Our research interests in theoretical physics broadly focus on understanding nonequilibrium quantum dynamics in various mesoscopic systems. During the last five years, we have significantly contributed to developing waveguide quantum electrodynamics (WQED), understanding thermalization and quantum chaos in disordered many-body models, topological characterization of non-Hermitian systems, and performing precision measurements using spin noise spectroscopy (SNS).
The WQED is a relatively new research discipline [1] where strong, effective interactions between propagating single photons can be realized by employing strong light-matter coupling inside waveguide geometries. These cavity-free systems feature intrinsically nonequilibrium, quantum many-body dynamics. We developed different theoretical tools using multi-particle scattering theories [2,3,4] and the Heisenberg-Langevin equations approach [4,5,6] for studying light propagation in WQED systems. We studied electromagnetically induced transparency [3] and correlated light transmission in these systems [4,6]. We proposed new all-optical devices, e.g., a nonlinear optical isolator/diode [2,5] and two-photon optical probe, and investigated the functionality of already proposed devices, e.g., a single-photon router, a quantum amplifier. Our proposal of the nonlinear optical diode [2,5] was recently experimentally realized using superconducting circuits (PRL 121, 123601 (2018)). In an article in Rev. Mod. Phys. [1], we discussed the topic of strongly interacting photons when confined to one dimensional geometry from empirical and theoretical perspectives.
We have extended the recent effort to identify microscopic mechanisms of quantum chaos to many-body fermionic [7], bosonic [8], and mixed [9] systems in the presence or absence of conserved particle numbers. A new dynamical chaos mechanism has been found, which maps the spectral form factor to an average recurrence probability of a classical Markov chain with transition probabilities given as square-moduli of hopping amplitudes. We have discovered universal non-abelian symmetries of the Markov matrices, whose subleading eigenvalues determine the system-size scaling of the Thouless timescales to reach universal random matrix theory form for the spectral form factor in these systems. These results provide a valuable tool for investigating the ergodic phase of long-range interacting systems with the disorder, extensively researched for many-body localization transition [10].
We have shown interplay of topology and nonequilibrium in quantum systems, particularly the role of topology in characterizing nonequilibrium dynamics or vice versa. We have demonstrated the absence of thermalization in Josephson junctions of topological superconductors [11] and correlated magnets with gapped spectrum [12]. We have suggested that contrary to adiabatic and cyclic geometric phase for Hermitian systems, both adiabatic and nonadiabatic/dynamical descriptions are required to understand the topology of the phases in non-Hermitian multipartite systems. We have proposed intriguing composite metallic and insulating phases with the topology of Möbius strip and Penrose triangle in such non-Hermitian systems [13,14]. We also provide a new interpretation of the Skin effect in these systems through special open boundary conditions [13]. We have further shown special dynamical signatures of these composite topological phases [15].
The SNS is a relatively non-invasive measurement tool to monitor spontaneous spin fluctuations or “spin noise” of a material with an off-resonant linearly polarized laser beam. The SNS was mainly developed at Los Alamos to passively detect dynamical spin properties of dilute atomic gases, bulk semiconductors, and nanostructures in equilibrium. We have collaborated with the researchers at Los Alamos to extend SNS applications to measure correlations between different spin resonances and cross-correlations among various spin species in an atomic mixture [16]. Along with our experimental collaborators at RRI, we set up the SNS in India for the first time. We used it to perform precision measurements of different spin properties of atomic vapors and ultracold atoms in and out of equilibrium [17,18].
References:
1. D. Roy, C. M. Wilson and O. Firstenberg, Rev. Mod. Phys. 89, 021001 (2017)
2. D. Roy, Phys. Rev. B 81, 155117 (2010)
3. D. Roy, Phys. Rev. Lett. 106, 053601 (2011)
4. A. Vinu and D. Roy, Phys. Rev. A 107, 023704 (2023)
5. D. Roy, Phys. Rev. A 96, 033838 (2017)
6. P. Manasi and D. Roy, Phys. Rev. A 98, 023802 (2018)
7. D. Roy and T. Prosen, Phys. Rev. E 102, 060202(R) (2020)
8. D. Roy, D. Mishra and T. Prosen, Phys. Rev. E 106, 024208 (2022)
9. V. Kumar and D. Roy, Phys. Rev. E 109, L032201 (2024)
10. R. Singh, R. Moessner and D. Roy, Phys. Rev. B 95, 094205 (2017)
11. N. Bondyopadhaya and D. Roy, Phys. Rev. B 99, 214514 (2019)
12. M. Ljubotina, D. Roy and T. Prosen, Phys. Rev. B 106, 054314 (2022)
13. V. M. Vyas and D. Roy, Phys. Rev. B 103, 075441 (2021)
14. R. Nehra and D. Roy, Phys. Rev. B 105, 195407 (2022)
15. R. Nehra and D. Roy, Phys. Rev. B 109, 094311 (2024)
16. D. Roy, L. Yang, S. A. Crooker and N. A. Sinitsyn, Scientific Reports 5, 9573 (2015)
17. M. Swar, D. Roy, Dhanalakshmi D, S. Chaudhuri, S. Roy and H. Ramachandran, Optics Express 26, 32168 (2018)
18. M. Swar, D. Roy, S. Bhar, S. Roy and S. Chaudhuri, Phys. Rev. Res. 3, 043171 (2021)
Our current and past research activities in nonequilibrium quantum dynamics are summarized in Research themes.