Talks

Control and manipulation of quantum states by measurements and bath engineering in open quantum systems, and associated phenomena, such as measurement-induced phase transitions, have emerged as new paradigms in many-body physics. Here, taking a prototypical example of Josephson junction arrays (JJAs), I will discuss how repetitive monitoring can transform an insulating state in these systems to a superconductor and vice versa. To this end, we study the effects of continuous weak measurements and feedback control on isolated JJAs in the absence of any external thermal bath. The monitoring due to combined effect of measurements and feedback, inducing non-unitary evolution and dissipation, leads to a long-time steady state characterized by an effective temperature in a suitably defined semiclassical limit. However, we show that the quantum dissipation due to monitoring has fundamental differences with equilibrium quantum and/or thermal dissipation in the well-studied case of JJAs in contact with an Ohmic bath. In particular, using a variational approximation, and by considering the semiclassical, strong measurement/feedback and weak-coupling limits, we demonstrate that this difference can give rise to re-entrant steady-state phase transitions, resulting in transition from an effective low-temperature insulating normal state to superconducting state at intermediate temperature. Our work emphasizes the role of quantum feedback, that acts as an additional knob to control the effective temperature of non-equilibrium steady state leading to a phase diagram, not explored in earlier works on monitored and open quantum systems.

We investigate the dynamics of a quantum system subjected to a time-dependent and conditional resetting protocol. Namely, we ask what happens when the unitary evolution of the system is repeatedly interrupted at random time instants with an instantaneous reset to a specified set of reset configurations taking place with a probability that depends on the current configuration of the system at the instant of reset? Analyzing the protocol in the framework of the so-called tight-binding model describing the hopping of a quantum particle to nearest-neighbor sites in a one-dimensional open lattice, we obtain analytical results for the probability of finding the particle on the different sites of the lattice. We explore a variety of dynamical scenarios, including the one in which the resetting time intervals are sampled from an exponential as well as from a power-law distribution. Under exponential resetting, the system relaxes to a stationary state characterized by localization of the particle around the reset sites. The choice of the reset sites plays a defining role in dictating the relative probability of finding the particle at the reset sites as well as in determining the overall spatial profile of the site-occupation probability. Furthermore, analyzing the case of power-law resetting serves to demonstrate that the attainment of the stationary state in this quantum problem is not always evident and depends crucially on whether the distribution of reset time intervals has a finite or an infinite mean. 

There is more matter than antimatter in the universe. This asymmetry requires three conditions: 1- Baryon (or lepton) number violation, 2- C and CP violation and 3- out-of-thermal equilibrium conditions in the early universe, before BBN. Although the SM does not have any out-of-equilibrium process, it does provide baryon number violation and CP violation. Still, it is commonly accepted that the SM CP violation is not enough for producing the observed baryon asymmetry. I will present one new physics model in which the SM CP violation, that is measured in the B meson system, is in fact enough to generate the baryon asymmetry. 

The 21-cm line from neutral hydrogen holds immense potential as a probe of early astrophysics and cosmic evolution. Realizing the potential of this observational probe, however, is limited by our ability to control systematic effects in the data and model the cosmological 21-cm signal. In this talk, I will provide an overview of the observational prospects made possible through the cosmic 21-cm signal and discuss the challenges confronting interferometric experiments that are targeting a high-redshift detection. I will focus on recent developments from the Hydrogen Epoch of Reionization Array (HERA), covering our latest upper limits on the 21-cm power spectrum and their implications for early X-ray heating of the intergalactic medium. I will additionally discuss our latest efforts to understand and mitigate mutual coupling, a complex systematic effect in the data that threatens to thwart our efforts to detect the cosmological 21-cm signal. I will conclude by discussing the path forward with HERA, with a preliminary view of what to expect from forthcoming analyses.

If two parties have access to entangled parts of a quantum state, the common lore suggests that when measurements are made by one of the parties and its outcomes are classically communicated to the other party, it leaves telltale signatures on the state of the part accessible to the other party. Here we show that this lore is not necessarily true -- in generic scrambling dynamics within a tripartite setting (with the $R$, $S$ and $E$ labelling the three parts), a new kind of dynamical phase emerges, wherein local measurements on $S$ are invisible to one of the remaining two parts, say $R$, despite there existing non-trivial quantum correlations and entanglement between $R$ and $S$. At the heart of this lies the fact that information scrambling transmutes local quantum information into a complex non-local web of spatiotemporal quantum correlations. This non-locality in the information then means that ignorance of the state of part $E$ can leave $R$ and $S$ with sufficient information for them to be quantum correlated or entangled but not enough for measurements on $S$ to have a non-trivial backaction on the state of $R$. This new dynamical phase is sandwiched between two conventionally expected phases where the $R$ and $S$ are either disentangled from each other or are entangled along with non-trivial measurement backaction. This provides a new characterisation of entanglement phases in terms of their response to measurements instead of the more ubiquitous measurement-induced entanglement transitions. Our results have implications for the kind of tasks that can be performed using measurement feedback within the framework of quantum interactive dynamics.

