Quantum systems subject to driving fields are ubiquitous in many areas of research nowadays, such as quantum thermodynamics and quantum control
theory. In several situations, as in the case of quantum heat engines, in addition to the driving fields the system also interacts
with a surrounding environment, which is responsible for dissipative effects and decoherence.
On the one hand, many standard analytical results that are used to study open quantum systems, need to be suitably modified in order to address the same problem in the case of time-dependent Hamiltonians.
On the other hand, the interplay between the environmental noise and the driving can lead to new phenomena, such as the generation of coherence stemming from the interaction with the environment.

Our work makes a step forward to understand these problems.

In this paper, we derive a time-dependent Markovian master equation, which governs the evolution of a N-level system weakly coupled to an
environment and subject to driving. The derivation makes use of a rigorous technique to treat weakly-coupled open quantum systems, called Van Hove limit, and extends it to treat the case of time-dependent Hamiltonians. In addition, we also derive precise bounds on the validity of the aforementioned equation, which can be computed from the system Hamiltonian and the environment spectral density.

To prove the usefulness of our result, we study two paradigmatic cases. First, we analyze a periodically driven qubit, which is particularly hard to treat with already known methods. Afterward, we consider the more structured case of two interacting qubits. In both cases, the dynamics given by our master equation is benchmarked against numerically exact results obtained from tensor network simulations, which support the reliability of our approach.

The combination of chain-mapping and tensor-network techniques provides a powerful tool for the numerically exact simulation of open quantum systems interacting with structured environments. However, these methods suffer from a quadratic scaling with the physical simulation time, and therefore, they become challenging in the presence of multiple environments. This is particularly true when fermionic environments, well-known to be highly correlated, are considered. In this work, we first illustrate how a thermo-chemical modulation of the spectral density allows replacing the original fermionic environments with equivalent, but simpler, ones. Moreover, we show how this procedure reduces the number of chains needed to model multiple environments. We then provide a derivation of the fermionic Markovian closure construction, consisting of a small collection of damped fermionic modes undergoing a Lindblad-type dynamics and mimicking a continuum of bath modes. We describe, in particular, how the use of the Markovian closure allows for a polynomial reduction of the time complexity of chain-mapping based algorithms when long-time dynamics are needed.

The exact simulation of quantum systems interacting with an environment beyond the weak coupling regime is a hard problem because the resources needed scale exponentially with the size of the {System + Environment}. In the last years, clever innovative solutions have been brought forward to circumvent this central issue. One of them is the T-TEDOPA method that combines environment chain mapping on the analytical side, and tensor networks methods on the numerical side. We introduce MPSDynamics.jl, a versatile, free, and open-source Julia package implementing the T-TEDOPA method to perform the numerically exact simulation of the dynamics of open quantum systems even in the non-perturbative, non-Markovian regimes. The package is accessible on Github and is accompanied by an extensive documentation. It handles bosonic and fermionic environments, time-dependent Hamiltonians, correlated environments, multiple environments and joint system-environment observables. Everyone interested is welcome to contribute to development of the package.

Giant artificial atoms are promising and flexible building blocks for the implementation of analog quantum simulators. They are realized via a multilocal pattern of couplings of two-level systems to a waveguide, or to a two-dimensional photonic bath. A hallmark of giant-atom physics is their non-Markovian character in the form of self-coherent feedback, leading, e.g., to nonexponential atomic decay. The timescale of their non-Markovianity is essentially given by the time delay proportional to the distance between the various coupling points. In parallel, with the state-of-the-art experimental setups, it is possible to engineer complex phases in the atom-light couplings. Such phases simulate an artificial magnetic field, yielding a chiral behavior of the atom-light system. Here, we report a surprising connection between these two seemingly unrelated features of giant atoms, showing that the chirality of a giant atom controls its Markovianity. In particular, by adjusting the couplings’ phases, a giant atom can, counterintuitively, enter an exact Markovian regime, irrespectively of any inherent time delay. We illustrate this mechanism as an interference process and via a collision model picture. Our findings significantly advance the understanding of giant atom physics, and open new avenues for the control of quantum nanophotonic networks.

