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).

Quantum theory contravenes classical macrorealism by allowing a system to be in a superposition of two or more physically distinct states, producing physical consequences radically different from that of classical physics. Motivated by this, we construct superpositions between time evolution unitaries and study the dynamics of a qubit under such superposed unitary operators. We find that the superposition of unitaries significantly affects the trajectory of the qubit in the Bloch sphere by shifting the path of evolution and making the speed of evolution non-linear in time. The qubit spends more time near the poles of the Bloch sphere and passes through the equator rather quickly. This  remarkably enhances the endurance against dephasing noise, making the superposed unitaries suitable for robust quantum control tasks. Moreover, we observe an extreme violation of Leggett-Garg inequalities beyond the temporal Tsirelson's bound, which increases with increasing superposition between the unitaries. This shows stronger temporal correlations achieved by the superposed unitares. Using an NMR quantum register, we experimentally demonstrate the superposition of unitaries with the help of an ancillary qubit and verify our theoretical predictions.

Quantum walks, the quantum analogs of classical random walks, have become powerful tools in quantum information processing, offering unique advantages in areas such as quantum computation, search algorithms, and quantum transport. While homogeneous quantum walks with uniform coin operations have been well studied, introducing inhomogeneity - by varying the coin operator or evolution across time and space opens new avenues for controlling the dynamics and properties of quantum systems. Our research has explored the impact of such inhomogeneous quantum walks, yielding two significant results. First, we demonstrated Parrondo's paradox in discrete-time quantum walks using space- and time-dependent coins, achieving paradoxical outcomes without requiring higher-dimensional coins or decoherence, thus enhancing the practicality of implementations [1]. Second, by introducing a Gaussian-profiled coin rotation angle, we showed that this configuration not only improves localization of the walker's probability distribution but also generates maximal entanglement rapidly and a correlation that is robust against decoherence [2]. These findings underscore the potential of inhomogeneous quantum walks for more efficient and resilient quantum technologies.