vapor-deposition process, only changing the reactive gas environment in the presence of water vapor [2-5]. Our novel approach offers greater flexibility for the continuous growth of multi-junction TMDs lateral heterostructures, controlled 1D interfaces, alloying, and layer numbers. The extent of the spatial modulation of individual TMD domains and their optical and electronic transition characteristics across the heterojunctions are studied in detail. Electrical transport measurements revealed diode-like responses across the 2D lateral junctions, which were found promising for electroluminescence at room temperature[2-3]. Using photon energy-resolved photoconductivity mapping, long-term carrier accumulation in MoS₂-WS₂ lateral heterostructures was observed.5 At the onset of photoexcitation, local carrier density was increased by two orders of magnitude and persisted for up to several days. Temperature-dependent photoluminescence from neutral exciton, trion, and defect-bound exciton provides a better understanding of the optical properties of these as-grown 2D lateral heterostructures. These studies will further supplement the quantitative evaluation of optical properties of various 2D heterostructures to develop more complex and atomically thin superlattices and exotic devices.

Laser-mediated Explosive Synthesis and Transfer of graphene electrodes for triboelectric nanogenerators

Triboelectric Nanogenerators (TENGs) have emerged as promising devices to harvest energy from surrounding mechanical perturbations. Different device structures and a range of tribomaterials have been tested by researchers over the last decade, whereas the role of the electrodes in TENGs remains not fully understood. A range of options has been reported in the literature including traditional metals, metal adhesive tapes, liquid metals, and silver nanowires as charge collectors in TENG; however, there are drawbacks associated with each of them. To tackle these hurdles, graphene-related materials (GRMs) have shown potential as efficient TENG electrodes. Most traditional graphene synthesis routes are complex, expensive, and incompatible with the commonly used polymeric tribomaterials. Moreover, tedious transfer processes are required to deposit and use graphene as an electrode. In this talk, I will present a novel laser-assisted technique that facilitates the simultaneous synthesis and transfer of GRMs onto any desired substrate including polymers, metals, ceramics, and textiles. The whole process takes place at ambient conditions and can be scaled up to satisfy industrial needs. The graphene-based composites or doping can be achieved with a careful decomposition of proper precursor. As a proof of concept, pure graphene, and nanocomposites of graphene SiO x nanoparticles, have been directly deposited on PDMS and tested as electrodes for triboelectric nanogenerators. Different combinations of metals have been tested as TENG electrodes. By replacing gold electrodes with GRMs-based electrodes, an enhancement of ~800% is observed in the instantaneous power density of the TENGs. Further, the simple and sustainable fabrication process allows for easy assembly of graphene electrodes for energy harvesting applications. Come and join us for the seminar to know more about the synergy between GRM electrodes and TENGs. The main emphasis will be on the laser-assisted synthesis of sponge-like graphene and its implications for the enhanced performance of TENG devices.

FriQuant Seminar: Nobel Prize in Physics 2022

As a tribute to his former professor Alain Aspect, Dr. Syamsundar will take you on a journey through the fascinating works of the Nobel laureates starting from the conceptualization of quantum entanglement to its validation and potential application for the development of quantum information technologies.

Thermodynamic signatures of Quantum Entanglement

Quantum Information theory shares profound similarities with Thermodynamics when considered in in finite limit scenario. For instance, state transformations in both the cases are governed by some no-go principles. While in thermodynamics they always obey the 2 nd law, in entanglement theory local manipulations and classical communications cannot increase entanglement. Although there is no apparent connection between these no-go's, existence of peculiar quantum correlations established a deeprooted link between these two theories. During recent past, significant efforts have been devoted in investigating the connection of quantum correlations with the thermodynamic quantity, in particular the extractable work. The work extraction from a quantum system can be obtained in two different ways -- (i) first, in the presence of bath (analogous to the Isothermal process) & (ii) secondly, from the system itself (analogous to the Adiabatic process). While the first kind of work extraction relates to the information theoretic quantity, i.e., von Neuman entropy, the second kind of work is depended on energy of the system. In this talk I will discuss the second aspect of work extraction which is also known as ergotropy. In such a case the entropy of the system does not change as the state is evolved under unitary operations. Remarkably, quantum correlations play a crucial role in ergotropic work extraction. In the emerging era of quantum technology, identification, characterization, and quantification of entanglement are of extreme practical relevance. In our work. we propose some thermodynamic quantities that capture signatures of entanglement for bipartite as well as for multipartite systems. As the proposed thermodynamic measures are solely energy dependent it welcomes experimental implementation. Furthermore, the work function of a quantum battery is greatly relied on ergotropy. Thus, it constitutes a promising research endeavor to study the connections among ergotropy, entangled states, and entangled measurements.
Main References: 1. MA, Tamal Guha, and Preeti Parashar; Bound on ergotropic gap for bipartite separable states, Phys. Rev. A 99, 052320 (2019).
2. Samgeeth Puliyil, Manik Banik, and MA; Thermodynamic Signatures of Genuinely Multipartite Entanglement, Phys. Rev. Lett. 129, 070601 (2022).

Related works:
1. Amit Mukherjee, Arup Roy, Some Sankar Bhattacharya, and Manik Banik; Presence of quantum correlations results in a nonvanishing ergotropic gap, Phys. Rev. E 93, 052140 (2016).
2. MA, Tamal Guha, and Preeti Parashar; Independence of work and entropy for equal-energetic finite quantum systems: Passive-state energy as an entanglement quantifier, Phys. Rev. E 102, 012145 (2020).
3. MA, Tamal Guha, and Preeti Parashar; Structure of passive states and its implication in charging quantum batteries, Phys. Rev. E 102, 022106 (2020).

