- Quantum coherence enables hybrid multitask and multisource regimes in
autonomous thermal machinesby Kenza Hammam on August 30, 2023 at 3:12 pm
Non-equilibrium effects may have a profound impact on the performance of thermal devices performing thermodynamic tasks such as refrigeration or heat pumping. The possibility of enhancing the […]
- Thermodynamics of hybrid quantum rotor devicesby Heather Leitch on April 17, 2023 at 10:00 am
We investigate the thermodynamics of a hybrid quantum device consisting of two qubits collectively interacting with a quantum rotor and coupled dissipatively to two equilibrium reservoirs at […]
- Non-equilibrium thermodynamics of gravitational objective-collapse
modelsby Simone Artini on February 5, 2025 at 1:47 pm
We investigate the entropy production in the Di\'osi-Penrose (DP) model, one of the most extensively studied gravity-related collapse mechanisms, and one of its dissipative extensions. To this end, we analyze the behavior of a single harmonic oscillator, subjected to such collapse mechanisms, focusing on its phase-space dynamics and the time evolution of the entropy production rate, a central quantity in non-equilibrium thermodynamics. Our findings reveal that the original DP model induces unbounded heating, producing dynamics consistent with the Second Law of thermodynamics only under the assumption of an infinite-temperature noise field. In contrast, its dissipative extension achieves physically consistent thermalization in the regime of low dissipation strength. We further our study to address the complete dynamics of the dissipative extension, thus including explicitly non-Gaussian features in the state of the system that lack from the low-dissipation regime, using a short-time approach.
- Positivity and Entanglement in Markovian Open Quantum Systems and Hybrid
Classical-Quantum Theories of Gravityby Oliviero Angeli on January 27, 2025 at 5:53 pm
Markovian master equations underlie many areas of modern physics and, despite their apparent simplicity, they encode a rich and complex dynamics which is still under active research. We identify a class of continuous variable Markovian master equations for which positivity and complete positivity become equivalent. We apply this result to characterize the positivity of the partially transposed evolution of bipartite Gaussian systems, which encodes the dynamics of entanglement. Finally, the entangling properties of models of classical gravity interacting with quantum matter are investigated in the context of the experimental proposals to detect gravitationally induced entanglement. We prove that entanglement generation can indeed take place within these models. In particular, by focusing on the Di\'osi-Penrose model for two gravitationally interacting masses, we show that entanglement-based experiments would constrain the free parameter of the model $R_0$ up to values six orders of magnitude stronger than the current state of the art.
- Improved bounds on collapse models from rotational noise of LISA
Pathfinderby Davide Giordano Ario Altamura on January 15, 2025 at 5:35 pm
Spontaneous wavefunction collapse models offer a solution to the quantum measurement problem, by modifying the Schr\"odinger equation with nonlinear and stochastic terms. The Continuous Spontaneous Localisation (CSL) model is the most studied among these models, with phenomenological parameters that are constrained by experiments. Here, we exploit the recent analysis of LISA Pathfinder's angular motion data to derive a tighter constraint than previously achieved with translational motion. Moreover, we identify the general conditions for preferring rotational measurement over translational ones for constraining the CSL model.
- Machine Learning-Enhanced Characterisation of Structured Spectral
Densities: Leveraging the Reaction Coordinate Mappingby Jessica Barr on January 13, 2025 at 5:02 pm
Spectral densities encode essential information about system-environment interactions in open-quantum systems, playing a pivotal role in shaping the system's dynamics. In this work, we leverage machine learning techniques to reconstruct key environmental features, going beyond the weak-coupling regime by simulating the system's dynamics using the reaction coordinate mapping. For a dissipative spin-boson model with a structured spectral density expressed as a sum of Lorentzian peaks, we demonstrate that the time evolution of a system observable can be used by a neural network to classify the spectral density as comprising one, two, or three Lorentzian peaks and accurately predict their central frequency.
- Entropy Density Benchmarking of Near-Term Quantum Circuitsby Marine Demarty on December 23, 2024 at 10:01 pm
Understanding the limitations imposed by noise on current and next-generation quantum devices is a crucial step towards demonstrations of quantum advantage with practical applications. In this work, we investigate the accumulation of entropy density as a benchmark to monitor the performance of quantum processing units. A combination of analytical methods, numerical simulations, and experiments on programmable superconducting quantum circuits is used to build a simple yet practical heuristic model of entropy accumulation based on global depolarising noise. This demonstrates the potential of such an approach to construct effective heuristic models. The monitoring of entropy density not only offers a novel and complementary approach to existing circuit-level benchmarking techniques, but more importantly, it provides a much needed bridge between circuit-level and application-level benchmarking protocols. In particular, our heuristic model of entropy accumulation allows us to improve over existing techniques to bound the circuit size threshold beyond which quantum advantage is unattainable.