Parahydrogen-induced polarization (PHIP) is a potent technique for generating target molecules with high nuclear spin polarization. The PHIP process involves a chemical reaction between parahydrogen and a target molecule, followed by the transformation of nuclear singlet spin order into magnetization of a designated target nucleus through magnetic field manipulations. Although the singlet-to-magnetization polarization transfer process works effectively at moderate concentrations, it is observed to become much less efficient at high molar polarization, defined as the product of polarization and concentration. This strong dependence on the molar polarization is attributed to interference due to the field produced by the sample magnetization during polarization transfer, which leads to complex dynamics and can severely affect the scalability of the technique. We address this challenge with a pulse sequence that suppresses the influence of the distant dipolar field, while simultaneously achieving singlet-to-magnetization polarization transfer to the desired target spins, free from restrictions on the molar polarization.

We investigate the nonequilibrium steady state of the anisotropic open quantum Rabi model, which exhibits first-order and second-order dissipative phase transitions upon varying the degree of anisotropy between the coupling strengths of rotating and counter-rotating terms. Using both semiclassical and quantum approaches, we find a rich phase diagram resulting from the interplay between the anisotropy and the dissipation. First, there exists a bistable phase where both the normal and superradiant phases are stable. Second, there are multicritical points where the phase boundaries for the first- and second-order phase transitions meet. We show that a new set of critical exponents governs the scaling of the multicritical points. Finally, we discuss the feasibility of observing the multicritical transitions and bistability using a pair of trapped ions where the anisotropy can be tuned by controlling the intensity of the Raman transitions. Our study enlarges the scope of critical phenomena that may occur in finite-component quantum systems, which could be useful for applications in critical quantum sensing.

We present a quantum sensing technique that utilizes a sequence of π pulses to cyclically drive the qubit dynamics along a geodesic path of adiabatic evolution. This approach effectively suppresses the effects of both decoherence noise and control errors while simultaneously removing unwanted resonance terms, such as higher harmonics and spurious responses commonly encountered in dynamical decoupling control. As a result, our technique offers robust, wide-band, unambiguous, and high-resolution quantum sensing capabilities for signal detection and individual addressing of quantum systems, including spins. To demonstrate its versatility, we showcase successful applications of our method in both low-frequency and high-frequency sensing scenarios. The significance of this quantum sensing technique extends to the detection of complex signals and the control of intricate quantum environments. By enhancing detection accuracy and enabling precise manipulation of quantum systems, our method holds considerable promise for a variety of practical applications.

Macrorealism formalizes the seemingly intuitive notion that, in contrast with the principles of quantum mechanics, a physical system can be in a definite state at any given time and moreover its dynamical evolution is independent of the measurements performed on it. In this study, we carry out a comparative analysis between three-time Leggett–Garg-type inequalities and the conditions of no-signaling-in-time and arrow-of-time for macrorealism within the context of meson oscillations. Our findings indicate that, under given initial conditions, no violations of Leggett–Garg inequalities are observed. However, no-signaling-in-time conditions are found to be violated, thereby revealing the impossibility of applying a macrorealistic description to the physics of meson oscillations.

We investigate the relation between quantum nonlocality and gravity at the astrophysical scale, both in the classical and quantum regimes. Considering a gedanken experiment involving particle pairs orbiting in the strong gravitational field of ultra-compact objects, we find that the violation of Bell inequality acquires an angular modulation factor that strongly depends on the nature of the gravitational source. We show how such gravitationally-induced modulation of quantum nonlocality readily discriminates between black holes (both classical and inclusive of quantum corrections) and string fuzzballs, i.e., the true quantum description of ultra-compact objects according to string theory. These findings promote Bell nonlocality as a potentially key tool in putting quantum gravity to the test.

In a theoretical study, we investigate the spin dynamics of interacting nitrogen-vacancy (NV) centers and quadrupolar I=3/2 nuclear spins, specifically ¹¹B spins in hexagonal boron nitride (h-BN) nanosheets located near the microdiamond surface. We demonstrate the possibility of obtaining external spin-polarization by magnetic-field sweeps across the level anticrossings around zero field. To achieve this, we leverage crystal strains to establish a polarization transfer mechanism that remains robust against variations in NV orientation, crystal strain inhomogeneity, and electron-nuclear effective couplings. These results pave the way for hyperpolarization of spins in nanomaterials near the diamond surface without experiencing polarization loss to intrinsic nuclear spin-1/2 species, such as ¹³C and ¹H nuclear spins in diamond. The ¹¹B spins in h-BN nanosheets, with their extended relaxation time and large surface area, present a promising alternative for relayed nuclear polarization to the liquid phase and for the development of quantum simulators based on surface nuclear spins.