Colloquium: Weak measurements on Spin optical effects

The weak value amplification (WVA) concept, introduced by Aharonov, Albert, and Vaidman, has proven to be fundamentally important and extremely useful for numerous metrological applications. This quantum mechanical concept can be understood using the wave interference phenomena and can therefore be realized in classical optical settings also. In this talk, I shall illustrate how the WVA concept can be formulated within the realm of classical electromagnetic theory of light and discuss its use for the amplification of tiny spin orbit interaction effects of classical light beam. I shall present our recent experimental work on the realization of WVA in standard path interference by introducing a weak coupling between the path degree of freedom of an interferometer and the polarization degree of freedom of light. Taking example of Fano resonance, it will be shown how WVA of an appropriate weak interaction parameter may naturally evolve in a rich variety of non-trivial wave phenomena that originate from fine interference effects. In this regard, our recent work on extending weak measurements into the domain of plasmonics, on demonstrating weak measurements using spectral line shape of resonance as pointer in precisely designed metamaterials, observing natural WVA of Faraday effect in Fano resonances from hybrid magneto-plasmonic systems will be highlighted.

Dynamical construction of Quadrupolar and Octupolar topological superconductors

We propose a three-step periodic drive protocol to engineer two-dimensional~(2D) Floquet quadrupole superconductors and three-dimensional~(3D) Floquet octupole superconductors hosting zero-dimensional Majorana corner modes~(MCMs), based on unconventional d-wave superconductivity. Remarkably, the driven system conceives four phases with only 0 MCMs, no MCMs, only anomalous ?? MCMs, and both regular 0 and anomalous ?? MCMs. To circumvent the subtle issue of characterizing 0 and ?? MCMs separately, we employ the periodized evolution operator to architect the dynamical invariants, namely quadrupole and octupole motion in 2D and 3D, respectively, that can distinguish different higher order topological phases unambiguously. Our study paves the way for the realization of dynamical quadrupolar and octupolar topological superconductors.
Ref: Phys. Rev. B 105, 155406 (2022)

Colloquium: Decoding and Encoding of Molecular Information in Distributed Cellular Systems

It is often useful to think about Cells and Tissues as Distributed Computing Systems, especially in the context of the processing of noisy molecular information. I will illustrate this in two parts. In the first, I will talk about cellular compartmentalization and receptor promiscuity as a strategy for accurate inference of position during Morphogenesis. In the second, I will discuss the synthesis of a complex Glycan code in the Golgi cisternae, and how cisternal number and enzyme promiscuity achieves the target distribution with high fidelity.

Effects of Defects on Structural, electronic, magnetic and transport properties of Graphene/h-Boron Nitride (G/h-BN) heterostructures

In the present work, we discuss the effects of B, N and C sites vacancy defects on structural, electronic, magnetic and transport properties of G/h-BN heterostructures (HS) material by first-principles calculations based on spin-polarized DFT method with DFT-D2 approach. Our investigations show that G/h-BN and B, N, & C sites vacancy defected G/h-BN are stable 2D vdWs HS materials. Further, we find that G/h-BN and B & N sites vacancy defected G/h-BN HS materials have semimetallic properties and 1C vacancy defected G/h-BN HS is an n-type semiconductor and 2C vacancy defected G/h-BN HS has metallic properties. Moreover, although Graphene and h-BN are nonmagnetic materials, G/h-BN with B, N & C sited vacancy defected HS materials have magnetic properties even though, in some cases, very weak. From the study of transport properties, it is found that Graphene, h-BN, G/h-BN and B, N & C sites vacancy defected HS materials are promising candidates in the fields of thermoelectricity.

Transport in ordered and disordered charged harmonic oscillator systems in presence of a magnetic field

We will consider heat transport across a harmonic chain of charged particles, with transverse degrees of freedom, in the presence of a uniform and disordered magnetic field. For an open-chain connected to heat baths at the two ends, we obtain the nonequilibrium Green???s function expression for the heat current for any spatial configuration of the magnetic field. We will then consider the uniform magnetic field case, and find an exact expression for the current in the thermodynamic limit, for the cases of free and fixed boundary conditions. In this limit, we find that at a small frequency, the effective transmission, has the frequency-dependence and for fixed and free boundary conditions respectively. This is in contrast to the zero magnetic field case where the transmission has the dependence and for the two boundary conditions respectively. This is of interest as the low-frequency behaviors of the transmission partly determine the scaling laws for the disordered case. We will then discuss the disordered magnetic field case (or equivalently disordered charge) where Anderson localization causes suppression of the heat transmission. However, for this system, the localization length diverges as the normal mode frequency approaches zero. Therefore, the low-frequency modes contribute to the transmission, and the heat current goes down as a power law with the system size. This power law is determined by the small frequency behavior of some Lyapunov exponent, and the transmission in the thermodynamic limit for the uniform case for small frequency. We find that the Lyapunov exponent for the system behaves as for and for. Therefore, we obtain different power laws for current vs N depending on and the boundary conditions.

Tensor networks for two-dimensional frustrated Ising models: partial lifting of a macroscopic degeneracy

Despite their apparent simplicity, classical antiferromagnetic Ising models on frustrated lattices give rise to exotic phases of matter, in particular due to their macroscopic ground-state degeneracy. They also represent an interesting numerical challenge for classical Monte Carlo approaches, requiring the design of ad-hoc cluster updates [1??"3]. Motivated by the dipolar Ising antiferromagnet on the kagome lattice [1, 2, 4] as well as by the success of tensor networks for computing the residual entropy of ice and of nearest-neighbor frustrated Ising models [5], I will discuss the ground-state phase dia- gram of the J1 ??' J2 ??' J3 Ising antiferromagnet on the kagome lattice for J1 ??? J2, J3 [6]. Exact results for the ground-state energies can be established using Kanamori???s method of inequalities [7], combined with Monte Carlo simulations [3]. When all the couplings are antiferromagnetic, the model has three macroscopically degenerate phases, whose residual entropies can be obtained with a very high numerical precision using tensor network approaches which I will introduce in some detail [8]. To ensure the convergence of standard approximate contraction algorithms for tensor networks describing frustrated Ising models, we have to first obtain the ground-state local rule [8, 9]; these rules provide further insight into the various ground states of this highly frustrated model.
1 I. A. Chioar, N. Rougemaille, and B. Canals, Phys. Rev. B 93, 214410 (2016).
2 J. Hamp, R. Moessner, and C. Castelnovo, Phys. Rev. B 98, 144439 (2018).
3 G. Rakala and K. Damle, Phys. Rev. E 96, 023304 (2017).
4 L. F. Cugliandolo, L. Foini, and M. Tarzia, Phys. Rev. B 101, 144413 (2020).
5 L. Vanderstraeten, B. Vanhecke, and F. Verstraete, Phys. Rev. E 98, 042145 (2018).
6 J. Colbois, B. Vanhecke, et al., arXiv:2206.11788 (2022).
7 J. Kanamori, Prog. Theor. Phys. 35, 16??"35 (1966).
8 B. Vanhecke, J. Colbois, et al., Phys. Rev. Research 3, 013041 (2021).
9 W. Huang et al., Physical Review B 94, 134424 (2016).

