Scientific Series

In this talk I will discuss recent advances in Neural-Network parametrisations of pure and mixed quantum states that can be efficiently sampled.  In the first part I will generalise the Neural Density Operator ansatz originally proposed by Torlai and Melko by combining two general ingredients that can be used to construct deep, autoregressive ansatze that automatically enforce positive definitness.  In the second part, instead, I will show that any matrix product state can be exactly represented by a recurrent neural network with a linear memory update. I will then discuss how to generalise this linear RNN architecture to 2D lattices, comparing both approaches to standard DMRG calculations.

Zoom link:  https://pitp.zoom.us/j/98833163484?pwd=bEZTeTB0c1l1QmkrQXdEc3dBQ0dJZz09

The axion solution to the strong CP problem has many virtues but it suffers from the so-called quality problem. This problem is avoided in heavy axion realizations which, however, are harder to probe experimentally. I will show how generic heavy axion models can be tested in current and future gravitational wave (GW) observatories, due to the cosmological GW background produced by the axionic topological defects. Moreover, the resulting GW signal is correlated with a sizable strong CP phase, within reach of upcoming neutron electric dipole moment measurements.

Zoom link:  https://pitp.zoom.us/j/99150077172?pwd=UWhkZVdzV2lCQittWXVtMnludXBkUT09

In the standard cosmological paradigm, the initial condition follows Gaussian statistics. At later times, gravitational evolution induces nonlinearities in the large-scale structure, information that was fully captured by the two-point statistics at the early times gets spread into higher-order statistics. Whilst current standard cosmological analyses have focused on two-point statistics, higher-order statistics help further to tighten constraints by breaking parameter degeneracies as well as to probe the primordial Universe. In this talk, I will present our recent progress on the N-point Correlation Function (NPCF), including an analytical Gaussian covariance formalism, a first detection of the 4-point correlation function from nonlinear structure formation. Finally, I will focus on our recent analysis of parity-odd mode using the data from Baryon Oscillation Spectroscopic Survey (BOSS) and discuss the implication of parity-search at cosmological scales with large scale structure.

Zoom Link: https://pitp.zoom.us/j/92321535422?pwd=VlF4cHpvUit4bmR0eXYyczI5Qmw4dz09

Any consistent regularization scheme induces an apparent violation of gauge invariance in non-anomalous chiral gauge theories. This violation shows up in perturbative calculations, and can be removed by including appropriate finite counterterms. In this talk I will discuss the derivation of such counterterms for a renormalizable chiral gauge theory.  I will use the background field method, which ensures background gauge invariance in the quantized theory. As a concrete application, I will show the finite counterterm at one loop in the Standard Model, within dimensional regularization and the Breitenlohner-Maison-'t Hooft-Veltman prescription for gamma5. 

Zoom Link: https://pitp.zoom.us/j/95507223984?pwd=Y0FDTEJpNDlVaE9Ba1ZNMURXeS9rdz09

Understanding the reason behind the observed accelerating expansion of the Universe is one of the most notable puzzles in modern cosmology, and conceivably in fundamental physics. In the upcoming years, near future surveys will probe structure formation with unprecedented precision and will put firm constraints on the cosmological parameters, including those that describe properties of dark energy. In light of this, in the first part of my talk, I'm going to show a systematic extension of the Effective Field Theory of Dark Energy framework to non-linear clustering. As a first step, we have studied the k-essence model and have developed a relativistic N-body code, k-evolution.

I'm going to talk about the k-evolution results, including the effect of k-essence perturbations on the matter and gravitational potential power spectra and the k-essence structures formed around the dark matter halos. In the second part of my talk, I'm going to show that for some choice of parameters the k-essence non-linearities suffer from a new instability and blow up in finite time.

This talk is based on: arXiv:2204.13098, arXiv:2205.01055, arXiv:2107.14215, arXiv:2007.04968, arXiv:1910.01105, arXiv:1910.01104.

Zoom Link: https://pitp.zoom.us/j/99797451101?pwd=dituM2d2MDFCbDgyVXJ4c2s1NVoyUT09

An exciting development in outer solar system studies is the discovery of a new class of Kuiper belt objects with orbits that lie outside that of Neptune and have semimajor axes in excess of 250 A.U. The alignment of the major axes of these objects and other orbital anomalies are the basis for the Planet Nine hypothesis that an undiscovered giant planet orbits the sun at a distance of around 500 A.U. We show that a modified gravity theory known as MOND (Modified Newtonian Dynamics) provides an alternative explanation for the observed alignment, owing to significant quadrupolar and octupolar terms in the MOND galactic field within the solar system that are absent in Newtonian gravity. Using the well-established secular approximation of solar system dynamics we predict a population of Kuiper belt objects whose major axes are aligned with the direction to the center of the galaxy and that have additional clustering in orbital parameters. These features are exhibited by known Kuiper belt objects of the newly discovered class in support of the MOND hypothesis. Thus MOND, originally developed to explain galaxy rotation without invoking dark matter, may also be observable in the outer solar system.