Noise in quantum devices is generally considered detrimental to computational accuracy. However, the recent proposal of noise-assisted simulation has demonstrated that noise can be an asset in digital quantum simulations of open systems on noisy intermediate-scale quantum (NISQ) devices. In this context, we introduce an optimized decoherence rate control scheme that can significantly reduce computational requirements by multiple orders of magnitude, in comparison to the original noise-assisted simulation. We further extend this approach to encompass Lindblad equations with time-dependent coefficients, using only quantum error characterization and mitigation techniques. This extension allows for the perturbative simulation of non-Markovian dynamics on NISQ devices, eliminating the need for ancilla qubits or midcircuit measurements. Our contributions are validated through numerical experiments on an emulated IBM Quantum device. Overall, our paper offers valuable optimizations that bring current quantum processors closer to effectively simulating realistic open systems.

Nuclear spin hyperpolarization techniques, such as dynamic nuclear polarization (DNP) and parahydrogen-induced polarization (PHIP), have revolutionized nuclear magnetic resonance and magnetic resonance imaging. In these methods, a readily available source of high spin order, either electron spins in DNP or singlet states in hydrogen for PHIP, is brought into close proximity with nuclear spin targets, enabling efficient transfer of spin order under external quantum control. Despite vast disparities in energy scales and interaction mechanisms between electron spins in DNP and nuclear singlet states in PHIP, a pseudo-spin formalism allows us to establish an intriguing equivalence. As a result, the important low-field polarization transfer regime of PHIP can be mapped onto an analogous system equivalent to pulsed-DNP. This establishes a correspondence between key polarization transfer sequences in PHIP and DNP, facilitating the transfer of sequence development concepts. This promises fresh insights and significant cross-pollination between DNP and PHIP polarization sequence developers.

We propose and analyze a driven-dissipative quantum sensor that is continuously monitored close to a dissipative critical point. The sensor relies on the critical open Rabi model with the spin and phonon degrees of freedom of a single trapped ion to achieve criticality-enhanced sensitivity. Effective continuous monitoring of the sensor is realized via a co-trapped ancilla ion that switches between dark and bright internal states conditioned on a ‘jump’ of the phonon population which, remarkably, achieves nearly perfect phonon counting despite a low photon collection efficiency. By exploiting both dissipative criticality and efficient continuous readout, the sensor device achieves highly precise sensing of oscillating electric field gradients at a criticality-enhanced precision scaling beyond the standard quantum limit, which we demonstrate is robust to the experimental imperfections in real-world applications.

The main limitation of future quantum technologies is the decoherence resulting from the interaction of the different working units of quantum devices with external uncontrollable environments (e.g. the electromagnetic field, lattice vibrations…). Usually different units are described as interacting with different environments that do not couple with one another, and these environments are responsible for local dissipation and decoherence. However the more complex quantum devices will become, the closer their different components will be. In that context, the assumption of distinct local environments breaks down and we need to consider the interaction of functional units with a common environment. In that case, the energy dissipated by one part of the system could, for instance, be absorbed later by another part. This makes the description of such global environments fundamentally more complex than local ones because their inner dynamics cannot be neglect if one wants to understand the dynamics of the system. Using tensor networks methods to represent and time-evolve the quantum state of the system and environment together, we are able to uncover processes that are happening on new time-and-length scales because of the propagation of energy/information inside of the environment. The new phenomenology of physical processes, resulting from considering quantum systems interacting with a common environment, has important consequences for the design of nanodevices as it gives access to new control, sensing and cross-talk mechanisms.