Quantum measurements are described as instantaneous projections in textbooks. They can be stretched out in time using weak measurements, whereby one can observe the evolution of a quantum state towards one of the eigenstates of the measured operator. This evolution is a continuous nonlinear stochastic process, generating an ensemble of quantum trajectories. In particular, the Born rule can be interpreted as a fluctuation-dissipation relation. We experimentally observe the entire quantum trajectory distribution for weak measurements of a superconducting transmon qubit in circuit QED architecture, quantify it, and demonstrate that it agrees very well with the predictions of a single-parameter white-noise stochastic process. This characterisation of quantum trajectories is a powerful clue to unraveling the dynamics of quantum measurement, beyond the conventional axiomatic quantum theory. We emphasise the key quantum features of this framework, and their implications.

Noise in quantum hardware poses the biggest challenge to realizing robust and scalable quantum computing devices. While conventional quantum error correction (QEC) schemes are relatively resource-intensive, approximate QEC (AQEC) promises a comparable degree of protection from specific noise channels using fewer physical qubits [1 ]. However, unlike standard QEC, the AQEC framework faces hurdles in reliable syndrome measurements due to the overlapping syndrome subspaces leading to the violation of the distinguishability criterion of error subspaces. Our work [2 ] provides an algorithm for discriminating overlapping syndrome subspaces based on the Gram-Schmidt-like orthogonalization routine. In the recovery, we map these orthogonal and disjoint subspaces to the code space followed by a recovery like the perfect recovery [1 , 3 ], or the Petz map [4, 5]. We further prove that this evolved recovery utilizing the Petz map (which we call the canonical Petz map ) gives optimal protection on the information regarding the measure of entanglement fidelity. We show that the performance of the canonical Petz map is similar to that of the Fletcher recovery [ 6 ]. 

[1] D. W. Leung, M. A. Nielsen, I. L. Chuang, and Y. Yamamoto, Approximate quantum error correction can lead to better codes, Physical Review A 56, 2567 (1997).
[2] D. Biswas and P. Mandayam, Efficient syndrome detection for approximate quantum error correction – road towards the optimal recovery, Manuscript is under preparation (2025).
[3] M. A. Nielsen and I. L. Chuang, Quantum Computation and Quantum Information (Cambridge University Press, 2000).
[4 ] H. K. Ng and P. Mandayam, Simple approach to approximate quantum error correction based on the transpose channel, Phys. Rev. A 81, 062342 (2010).
[5] H. Barnum and E. Knill, Reversing quantum dynamics with near- optimal quantum and classical fidelity, Journal of Mathematical Physics 43, 2097 (2002).
[6] A. S. Fletcher, P. W. Shor, and M. Z. Win, Channel-adapted quantum error correction for the amplitude damping channel, IEEE Transactions on Information Theory 54, 5705 (2008).

While a generic open quantum system decays to its steady state, continuous time crystals (CTCs) develop spontaneous oscillation and never converge to a stationary state. Just as crystals develop correlations in space, CTCs do so in time. Here, we introduce a Continuous Quasi Time Crystals (CQTC). Despite being characterized by the presence of non-decaying oscillations, this phase does not retain its long-range order, making it the time analogous of quasi-crystal structures. We investigate the emergence of this phase in a system made of two coupled collective spin sub-systems, each developing a CTC phase upon the action of a strong enough drive. The addition of a coupling enables the emergence of different synchronized phases, where both sub-systems oscillate at the same frequency. In the transition between different CTC orders, the system develops chaotic dynamics with aperiodic oscillations. These chaotic features differ from those of closed quantum systems, as the dynamics is not characterized by a unitary evolution. At the same time, the presence of non-decaying oscillations makes this phenomenon distinct from other form of chaos in open quantum system, where the system decays instead. We investigate the connection between chaos and this quasi-crystalline phase using mean-field techniques, and we confirm these results including quantum fluctuations at the lowest order.