QTIH FriQuant Seminar: Magnetic Proximity effect: Molecular spin-interface to Topological insulators

In this talk, we will discuss about the magnetic proximity effect which is a mechanism to locally induce spin-correlations in an adjacent interface layer that can allow control and tunability of the electronic and magnetic states of the interface system. In this context, we will first present our experimental results of a molecular crane-pulley effect in a monolayer metal-phthalocyanine molecule chemisorbed on a Fe surface. Here, the strong hybridization of the molecular orbitals with the spin-polarized bands of the ferromagnet renders a phenomenon of surface magnetic hardening, promoting a robust form of exchange-bias effect at lower temperatures. In the second part of the talk, we will present our reports of the inverse proximity effect at the interface between a topological insulator thin film and a Heisenberg ferromagnet, EuS. Here, we detect the presence of an interfacial EuS with enhanced interface magnetism, and subsequently, using planar Hall measurements, provide evidence for the formation of unconventional spin texture states in the interfacial EuS layer. At the end, we shall also briefly present our group???s recent research activity in Kagome non-collinear antiferromagnets.

Entanglements in Polymeric Liquids: Monodisperse Melts and Binary Blends

Viscoelastic properties of high molecular weight polymeric liquids are dominated by entanglements which anisotropically constrain chain motion. These "topological constraints" arise because polymer chains can slide past but not cut through each other. The idea that entanglements confine a chain to small fluctuations around a coarse-grained chain contour, the "primitive path", forms the basis for the highly successful but phenomenological "tube models". Towards establishing the microscopic foundation of the tube models, we introduced a method to directly determine the primitive path mesh of computer-generated long-chain polymer melts and solutions. This offered a way to obtain parameter-free, quantitative predictions for the plateau modulus. These predictions agree with experiment for all major classes of synthetic polymers [1,2]. The next level of complexity is that of binary blends, a miscible mixture of two different polymers. The dynamical heterogeneity induced by the composition fluctuations are believed to be behind the unique dynamics and rheology, still poorly understood, of binary blends [3]. Therefore, we have investigated the structure of entanglements in these blends. We started with the nature of the mixing rules for the plateau modulus or equivalently, in the context of the tube model, the tube diameter. The older literature ([3] and the references therein) appeared to suggest that binary blends were rather similar to monodisperse melts and that the harmonic mixing rule provided an adequate description of the experiments. However, recent work by Watanabe and coworkers [3-5] on a blend of Poly(isoprene) and Poly(tert-butylstyrene) has clearly demonstrated significant and qualitative deviations from the harmonic mixing rule. To provide a microscopic understanding of the experimental results, we have investigated binary blends of bead spring polymer chains. Motivated by the differences in the molecular characteristics of the two component polymers in the blend, we considered two types of blends: (1) blends of polymers that differ only in their stiffness and (2) blends of polymers that differ only in their monomer size. We found that, in both cases, the topological analysis yielded just one tube diameter for the blend. However, the mixing rules for the two cases were qualitatively different. The harmonic mixing rule provided a good description of the variation of the tube diameter with blend composition, similar to the findings in the older literature [3]. This mixing rule can be understood by a direct extension of the packing ansatz for monodisperse melts [5]. On the other hand, the simulation results for case (2) did not follow the harmonic mixing rule but were well described by an ad-hoc mixing rule proposed Watanabe and coworkers. We have reasons to believe that the difference between cases (1) and (2) can be traced to the difference in the nature of the mixing of the two components in these two cases. If time permits, I will briefly introduce our preliminary but intriguing findings regarding chain dynamics in binary blends.

[1] R. Everaers, SKS, G. S. Grest, C. Svaneborg, A. Sivasubramanian, and K. Kremer, Science 303, 823 (2004).
[2] SKS, G. S. Grest, K. Kremer, R. Everaers, J. Poly. Sci., Poly. Phys. Ed. 43, 917 (2005).
[3] H. Watanabe and O. Urakawa, ???Component Dynamics in Miscible Polymer Blends.??� In Functional Polymer Blends, edited by Vikas Mittal, 53??"126. CRC Press (2012).
[4] H. Watanabe, Q. Chen, Y. Kawasaki, Y. Matsumiya, T. Inoue and O. Urakawa, Macromolecules 44, 1570 (2011).
[5] Y. Matsumiya, N. Rakkapao and H. Watanabe, Macromolecules 48, 7889 (2015).

Bethe-Salpeter study of optical absorption in metal-organic-frameworks

Metal-organic frameworks functionalized with photoswitching azobenzene molecules are shown to be promising materials for light-tunable gas capture. The microscopic mechanism of CO2 capture and release in azobenzene functionalized MOF-5 (PCN-123) is unravelled by calculating the potential energy surfaces of cis and trans geometrical configurations of embedded azobenzene along relevant degrees of freedom. The blocking and unblocking of metal-node, a prominent adsorption site, by the photoswitch in cis and trans geometry, respectively, is found to be responsible for a large change in CO2 uptake by the PCN-123 MOF. To achieve high efficiency of gas capture using such MOF, higher yields of either isomer must be achieved using light of wavelengths corresponding to optical absorption in either Somers. To this end, a detailed description of the optical absorption of the PCN-123 MOF and its parent MOF is obtained using many-body perturbation theory (MBPT) methods, namely GW and Bethe-Salpeter equation (BSE). The optical excitations associated with the MOF and with the azobenzene functionalization in cis and trans geometry are separated in energy, allowing selective absorption and isomerization in the case of either isomer. The use of MBPT methods also allows for quantitative estimation of the electronic bandgap. The parent MOF of PCN-123 (MOF-5) was earlier considered a semiconductor by several computational and experimental studies. The GW/BSE calculations carried out here suggest that it is a wide gap (8 eV) insulator with its optical excitations featuring strong excitonic binding larger than 3 eV.