Quantum mechanics is a cornerstone of modern physics. Just as the 19th century was called the Machine Age and the 20th century the Information Age, the 21st century promises to go down in history as the Quantum Age. Quantum Computing promises unprecedented speed in solving certain classes of problems while Quantum Cryptography promises unconditional security in communications. In this talk, I will discuss the world of single and entangled photons and also discuss ongoing work towards quantum computing, quantum information and quantum cryptography in our Quantum Information and Computing lab at the Raman Research Institute, Bengaluru. I will end with our broad vision for the future, which includes establishment of long distance secure quantum communications in India and beyond involving satellite based, fibre based as well as integrated photonics based approaches towards the global quantum internet.

Zoom Link: https://pitp.zoom.us/j/94788493307?pwd=QTlUaTY4Nm1IS3hyeUFIbVFKV3RMQT09

New massive "dark" vector bosons are ubiquitous in BSM physics.  I'll discuss the effective theories, interactions, and applications of theories with a (single) massive vector boson with a Stückelberg mass, with the goal of providing a clear and self-consistent approach to the (sometimes confusing) subject.  The critical role of possible couplings of the longitudinal mode will be emphasized, demarcating when theories have amplitudes that grow with energy.  In such theories, I'll identify the cutoff scale, as well as show the potential of longitudinally-enhanced observables given specific couplings of the vector boson.  The Stückelbergian approach to massive vectors also provides insight into generalized dark vector boson theories; I'll demonstrate a specific application to an "electrophobic" vector field that can evade many terrestrial experiment constraints that would otherwise apply to a dark photon.

Zoom Link: https://pitp.zoom.us/j/96937794653?pwd=cjIvM0hxQmo2ZEhuamhxeWx3cVdoZz09

Inflation can be viewed as a natural "cosmological particle detector" which can probe energies as high as its Hubble scale. In this talk, I study the imprints of heavy relativistic particles during inflation on primordial correlators in situations where the scalar fluctuations have a reduced speed of sound. Breaking dS boosts allows new types of footprints of massive fields to emerge. In particular, I show that heavy particles that are lighter than Hubble divided by the speed of sound leave smoking gun imprints in the three-point function of curvature perturbations (due to the exchange of those fields) in the form of resonances in the squeezed limit which are vividly distinct from the previously explored signatures of heavy fields in de Sitter correlators. Throughout I use and extend the cosmological bootstrap techniques derived from locality, unitarity, and analyticity in order to find fully analytical formulae for the desired boost breaking correlators.

The cosmic radio background (CRB) contains diffuse, discrete and cosmological emission sources from the universe. In this talk, I will discuss  wether an additional contribution coming from low frequency photons is produced by free-free emission during the recombination era of the univers. These photons do not readily thermalise with the electron-proton plasma and therefore might  survive as a small non-thermal relic in the current universe.  Inclusion of such a contribution can alleviate the observed tension between existing CRB models and explain radio source counts between $100 MHz$ to $10 GHz$ to a very high degree of precision.  I will also discuss the challenges and future work we must considere in order to establish wether the signal is created or not.

Zoom Link: https://pitp.zoom.us/j/99032512514?pwd=eldocVA2VHhoUGVudmtFN3F4NHNEQT09

I will discuss a recent result on an intimate link between two a priori distinct phenomena: quantum nonlocality without entanglement and classically-achievable indefinite causal order. The first phenomenon refers to a multipartite scenario where the parties are unable to perfectly discriminate orthogonal product states drawn from an ensemble of quantum states by using local operations and classical communication (LOCC). The second (hypothetical) phenomenon refers to a multipartite scenario where the parties can communicate classically but the local operations of each party are in the future of the other parties, i.e., they cannot be ordered causally. Specifically, I will show how three separated parties with access to a classical process exhibiting indefinite causal order---the AF/BW process---can perfectly discriminate the states in an ensemble---the SHIFT ensemble---that exhibits quantum nonlocality without entanglement. Time permitting, I will discuss the generalization of this result beyond the tripartite case and comment on its connection with separable operations that are outside LOCC. 

Based on joint work with Ämin Baumeler, arXiv:2202.00440.

Zoom Link: https://pitp.zoom.us/j/93727212623?pwd=cjVRL3cvMmhicDRic3lXRFBkNi9xZz09