Search Talks

Feedback issues relying on classical controllers (optimizing QND measurement via Markovian feedback, quantum state stabilization via Bayesian feedback) and on quantum controllers (stabilization of Schrödinger cats via autonomous feedback).

We introduce a numerical method to sample the distributions of charge, heat, and entropy production in open quantum systems coupled strongly to macroscopic reservoirs, with both temporal and energy resolution and beyond the linear-response regime. Our method exploits the mesoscopic-leads formulation, where macroscopic reservoirs are modeled by a finite collection of modes that are continuously damped toward thermal equilibrium by an appropriate Gorini-Kossakowski-Sudarshan-Lindblad master equation. Focussing on non-interacting fermionic systems, we access the time-resolved full counting statistics through a trajectory unraveling of the master equation. We show that the integral fluctuation theorems for the total entropy production, as well as the martingale and uncertainty entropy production, hold. Furthermore, we investigate the fluctuations of the dissipated heat in finite-time information erasure. Conceptually, our approach extends the continuous-time trajectory description of quantum stochastic thermodynamics beyond the regime of weak system-environment coupling.

In this work, we develop a panoramic schematic of a quantum thermoelectric circuit theory in the steady state regime. We establish the foundations of the said premise by defining the analogs of Kirchhoff's laws for heat currents and temperature gradients. We further show that our approach encompasses various circuits like thermal diode, transistor, and Wheatstone bridge. Additionally, we have been able to develop a model of a quantum thermal step transformer. We also construct a novel model of a thermal adder circuit, paving the way to develop thermal integrated circuits. This sheds new light on the present architecture of quantum device engineering.

We describe the smallest quantum error correcting (QEC) code to correct for amplitude-damping (AD) noise, namely, a 3-qubit code that corrects up to first order in the damping strength. We generalize this construction to create a family of codes that correct AD noise up to any fixed order. We underpin the fundamental connection between the structure of our codes and the noise structure via a relaxed form of the Knill-Laflamme conditions, that are different from existing formulations of approximate QEC conditions. Although the recovery procedure for this code is non-deterministic, our codes are optimal with respect to overheads and outperform existing codes to tackle AD noise in terms of entanglement fidelity. This alternate formulation of approximate QEC in fact leads us to a new class of quantum codes tailored to AD noise and also gives rise to a noise-adapted quantum Hamming bound for AD noise.

We devise an autonomous quantum thermal machine consisting of three pairwise-interacting qubits, two of which are locally coupled to thermal reservoirs. The machine operates autonomously, as it requires no time-coherent control, external driving or quantum bath engineering, and is instead propelled by a chemical potential bias. Under ideal conditions, we show that this out-of-equilibrium system can deterministically generate a maximally entangled steady-state between two of the qubits, or any desired pure two-qubit entangled state, emerging as a dark state of the system. We study the robustness of entanglement production with respect to several relevant parameters, obtaining nearly-maximally-entangled states well-away from the ideal regime of operation. Furthermore, we show that our machine architecture can be generalised to a configuration with 2n−1 qubits, in which only a potential bias and two-body interactions are sufficient to generate genuine multipartite maximally entangled steady states in the form of a W state of n qubits.

In this talk, we describe recent NMR experiments demonstrating fast quantum state preparation. In particular, we describe counter-diabatic drive, quantum alternating operator ansatz, feedback-assisted quantum control, as well as nonlinear evolutions via ancilla-assisted superposition of unitaries.

Exploring continuous time crystals (CTCs) within the symmetric subspace of spin systems has been a subject of intensive research in recent times. Thus far, the stability of the time-crystal phase outside the symmetric subspace in such spin systems has gone largely unexplored. Here, I present results relating the effect of including the asymmetric subspaces on the dynamics of CTCs in a driven dissipative spin model. This results in multistability, and the dynamics becomes dependent on the initial state. Remarkably, this multistability leads to exotic synchronization regimes such as chimera states and cluster synchronization in an ensemble of coupled identical CTCs. Interestingly, it leads to other nonlinear phenomena such as oscillation death and signature of chaos.

(based on work with coauthors reported in Phys. Rev. Lett. 133, 260403, 2024)

Quantum measurements give rise to back-action on the measured system. Tuning the quantum measurement dynamics, and repeating the measurement protocol irrespective of the detectors’ readouts, may be employed to engineer a stable target state. Such a scheme is referred to as a passive quantum steering protocol. The ground state of a given Hamiltonian may or may not be steerable, depending on whether the Hamiltonian is non-frustrated or frustrated. We will discuss how cooling to the ground state may be facilitated even when acting on (measuring) small parts of the system ( “dilute cooling”).
We will also discuss how close to the ground state one can get in the presence of non-steerable frustrated Hamiltonians.

Spin ensembles are promising quantum technological platforms, but their utility relies on the ability to perform quantum error correction (QEC) for decoherences in these systems. Typical QEC for ensembles requires addressing individually resolved qubits, but this is practically challenging in most realistic architectures. Here, we propose QEC schemes for unresolvable spin ensembles. By using degenerate superpositions of excited states, which are fundamentally mixed, we find codes that can protect against both individual and collective errors, including dephasing, decay, and pumping. We show how information recovery can be achieved with only collective measurement and control, and illustrate its applications in extending memory lifetime and loss-tolerant sensing.

We investigate the thermodynamic uncertainty relation (TUR), i.e., a trade-off between entropy production rate and relative power fluctuations, for nondegenerate three-level and degenerate four-level maser heat engines. In the nondegenerate case, we consider two slightly different configurations of the three-level maser heat engine and contrast their degree of violation of the standard TUR. We associate their different TUR-violating properties to the phenomenon of spontaneous emission, which gives rise to an asymmetry between them. Furthermore, in the high-temperature limit, we show that the standard TUR relation is always violated for both configurations. For the degenerate four-level engine, we study the effects of noise-induced coherence on the TUR. We show that, depending on the parametric regime of operation, noise-induced coherence can either suppress or amplify the relative power fluctuations.