Colloquium: Inducing organization in intrinsically disordered polymeric systems: Lessons from bacterial chromosome organization

In recent times, researchers are using statistical physics to understand the emergent dynamics and organization in living systems, such as organization of the DNA within the living cell.

The mechanism and driving forces of chromosome segregation in the bacterial cell cycle of E. coli is one of the least understood events in its life cycle [1,2,3]. Using principles of entropic repulsion between polymer loops confined in a cylinder, we use Monte Carlo simulations to show that the segregation is spontaneously enhanced by the adoption of a certain DNApolymer architecture as replication progresses. Secondly, the chosen polymer-topology ensures its self-organization along the cell axis while segregation is in progress, such that various chromosomal segments (loci) get spatially localized as seen in vivo. The evolution of loci positions matches the corresponding experimentally reported results. Additionally, the contact map generated using our bead-spring model reproduces the macro-domains of the experimental Hi-C maps.

Thus we have proposed a framework [4] which reconciles many spatial organizational aspects of E. coli chromosomes as seen in-vivo and provides a consistent mechanistic understanding of the process underlying the segregation of bacterial chromosomes. Certain proteins are expected to contribute to changing the DNA-polymer architecture. We also studied other polymer architecture and saw that the same mechanism could be used to explain the experimental data of the C.crescentus chromosome [5].

[1] Jay K. Fisher, A. Bourniquel, G. Witz, B. Weiner, M. Prentiss, and N. Kleckner.
Four-dimensional imaging of e. coli nucleoid organization and dynamics in living cells.
Cell, 153(4):882??"895, 2013.

[2] A. Japaridze, C. Gogou, J. W. J. Kerssemakers, H. M. Nguyen, and Cees Dekker.
Direct observation of independently moving replisomes in Escherichia coli.
Nature Communications, 11(1), June 2020.

[3] Virginia S. Lioy, Ivan Junier, and Fr??d??ric Boccard.
Multiscale dynamic structuring of bacterial chromosomes.
Annual Review of Microbiology, 75(1), August 2021.

[4] D.Mitra, S. Pande, Apratim Chatterji.
DNA-polymer architecture orchestrates the segregation and spatial organization of bacterial chromosomes during replication.
Soft Matter (2022)

[5] D.Mitra, S. Pande, Apratim Chatterji.
Modified topology of ring polymers in confinement leads to spatial organization

QTIH FriQuant Series: Hybrid Quantum Spintronics

With the promise of lower-dissipation and longer quantum coherence time offered by the spin, several fundamental and technological advances have been made in the field of classical and quantum spintronics. However, thus far, the advances in the classical and the quantum domains have largely occurred independent of each other. In this talk, taking the example of integrating dynamic excitations of magnets (magnons) with quantum spins hosted in solid state platforms (e.g. NV center), we will present an emerging direction??"hybrid quantum spintronics??" which merges these advances to explore new phenomena and devices at the interface of classical and quantum spintronics. On the one hand, we show how quantum spins can give rise to previously unavailable nanoscale probes for exploring a wide range of magnetic and electric phenomena [1,2]. On the other hand, we demonstrate how magnons can act as novel on-chip nanoscale drives for enabling scalable and non-reciprocal quantum circuits [3.4].
[1] Du, et. al. Science 357, 195 (2017)
[2] Solanki, et. al. Phys. Rev. Res. Lett. 4, L012025 (2022)
[3] Rustagi et. al. Phys. Rev. B. Rapid Comm. 102, 220403 (2020)
[4] Rustagi et. al. arXiv 2203.03652 (2022); under review Phys. Rev. Appl.

QTIH FriQuant Series: Valleys in flatlands under high magnetic fields

The technological and communication revolution around us could not be possible without fundamental understanding of semiconductors and their nanostructures. Recently, Moore's law which has held its ground for many decades is showing signs of saturation. 2D semiconductors and their striking properties involving valley magnetic moments promise to contribute towards the next phase of quantum technological revolution [1]. However, an understanding of these new degrees of freedom is necessary. In this talk, I will discuss our various contributions in experimental explorations of the valley degrees of freedom of 2D materials, in the last few years. We used micro-optical spectroscopy under high magnetic fields (up to 30 T) and low temperatures (~4 K) to discover many new effects in 2D semiconductors such as valley Zeeman splitting and valley polarization in monolayer MoTe2 [2], magnetic control of valley coherence in monolayer WS2 [3], interlayer excitons in bilayer and bulk MoTe2, MoS2, and MoSe2 [4], and valley polarization of trions and excitons in monolayer MoSe2 [1]. I will also discuss our new Faraday Rotation spectroscopy technique which is promising in investigations of 2D semiconductors and magnetic materials under low magnetic fields, with high precision [5]. Our studies are promising in exploring new physical dimensions and quantum-device-based applications of 2D materials.

[1] Invited perspective: A. Arora, J. Appl. Phys., 129 (12), 120902 (2021)
[2] Arora et al., Nano Lett. 16, 3624 (2016)
[3] Schmidt-Arora et al., Phys. Rev. Lett. 117, 077402 (2016)
[4] Arora et al., Nat. Commun. 8, 639 (2017)
[5] Carey et al., arXiv: 2204.12809 (2022)

Quantum transport and discrete time crystals

Discrete time crystals are periodically driven quantum many-body systems which exhibit oscillatory dynamics with a time-period that is larger than the driving period. Originally proposed for closed quantum systems and observed essentially only by optical means, open quantum systems also exhibit discrete time-crystal behavior, but only under very special circumstances. The question of whether time crystal behavior can be observed in transport experiments is still open, very timely and pertinent. Here we show the existence of the discrete time crystals in a dissipative spin-full Fermi-Hubbard chain and a spin-less Hubbard ladder both representing a quantum dot array attached to external electrodes. A stable and a meta-stable discrete time-crystal appear for weak and moderately strong system-lead couplings, respectively, for sufficiently long and experimentally relevant time-scales. We further show that both of these can be detected by measuring the transport current through the leads, spin-current for the spin-full Fermi-Hubbard chain and charge-current for the spin-less Hubbard ladder. We rigorously prove the condition for the survival of discrete time crystals, viz., the weak-local Floquet dynamical symmetry. The commonplace availability of a quantum dot array with multiple independent, electrically controllable sites makes our findings experimentally verifiable.
The talk will be based on, S. Sarkar and Y. Dubi, Communications Physics 5, 155 (2022), Nano Lett., 22, 11, 4445 (2022).