- Terrestrial Very-Long-Baseline Atom Interferometry: Summary of the
Second Workshopby Adam Abdalla on December 19, 2024 at 3:40 pm
This summary of the second Terrestrial Very-Long-Baseline Atom Interferometry (TVLBAI) Workshop provides a comprehensive overview of our meeting held in London in April 2024, building on the initial discussions during the inaugural workshop held at CERN in March 2023. Like the summary of the first workshop, this document records a critical milestone for the international atom interferometry community. It documents our concerted efforts to evaluate progress, address emerging challenges, and refine strategic directions for future large-scale atom interferometry projects. Our commitment to collaboration is manifested by the integration of diverse expertise and the coordination of international resources, all aimed at advancing the frontiers of atom interferometry physics and technology, as set out in a Memorandum of Understanding signed by over 50 institutions.
- Classical simulation of circuits with realistic
Gottesman-Kitaev-Preskill statesby Cameron Calcluth on December 17, 2024 at 6:00 pm
Classically simulating circuits with bosonic codes is a challenging task due to the prohibitive cost of simulating quantum systems with many, possibly infinite, energy levels. We propose an algorithm to simulate circuits with encoded Gottesman-Kitaev-Preskill states, specifically for odd-dimensional encoded qudits. Our approach is tailored to be especially effective in the most challenging but practically relevant regime, where the codeword states exhibit high (but finite) squeezing. Our algorithm leverages the Zak-Gross Wigner function introduced by J. Davis et al. [arXiv:2407.18394], which represents infinitely squeezed encoded stabilizer states positively. The runtime of the algorithm scales with the amount of negativity of this Wigner function, enabling fast simulation of certain large-scale circuits with a high degree of squeezing.
- Driving Enhanced Exciton Transfer by Automatic Differentiationby E. Ballarin on November 26, 2024 at 9:42 pm
We model and study the processes of excitation, absorption, and transfer in various networks. The model consists of a harmonic oscillator representing a single-mode radiation field, a qubit acting as an antenna, a network through which the excitation propagates, and a qubit at the end serving as a sink. We investigate how off-resonant excitations can be optimally absorbed and transmitted through the network. Three strategies are considered: optimising network energies, adjusting the couplings between the radiation field, the antenna, and the network, or introducing and optimising driving fields at the start and end of the network. These strategies are tested on three different types of network with increasing complexity: nearest-neighbour and star configurations, and one associated with the Fenna-Matthews-Olson complex. The results show that, among the various strategies, the introduction of driving fields is the most effective, leading to a significant increase in the probability of reaching the sink in a given time. This result remains stable across networks of varying dimensionalities and types, and the driving process requires only a few parameters to be effective.
- Strategies for entanglement distribution in optical fiber networksby Hannah McAleese on November 11, 2024 at 7:01 pm
Distributing entanglement over long distances remains a challenge due to its fragility when exposed to environmental effects. In this work, we compare various entanglement distribution protocols in a realistic noisy fiber network. We focus specifically on two schemes that only require the sending of a non-entangled carrier photon to remote nodes of the network. These protocols rely on optical CNOT gates and we vary the probability with which they can be successfully performed. Encoding our entangled states in photon polarization, we analyse the effect of depolarizing noise on the photonic states as the carrier passes through the fibers. Building a robust model of photon loss and calculating the distillable entanglement of the noisy states, we find the entanglement distribution rate. We discover that methods involving a separable carrier can reach a higher rate than the standard entanglement distribution protocol, provided that the success probability of the optical CNOT gates is sufficiently high.
- Thermodynamics of coupled time crystals with an application to energy
storageby Paulo J. Paulino on November 7, 2024 at 4:21 pm
Open many-body quantum systems can exhibit intriguing nonequilibrium phases of matter, such as time crystals. In these phases, the state of the system spontaneously breaks the time-translation symmetry of the dynamical generator, which typically manifests through persistent oscillations of an order parameter. A paradigmatic model displaying such a symmetry breaking is the boundary time crystal, which has been extensively analyzed experimentally and theoretically. Despite the broad interest in these nonequilibrium phases, their thermodynamics and their fluctuating behavior remain largely unexplored, in particular for the case of coupled time crystals. In this work, we consider two interacting boundary time crystals and derive a consistent interpretation of their thermodynamic behavior. We fully characterize their average dynamics and the behavior of their quantum fluctuations, which allows us to demonstrate the presence of quantum and classical correlations in both the stationary and the time-crystal phases displayed by the system. We furthermore exploit our theoretical derivation to explore possible applications of time crystals as quantum batteries, demonstrating their ability to efficiently store energy.