Talks

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.

We consider the problem of quantum error correction (QEC) for non-Markovian noise. We show that conditions for approximate QEC can be easily generalized for the case of non-Markovian noise, in the strong coupling regime where the noise map becomes non-completely-positive at intermediate times. While certain adaptive recovery schemes are ineffective against quantum non-Markovian noise, in the sense that the fidelity vanishes in finite time, a specific strategy based on the Petz map uniquely safeguards the code space even at the maximum noise limit. Focusing on the case of non-Markovian amplitude damping noise, we observe that the non-Markovian Petz map also outperforms the standard, stabilizer-based QEC code. Since implementing such a non-Markovian map poses practical challenges, we also construct a Markovian Petz map that achieves similar performance, with only a slight compromise on the fidelity.
[Based on arXiv:2411.09637]

The thermodynamics of quantum systems driven out of equilibrium has attracted increasing attention in the last decade, in connection with quantum information and statistical physics, and with a focus on non-classical signatures. While a first approach can deal with average thermodynamics quantities over ensembles, in order to establish the impact of quantum and environmental fluctuations during the evolution, a continuous quantum measurement of the open system is required. Such a framework has been developed during the last decade, with recent advances incorporating multiple conserved quantities, the assessment of thermodynamic quantities at stopping times using martingale theory, and the consideration of imperfect and partial monitoring schemes. These advances provide new universal relations in the form of fluctuation theorems and inequalities that refine our understanding of the second-law of thermodynamics in different senses.

Structure of dynamical models describing  open quantum systems including measurement back-action and decoherence: discrete-time models based on quantum channels and  left stochastic matrices;  continuous-time  models driven by Wiener processes (weak measurement) and Poisson processes (quantum jump and counting measurement).