Progress in the Classification of 2D Conformal Field Theory: The IISER Years

I will present a summary of some of the research that I carried out at IISER Pune in the period 2015-22, namely the new results in the important problem of classifying rational two dimensional conformal invariant quantum field theories (CFT). The problem has potential ramifications for the understanding of critical phenomena in statistical systems, in the physics of anyons and topological quantum computing, and in the construction of consistent backgrounds for string theory. It interfaces with the classical mathematics of modular forms and more recent mathematical developments in the area of vertex operator algebras/modular tensor categories/vector-valued modular forms (VVMF). The highlights that will be summarised are: (i) the discovery of a novel coset construction, providing an explicit construction of CFT's corresponding to given classes of VVMF, (ii) the development of a formalism to compute 4-point correlation functions in a universal way via meromorphic differential equations, (iii) the discovery of "quasi-characters" and the resulting complete classification of admissible VVMF with a single scaling field, (iv) progress in the classification of theories with two scaling fields, (ii) new combinatoric methods for computation of the modular transformation matrix.

QTIH FriQuant Series: Defect induced Complex Phenomena in two-dimensional materials

Synthesis of graphene has paved the path to the discovery of a huge pool of two-dimensional materials and their heterostructures offering novel functionalities and their tuning. In this regard, defects play a very important role in determining the electronic, structural and magnetic properties. Broad view regarding the following will be discussed
(i) Charge de-excitation dynamics controlled by defects in 2D MoS2 studied by density functional theory (DFT) based non-adiabatic molecular dynamics simulations.
(ii) Tuning molecular magnetism by vacancy defect in graphene.
(iii) Structural reconstructions and peculiar magnetism induced by Fe vacancy defects in a 2D magnet, Fe5GeTe2 by DFT based calculations of interatomic exchange interactions and Monte Carlo simulations.
Demonstration of the power of ab-initio theory to unravel complex phenomena in 2D systems.

Role of thermodynamic cost in information processing by cell signaling systems

Cell signaling systems are involved in sensing the changes in environment and enable the cell to elicit appropriate phenotypic responses. In most cases, the environmental ques ultimately lead to changes in gene expression triggered by a set of transcription factors. These transcription factors activated by the signaling pathway typically diffuse inside the nucleus to activate transcription of the required genes. However, the inherent stochasticity of the biochemical pathways associated with signaling processes severely limit the accuracy of estimating the environmental input by the transcription factors. To improve the accuracy, cells utilize multiple strategies namely reduction of noise, amplify the output or measurement of the input at several time points and multiple outputs. These strategies often impose extra thermodynamic cost through enhanced entropy production. Thermodynamic uncertainty relation (TUR) provides a general trade-off between fluctuation and entropy production rate. We explore this trade-off in various cell signaling systems using mathematical models coupled with single cell dynamics of transcription factors. Experimental observations coupled with TUR based theoretical models demonstrate the role of fluctuation and entropy production on cellular information processing.

Introduction to the abelian sandpile model of self-organized criticality - IV

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Introduction to the abelian sandpile model of self-organized criticality - III

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

QTIH FriQuant Series: Quantum Clocks for the Fundamental Science

Quantum clocks have reached an unprecedented level of accuracy (few parts in 1020), which are realized either by probing forbidden optical transitions of neutral atoms localized in an optical lattice or using a single atomic ion trapped in an electrodynamic trap. The tick-rate of such clocks are influenced by any unimaginably tiny perturbations of the energy states associated with their clock transitions, which are even caused by variations of the fundamental constants, breaking of fundamental symmetries, gravitational redshifts, propagation of the gravitational waves, cosmic microwave background and so on. Thus, intercomparison of the geographically distributed ultra-stable, highly accurate quantum clocks is expected to act like ???distributed & networked quantum sensors??� to test the foundations of science and the general theory of relativity. The seminar shall cover some of these scientific motivations to build a Ytterbium-ion based optical clock at the Precision & Quantum Measurement laboratory at IUCAA, Pune.

Levitated Optomechanics with Optical Tweezers

This talk will address our recent theoretical work on optical tweezers in vacuum and its experimental confirmation in the group of our collaborator N. Vamivakas. Specifically, the realization of a mechanical laser using the center-of-mass oscillations of an optically levitated nanoparticle will be described. The discussion will include threshold behavior, coherence, subthermal number squeezing, time dynamics, phase space characterization, higher order correlations and the role of stimulated emission in our single mode phonon laser. Based on this discussion, I will conclude that our device provides a pathway for engineering a coherent source of phonons on the mesoscale that can be applied to both fundamental problems in quantum mechanics as well as tasks of precision metrology.

