Talks

We consider the dynamics of a continuously monitored qubit in the limit of strong measurement rate where the quantum trajectory is described by a stochastic master equation with Poisson noise. Such limits are expected to give rise to quantum jumps between the pointer states associated with the non-demolition measurement. A surprising discovery in earlier work (Tilloy et al., Phys. Rev. A 92, 052111 (2015)) on quantum trajectories with Brownian noise was the phenomena of spikes observed in between the quantum jumps. Here, we show that spikes are observed also for Poisson noise. We consider three cases where the non-demolition is broken by adding, to the basic strong measurement dynamics, either unitary evolution or thermal noise or additional measurements. We present a complete analysis of the spike and jump statistics for all three cases using the fact that the dynamics effectively corresponds to that of stochastic resetting. We provide numerical results to support our analytic results. In addition, we propose protocols for the experimental detection of spikes.

In the presence of environmental decoherence, achieving unit fidelity in quantum state preparation is often unattainable. Monitoring the environment and performing feedback based on the results can enhance the maximum achievable fidelity, yet unit fidelity remains elusive in many scenarios. We derive a theoretical bound on the average fidelity in the ideal case of perfect environmental monitoring. The work focuses on the challenge of preparing Dicke states under collective noise, employing machine learning techniques to identify optimal control protocols. These protocols are then compared against the derived theoretical bound, offering insights into the limits of fidelity in continuously monitored quantum systems.

In continuous measurement, we never have empirical access to the true continuous signal, but rather, to a digitized (discretized) average of the true signal over a finite number time bins. If these time bins are really much smaller than all other time scales, one can reconstruct the quantum trajectory naively with an Euler scheme. Even then, it is just a (good) approximation that cannot be refined. Can one do better? I'll show that one can define a quantum state conditioned on the binned signal (i.e. on a finite list of signal averages), and that, quite surprisingly, this binned conditional state can be efficiently reconstructed. This allows more accurate (in the Bayesian sense) reconstruction of quantum trajectories from data, and can also be used for sampling of trajectories.

In January 2024, the Laser Interferometer Space Antenna (LISA) mission was officially adopted by the European Space Agency, marking a new era in gravitational wave astronomy. LISA will be the first space-based gravitational wave detector, designed to explore the cosmos in the millihertz frequency range. This talk will present the mission's key scientific objectives and how the scientific community is preparing for the exploitation of LISA data. I will discuss the anticipated source types and the fundamental questions they could help answer. Then, we will focus on Extreme Mass Ratio Inspirals (EMRIs), a class of sources where small compact objects orbit the massive black holes at the centers of galaxies. These systems hold immense scientific potential for the LISA mission, as they encode a detailed map of the spacetime around the massive black hole. I will discuss how future detections of EMRIs can be used to constrain parameters related to accretion disks and modifications of General Relativity. Finally, I will highlight the path forward in preparing for LISA's launch and how to get involved in contributing to the mission scientific success. 

Measurements are able to fundamentally affect quantum dynamics. We show that a continuously measured quantum many-body system can undergo a spontaneous transition from asynchronous stochastic dynamics to noise-free stable synchronization at the level of single trajectories. We formulate general criteria for this quantum phenomenon to occur and demonstrate that the number of synchronized realizations can be controlled from none to all. We additionally find that ergodicity is typically broken, since time and ensemble averages may exhibit radically different synchronization behavior. We further introduce a quantum type of multiplexing that involves individual trajectories with distinct synchronization frequencies. Measurement-induced synchronization appears as a genuine nonclassical form of synchrony that exploits quantum superpositions.

We will start by discussing page-curve entanglement dynamics in freely expanding fermionic gas [1]. We will then discuss time dynamics of entanglement entropy between a filled fermionic system and an empty reservoir when there are dephasing effects [2] under various geometries.  In this context, we will employ two different kinds of quantum trajectory approaches, namely stochastic unitary unraveling and quantum state diffusion. Our findings are expected to hold for a wide variety of generic interacting quantum systems and systems subject to environmental imperfections. 

[1] M. Saha, M. Kulkarni, A. Dhar, Phys. Rev. Lett. 133, 230402 (2024)
[2] K. Ganguly, P. Gopalakrishnan, A. Naik, B. K. Agarwalla, M. Kulkarni, arXiv:2501.12110 (2025)

We present a protocol for measurement-induced transition, where at each time-step we evolve a quantum Ising chain under transverse Ising Hamiltonian for time τ, and then make a global measurement with certainty. For system size ≤28, there is a transition in entanglement at some critical value τ = τc. We also calculate survival probability of the initial state and find that some quantity derived from this probability also shows a peak at τc. Using a recurrence relation, one can compute survival probability in our set-up for size upto 1000. It is found that the critical value of τ follows a scaling τc ∝1/√N. Hence, the transition occurs for finite size only. It will be interesting to investigate the size-dependence of critical point in other measurement-induced transitions.