Given a unitary evolution U on a multipartite quantum system and an ensemble of initial states, how well can U be simulated by local operations and classical communication (LOCC) on that ensemble? We answer this question by establishing a general, efficiently computable upper bound on the maximal LOCC simulation fidelity—what we call an “LOCC inequality.” We then apply our findings to the fundamental setting where U implements a quantum Newtonian Hamiltonian over a gravitationally interacting system. Violation of our LOCC inequality can rule out the LOCCness of the underlying evolution, thereby establishing the nonclassicality of the gravitational dynamics, which can no longer be explained by a local classical field. As a prominent application of this scheme we study systems of quantum harmonic oscillators initialized in coherent states following a normal distribution and interacting via Newtonian gravity, and discuss a possible physical implementation with torsion pendula. One of our main technical contributions is the analytical calculation of the above LOCC inequality for this family of systems. As opposed to existing tests based on the detection of gravitationally mediated entanglement, our proposal works with coherent states alone, and thus it does not require the generation of largely delocalized states of motion nor the detection of entanglement, which is never created at any point in the process.

Defect centers in a nanodiamond (ND) allow the detection of tiny magnetic fields in their direct surroundings, rendering them as an emerging tool for nanoscale sensing applications. Eumelanin, an abundant pigment, plays an important role in biology and material science. Here, for the first time, we evaluate the comproportionation reaction in eumelanin by detecting and quantifying semiquinone radicals through the nitrogen-vacancy color center. A thin layer of eumelanin is polymerized on the surface of nanodiamonds (NDs), and depending on the environmental conditions, such as the local pH value, near-infrared, and ultraviolet light irradiation, the radicals form and react in situ. By combining experiments and theoretical simulations, we quantify the local number and kinetics of free radicals in the eumelanin layer. Next, the ND sensor enters the cells via endosomal vesicles. We quantify the number of radicals formed within the eumelanin layer in these acidic compartments by applying optical relaxometry measurements. In the future, we believe that the ND quantum sensor could provide valuable insights into the chemistry of eumelanin, which could contribute to the understanding and treatment of eumelanin- and melanin-related diseases.

Conducting precise electronic-vibrational dynamics simulations of molecular systems poses significant challenges when dealing with realistic environments composed of numerous vibrational modes. Here, we introduce a technique for the construction of effective phonon spectral densities that capture accurately open-system dynamics over a finite time interval of interest. When combined with existing nonperturbative simulation tools, our approach can reduce significantly the computational costs associated with many-body open-system dynamics.

We briefly review recent developments in the study of the quantum nature of flavor mixing; in particular, the attention will be devoted to neutrino and neutral meson oscillations. We employ Leggett-Garg type inequalities and no-signaling-in-time conditions to probe the intrinsic quantumness of such a physical manifestation, showing how the analysis is not affected by the wave-packet spreading (for neutrinos) and the intrinsic particle instability (for mesons).

Measuring temperature inside living cells is an important endeavour as many questions remain on how heat diffuses in a media containing so many complex and dynamic nanostructures. It is also very relevant in biology as temperature affects reaction rates and protein structures. Therefore, nanothermometry may open new drug-discovery possibilities as heat could be used to intracellularly manipulate pathogenic processes. Temperature sensing can be performed with nitrogen-vacancy (NV) centres inside nanodiamonds (NDs) by measuring their optically-detected-magnetic resonance (ODMR) spectra. The zero-field splitting (ZFS) that can be extracted from such spectra responds linearly to a temperature change between 0 and 50°C. Using nanothermometers such as quantum dots, fluorescent molecules and NDs, many have reported up to a few degrees celsius differences between cellular compartments or upon metabolic stimulation. However, the fact that metabolic activity could cause temperature differences, up to a few degrees celsius, in single cells remains controversial. Arguments against this hypothesis are the technical difficulties in removing artefacts from the measurement and the lack of a theoretical model explaining such large gradients. Here, we report ND nanothermometry inside living macrophages with a view to understanding better how such temperature gradients could exist. We analyse the properties of the ND's ODMR spectra during a few hours inside macrophages and after cell death. The apparent change of ZFS frequencies could be interpreted as temperature changes but we show that such fluctuations can also be explained by an unstable electrical field caused by the ND's surface chemistry. Our findings are not only relevant for the current controversy but also open the possibility to simultaneously detect temperature and chemical changes on the ND's surface.

Quantum hypothesis testing—the task of distinguishing quantum states—enjoys surprisingly deep connections with the theory of entanglement. Recent findings have reopened the biggest questions in hypothesis testing and reversible entanglement manipulation.