Pre-thermalization and thermalization of isolated quantum systems

An isolated quantum many-body system is expected to evolve under its own unitary dynamics to attain thermal equilibrium predicted by statistical mechanics. Cold atom experiments have revealed an interesting scenario where some systems far from equilibrium relax quickly to an intermediate state (called the pre-thermal state) before slowly relaxing into their true thermal equilibrium. While prethermalization is conventionally discussed in the context of weakly perturbed integrable models, in this talk, I will discuss a very general yet conceptually simple theoretical framework to understand prethermalization in generic -possibly nonintegrable- systems. We show that prethermalization is ubiquitous when a perturbation breaks at least one local conservation law of the unperturbed system. I will discuss the prethermalization scenario in isolated and periodically driven quantum systems using a novel numerical technique called numerical linked cluster expansion. [1] K Mallayya, M Rigol, W De Roeck - PRX 2019 [2] K Mallayya, M Rigol - PRL 2019 [3] K Mallayya, M Rigol - PRB, 2021

Introduction to the abelian sandpile model of self-organized criticality - II

These lectures will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Colloquium: Controlling Nanoparticle Ordering by Directional Polymer Crystallization

Adding nanoparticles (NPs) to a polymer is a well-established means to improve the organic materials??? thermomechanical properties. To date much interest has been focused on amorphous polymer hosts ??" instead, here, we consider semicrystalline polymers which constitue more that 65% of all polymers sold. We first revisit our previous finding that nanoparticles (NPs), which are initially miscible with a polymer melt, can be ordered by polymer crystallization under ppropriate conditions. The key variable here is a Peclet number (Pe) that compares the time scale of NP motion to the rate of polymer crystallization. For large Pe the NPs are ???frozen??� in place during polymer crystallization. However, for Pe values less than 1 the NPs are moved by the growing crystals and selectively placed in the interlamellar, interfibrillar and interspherulitic regions. These results are in line with the Keith and Padden???s findings, expanded on by Russell et al., on the spatial dispersion of amorphous polymer defects in a semicrystalline morphology. The resulting samples have Young???s moduli that are increased by a factor of ~2 relative to ones where the NPs are randomly ordered. We believe that additional synergies in properties can be obtained if we could move from spherulitic polymer morphologies (which are locally anisotropic but spatially isotropic) to more directionally crystallized samples with a preferential orientation. To this end we employ the zone annealing methodology, a directional crystallization protocol popular in metallurgy and first applied to semicrystalline polymers by Lovinger and Gryte nearly 50 years ago. Using this method, we show that we can order and orient the NP assemblies into desired directions. In addition to these novel experimental findings, we propose a new theoretical framework that allows us to model the directional crystallization of polymers and the key variables that control them.

Data Science/Physics Seminar: The Spectrum of Wind Power Fluctuations

Wind is a variable energy source whose fluctuations threaten electrical grid stability and complicate dynamical load balancing. The power generated by a wind turbine fluctuates due to the variable wind speed that blows past the turbine. Indeed, the spectrum of wind power fluctuations is widely believed to reflect the Kolmogorov spectrum of atmospheric turbulence; both vary with frequency $f$ as $f^{-5/3}$. This variability decreases when aggregate power fluctuations from geographically distributed wind plants are averaged at the grid {\it via} a mechanism known as {\it geographic smoothing}. Neither the $f^{-5/3}$ wind power fluctuation spectrum nor the mechanism of geographic smoothing is understood. In this talk, I will chart out the non-equilibrium the character of wind power fluctuations, and explain the wind power fluctuation spectrum from the turbine through the grid scales. The $f^{-5/3}$ wind power fluctuation spectrum results from the largest length scales of atmospheric turbulence of order 200 km influencing the small scales where individual turbines operate. This long-range influence correlates outputs from geographically distributed wind plants over a range of frequencies that decreases with increasing inter-farm distance. Consequently, aggregate grid-scale power fluctuations remain correlated and are smoothed until they reach a limiting $f^{-7/3}$ spectrum, which is confirmed with field data. I will discuss the engineering and policy implications of these results and chart out the future directions I intend to pursue in the broad area of energy research.

QTIH FriQuant Series: Hybrid Quantum Registers: Efficient Control and Protection against Environmental Noise

Nuclear and electronic spins are attractive objects not only for spectroscopic studies, but also for emergent technologies like quantum information processing and sensing. The combination of different types of qubits allows one to take advantage of the favourable properties of each type. Examples include the nitrogen-vacancy (NV) center in diamond, where the electron spin interacts with 14N and 13C nuclear spins, or Si vacancies in SiC. One of the challenges for controlling hybrid systems is that the speed at which different spin species react to external control operations can differ by many orders of magnitude. It is therefore essential to develop techniques that result in sufficiently fast controls for the nuclear spins, whose gyromagnetic ratio is 3-4 orders of magnitude smaller than that of the electron spin. For this purpose, we have developed indirect control techniques that are based on sequences of microwave pulses applied to the electron spin in such a way that the motion of the electron spin generates a time dependent hyperfine interaction acting on the nuclear spin. Suitable sequences of microwave pulses can thus drive arbitrary evolutions of the nuclear spin, at rates that are far higher than those that can be achieved by radio-frequency pulses. In addition, these sequences can be optimised to protect the electron spin coherence against degradation by environmental noise.

Levitodynamics: Levitated nanoparticles in the study of stochastic thermodynamics, aerosols, materials science

Micro and nanoparticles can be individually levitated by different trapping mechanisms, among which optical tweezers and Paul traps are the most extended approaches. Even if they do not diffuse away due to the presence of the trap, trapped particles are still subjected to Brownian motion due to collisions with gas molecules. The analysis of the dynamics of trapped particles provides very useful information about the properties of the particles and the environment, allowing one to perform a variety of experiments in which a particle is used as an ultrasensitive probe. In this talk, I will present recent works that demonstrate the versatility and applicability of the technique in a large variety of fields, going from non-equilibrium dynamics to materials science to aerosol physics.

Magnetic resonance: spins as quantum sensors

When spin was established as a fundamental property of elementary particles, the goal of excitation and detection of transitions between different spin states was considered to be of purely academic interest. Successful experiments became possible thanks to advances of microwave and radio-frequency technology. As the instrumental capabilities improved, it became apparent that spins are an extremely versatile window into our environment, since they can report on a vast variety of chemical and physical parameters like molecular composition, temperature or motional processes. This led to a vast array of applications, first in physics, then chemistry, biology, materials science and medicine. While some fundamental physical principles limit the sensitivity of the technique, these limitations are constantly being pushed, sometimes to the limit of experiments with individual spins. Current developments in quantum information and quantum sensing continue to improve the sensitivity, spectral resolution and information content for an increasingly diverse range of applications.

External pressure tuning of strongly correlated materials

Exotic phenomena and unconventional phases often occur in regions of competing energy scales in correlated electron materials. The ground state of such materials can be tuned using non-thermal control parameters such as magnetic field, chemical substitution, and external pressure. In this regard, external pressure is an excellent tool to tune the ground state of materials without introducing additional disorders. Here, I will discuss various physical property measurement techniques under hydrostatic pressure. As an example, I will present our results on LuFe4Ge2, an itinerant antiferromagnet with a frustrated quasi-one-dimensional structure. Results from electrical transport, magnetic susceptibility, AC calorimetry, muon-spin resonance, M??ssbauer spectroscopy, and x-ray diffraction experiments under hydrostatic pressure provide a detailed temperature??"pressure phase diagram and elucidate the interplay of structure and magnetism in LuFe4Ge2.

Thermoelectric materials, applied strategies, and future directions

Thermoelectric (TE) materials are attractive due to their ability to convert waste heat into electric power. Significant developments in TE technology have been recorded over the past two decades. They comprise two key aspects: (i) basic understanding of electronic and thermal transport, facilitated by combination between theory and experiments; and (ii) optimizing the performance of various TE materials by integrating these interrelated transport parameters synergistically. An overview of used strategies for improving the TE performance of materials with exciting materials physics is covered in this talk. Interstitial engineering, nanocomposite, spacer engineering, charge carrier energy filtering, topological surface state contribution in TE materials, microstructural and surface modification engineering are some of the approaches examined. An intriguing aspect of the effects of microstructure on the TE transport properties of nanostructured Bi2Te3-based compounds is addressed, which is coupled with their topologically insulating (TI) nature. We create effective signals of TI states to increase TE performance by fine-tuning of grain sizes/misorientation. The structure-property correlation is strengthened by advanced characterization tools and a first principle calculation. Aside from this study, there is also discussion of other fascinating work that has been done in the before. These studies are significant for comprehending electron/phonon transport and developing high-performance thermoelectric materials.

Introduction to the abelian sandpile model of self-organized criticality - I

This lecture will introduce basic ideas of self- organized criticality , and the abelian sandpile model.

Strongly correlated phases in models with random interactions

Strong interactions between electrons gives rise to many fascinating emergent phases of matter. In certain situations it may result in a complete breakdown of quasiparticle picture leading to anomalous behavior and stark deviations from conventional theories. We will discuss how random models in the Sachdev-Ye-Kitaev (SYK) class have enhanced our understanding of such entangled phases of matter. As an important example, we will present a SYK-type random model of electrons for finite hole doping away from a Mott insulator. Using renormalization-group technique I will show that it hosts a deconfined critical point accompanied with a sharp change in charge-carrier density. In this approach we can calculate some exponents exactly. This model successfully captures the key aspects of high-Tc cuprates. I will also briefly discuss other examples such as an anomalous metal found in disordered superconducting films.

Colloquium: Experiments on solids under high magnetic fields

Experimental physics is challenging and fun. Scientific and technological progress in the modern world could not be imagined without careful experiments on semiconductors in the last 7 - 8 decades. As a result, semiconductors form hearts and brains of modern computation, optoelectronic and light-wave technologies. In this talk, I will describe experiments on semiconductors using light and strong magnetic fields which have provided vital tests to the theories of quantum mechanics. Classic textbook quantum phenomena such as 'particle in a box' and confinement effects, quantum mechanics of artificial hydrogen atoms etc. are routinely studied in the laboratory through such experiments. We will discuss the strongest magnetic fields which have been created in a laboratory. Alongside, recent progress in the area of research on 2D materials will be described, where every year, many fascinating discoveries are taking place setting up foundations of future quantum computation and communication technologies [1].

Spin liquid behaviour in the candidate material Sr_3CuSb_2O_9
Sumiran Pujari, IIT Bombay, Mumbai

New states of matter that are driven by quantum fluctuations have captured the interest of a large fraction of condensed matter physicists. Spin liquids are an exciting example of such a state in the context of magnetism. I will start with a general introduction to where the search for spin liquids fit in the science of magnetism. I will give a flavour of how theoretical ideas play out in practice by using the example of the material Sr_3CuSb_2O_9 that has been synthesized and studied recently at IIT Bombay.

Interplay of symmetry and strong correlations in excitonic insulator candidates

In low electron density materials, interactions can lead to highly correlated q]uantum states of matter with intriguing properties. Ta2NiSe5, an excitonic insulator (EI) candidate, exists in a novel broken symmetry phase below 327K, characterized by both robust exchange interaction and strong electron-lattice coupling. We study this broken symmetry phase of Ta2NiSe5 using a new spectroscopic probe, the quadrupole circular photogalvanic effect (QCPGE). The coupling of light to electrons in Ta2NiSe5 mediated by electric quadrupole/magnetic dipole coupling produces helicity-dependent DC current response even in the presence of bulk inversion symmetry, making it particularly sensitive to certain symmetry breaking effects. We show that the electronic exchange interaction in Ta2NiSe5 can lead to a triclinic structure with a broken symmetry. Our results provide an incisive probe of the symmetries of the low temperature phase of Ta2NiSe5 and add new symmetry constraints to the identification of a strongly correlated excitonic insulator phase in this material. Furthermore, the high sensitivity of QCPGE to subtle symmetry breaking effects in systems with bulk inversion symmetry will enable its use in studying other detailed structure-function relationships in complex crystalline systems.

Manipulation of Time Reversal Symmetry Breaking Superconductivity in Sr2RuO4 by Uniaxial Stress

Although the normal-state electronic structure of Sr 2 RuO 4 is known with exceptional precision, even after two decades of research, the symmetry of its certainly unconventional superconducting state is under strong debate, e.g. the long-time favoured spin-triplet p x ?? ip y state is ruled out by recent NMR experiments [1]. However, in general time-reversal-symmetry breaking (TRSB) superconductivity indicates complex two-component order parameters. Probing Sr 2 RuO 4 under uniaxial stress offers the possibility to lift the degeneracy between such components [2]. One key prediction for Sr 2 RuO 4 , a splitting of the superconducting and TRSB transitions under uniaxial stress, has not been observed so far. I will show results of muon spin relaxation (??SR) measurements on Sr 2 RuO 4 placed under uniaxial stress, wherein a large stress-induced splitting between the onset temperatures of superconductivity and TRSB was observed [3]. Moreover, at high stress beyond the Van Hove singularity, a new spin density wave ordered phase was detected for the first time. In order to perform ??SR measurements under uniaxial stress, a custom piezoelectric based pressure cell was developed [4]. This cell is going to be useful for a range of other materials, in which the Fermi surface or magnetic interaction strengths can be tuned leading to strong modifications of the electronic state.
References:
[1] A. Pustogow, et al., Nature 574, 72 (2019)
[2] C. Hicks, et al., Science 344, 283 (2014)
[3] V. Grinenko*, S. Ghosh* et al., Nature Physics 17, 748??"754 (2021)
[4] S. Ghosh et al., Review of Scientific Instruments 91, 103902 (2020)
[5] S Ghosh et al. APS March Meeting, Bulletin of the American Physical Society, Vol 66, 1, (2021)

Chromatin Loops and Random Walks: From biology to physics, and back Again

Random walks arise naturally in many biological contexts. In this talk, I will focus on some examples of non-trivial random walks which are relevant for loops in chromatin. In the first part of the talk, we will look at how large loops are formed between distant base pairs. Recent experiments suggest that loop formation is mediated by Loop Extrusion Factor (LEF) proteins like cohesin. Experiments on cohesin have shown that cohesins walk diffusively on DNA, and that nucleosomes act as obstacles to the diffusion, lowering the permeability and hence reducing the effective diffusion constant. An estimation of the times required to form the large loops typically seen in Hi-C experiments using these low effective diffusion constants seems to predict unphysically large looping times. In this talk, I'll propose a simple answer to this puzzle, and show that nucleosomes may play an unexpected role in regulating loop formation times. Motivated by this example, in collaboration with experimentalists, we design and conduct simple table-top experiments to investigate the role of obstacles on the first passage times of random walks. We formulate and solve the equations for mean first passage times in such cases, which shows non-trivial behaviour for different classes of obstacles. Finally, we will briefly discuss ongoing work on how the looped structure of chromatin affects search processes and search time for proteins walking on these complex networks.

Newly observed collective behaviour in systems of active Brownian particles

The ability of Active Brownian particles to undergo motility-induced phase separation (MIPS) even in the absence of any adhesive forces is well known. Detailed studies of such systems in the past have concluded that their phase behaviour is relatively simple, with only a homogeneous state and a MIPS state. However, we show that the phase behaviour of such systems is richer than what was previously conceived. More specifically, we observe another transition at high motility, where the particles form a percolated cluster. This transition follows all the characteristics of a standard percolation transition. In the second part of the talk, we show that the aggregates of such particles on walls undergo a morphological transition with a change in wall interactions. This transition is strikingly similar to the wetting-dewetting transitions in equilibrium systems.

QTIH Seminar: Route to a universal photonic quantum computer design

Any quantum computing architecture has a few ingredients: (i) the choice of qubit, (ii) the platform, (iii) the type of computation model, and (iv) the quantum error correction module containing the encoding and decoding procedure. In this talk I will discuss some aspects of a photonic quantum computer design that I have been investigating in the recent few years. In particular, I will present some recent interest and case for a continuous-variable encoded bosonic qubit approach in photonics, using the measurement-based model implemented using 3D cluster states. I will also highlight some of the many challenges both from the theoretical and the experimental side, including the difficulty in preparing non-Gaussian photonic states required for universal quantum computation. While photonics hardware seems amenable to scaling, which of the different competing platforms will eventually succeed in realizing fault-tolerant quantum computation is still completely open.

Quantum heat engines with Carnot efficiency at maximum power

Heat engines constitute the major building blocks of modern technologies. However, conventional heat engines with higher power yield lesser efficiency and vice versa and respect various power-efficiency trade-off relations. This is also assumed to be true for the engines operating in the quantum regime. Here we show that these relations are not fundamental. We introduce quantum heat engines that deliver maximum power with Carnot efficiency in the one-shot finite-size regime. These engines are composed of working systems with a finite number of quantum particles and are restricted to one-shot measurements. The engines operate in a one-step cycle by letting the working system simultaneously interact with hot and cold baths via semilocal thermal operations. By allowing quantum entanglement between its constituents and, thereby, a coherent transfer of heat from hot to cold baths, the engine implements the fastest possible reversible state transformation in each cycle, resulting in maximum power and Carnot efficiency. Finally, we propose a physically realizable engine using quantum optical systems. References:
(1) Mohit Lal Bera, Sergi Juli? -Farr??, Maciej Lewenstein, and Manabendra Nath Bera, Quantum heat engines with Carnot efficiency at maximum power, Physical Review Research 4, 013157 (2022).
(2) Mohit Lal Bera, Maciej Lewenstein, Manabendra Nath Bera, Attaining Carnot efficiency with quantum and nanoscale heat engines, npj Quantum Information 7, 1-7 (2021).

Cooper pairing without superconductivity: Phase fluctuations, Pseudogap state, Superinsulator and Bose Metal

Cooper pairing, where pairs of electrons with opposite momentum and opposite spins form a singlet bound state is usually associated with the onset of superconductivity. Within the celebrated Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity spin-zero Cooper pairs condense into a phase coherent state, giving rise to the zero resistance. However, an increasing number of experiments suggest that superconductivity is just one of the exotic states made of Cooper pairs. For example, it is now observed that in many superconductors Cooper pairing continues to persist above Tc even though the zero resistance state is destroyed by phase fluctuations. On the other hand, even at very low temperatures in many systems under the application of magnetic field one observes transition to a ???superinsulator??� where the conductance instead of resistance appears to vanish at a finite temperature. It is believed that in a superinsulator the Cooper pairs are in an eigenstate of number instead of phase and hence localized. Even more interestingly, in some systems Cooper pairs appear to exist in a dissipative metallic state, called Bose metal. In this talk, I will talk about these various novel states in the backdrop of the work done over the last ten years in Superconductivity Lab at TIFR.