arXiv subjects

Maria Maffei

Publications and source records attributed to Maria Maffei.

At least 19 recordsLinked to original sources

arXiv2026

Energetics of non-Gaussianity in single mode cavities

Non-Gaussian states play a central role in quantum technologies, making the ability to quantify non-Gaussianity essential. We introduce an energetic framework to characterize non-Gaussianity in single-mode bosonic states by decomposing the total energy into Gaussian and non-Gaussian contributions. For pure states, we show that the non-Gaussian component defines a valid measure of non-Gaussianity and establish its connection to the relative entropy of non-Gaussianity. As an illustration, we compare this measure with Wigner negativity and find that both are maximized in closely related parameter regimes. For mixed states, we demonstrate that the non-Gaussian contribution acts as a faithful witness of non-Gaussianity. Our results reveal an energetic fine structure underlying non-Gaussianity and may provide practical insights for the efficient generation of non-Gaussian states.

quant-ph↗

arXiv2026

Industry-ready spin-photon interfaces for hybrid photonic quantum computing

Hybrid photonic quantum computers, combining stationary matter qubits and flying photonic qubits, offer an intrinsically networked and resource-efficient route to large-scale, error-corrected quantum computation. Their core components are cavity-coupled matter qubits that act as light--matter interfaces, enabling: high-efficiency on-demand single-photon generation, stable near-unity photon indistinguishability and spin--multi-photon entanglement. Semiconductor quantum dots in microcavities are a leading platform for realizing such devices. Yet reaching the performance, reproducibility and spin-coherence thresholds for large-scale error correction remains a major challenge requiring industrial fabrication and control. Here we report thousands of monolithic semiconductor quantum-dot devices fabricated using a III--V pilot production-line process compatible with large-scale deployment. Systematic control of source parameters yields state-of-the-art efficiency and supports a path to optical losses below fault-tolerance thresholds. Using field-quadrature state reconstruction as a stringent joint test of efficiency and indistinguishability, we observe near-unity photon quantum purity stable over tens of minutes and a record single-photon Wigner-function negativity. We further demonstrate seven-partite spin--multi-photon entanglement and spin coherence extendable to microsecond timescales in the low-magnetic-field regime. Finally, photons from distant sources are as indistinguishable as photons emitted successively by a single source. These results establish foundry-compatible III--V quantum dots as a scalable platform for hybrid photonic quantum computing.

quant-ph↗

arXiv2026

Optimizing Wigner Negativity in Scattering Processes Using Energetic Cost Functions

Wigner negativity is a key resource for quantum technologies but is difficult to optimize in multimode scattering systems. We study the scattering of coherent pulses by a two-level emitter coupled to a one-dimensional waveguide and introduce energetic cost functions that enable the optimization of Wigner negativity without reconstructing the full Wigner function. By decomposing the scattered energy into coherent, thermal, squeezing, and non-Gaussian contributions, we identify an energetic witness that strongly correlates with the achievable negativity across all driving regimes. This approach singles out optimal output temporal modes and uncovers operating points generating appreciable Wigner negativity with sub-photon input energies. We further identify a maximal energy-efficiency regime at spectral mode matching, where the emitter effectively implements a vacuum-selective $\pi$ phase shift, realizing a giant optical nonlinearity. These results establish energetic optimization as a practical route to engineering Wigner-negative photonic states in waveguide quantum electrodynamics and related bosonic scattering platforms.

quant-ph↗

arXiv2026

An Energetic Constraint for Qubit-Qubit Entanglement

We analyze qubit-qubit entanglement from an energetic perspective and reveal an energetic trade-off between quantum coherence and entanglement. We decompose each qubit internal energy into a coherent and an incoherent component. The qubits' coherent energies are maximal if the qubit-qubit state is pure and separable. They decrease as qubit-qubit entanglement builds up under locally-energy-preserving processes. This yields a ``coherent energy deficit'' that we show is proportional to a well-known measure of entanglement, the square concurrence. In general, a qubit-qubit state can always be represented as a mixture of pure states. Then, the coherent energy deficit splits into a quantum component, corresponding to the average square concurrence of the pure states, and a classical one reflecting the mixedness of the joint state. Minimizing the quantum deficit over the possible pure state decompositions yields the square concurrence of the mixture. Our findings bring out new figures of merit to optimize and secure entanglement generation and distribution under energetic constraints.

quant-ph↗

arXiv2026

Hamiltonian Benchmark of a Solid-State Spin-Photon Interface for Computation

Light-matter interfaces are pivotal for quantum computation and communication. While typically analyzed using single-mode or open-quantum-system approximations, these models often neglect multi-mode field states and light-matter entanglement, hindering exact protocol modeling. Here, we solve the full Hamiltonian dynamics of a solid-state spin-photon interface for three key protocols: the generation of photon-number superpositions, a controlled photon-photon gate, and the production of photonic cluster states. By deriving exact fidelities, we identify fundamental performance limits. Our results reveal that while realistic imperfections severely limit photon-photon gates, they only slightly affect linear photonic clusters and are nearly harmless for photon-number state superpositions.

quant-ph↗

arXiv2025

Superradiant decay in non-Markovian Waveguide Quantum Electrodynamics

An array of initially excited emitters coupled to a one-dimensional waveguide exhibits superradiant decay under the Born-Markov approximation, manifested as a coherent burst of photons in the output field. In this work, we employ tensor-network methods to investigate its non-Markovian dynamics induced by finite time delays in photon exchange among the emitters. We find that the superradiant burst breaks into a structured train of correlated photons, each intensity peak corresponding to a specific photon number. We quantify the emitter-photon and emitter-emitter entanglement generated during this process and show that the latter emerges in the long-time limit, as part of the excitation becomes trapped within the emitters' singlet subspace. We finally consider the decay of the system's most radiant state, the symmetric Dicke state, and show that time delay can lead to decay rates exceeding those predicted by the Markovian approximation.

quant-ph↗

arXiv2025

Non-Markovian dynamics of generation of bound states in the continuum via single-photon scattering

The excitation of bound states in the continuum (BICs) in two- or multi-qubit systems lies at the heart of entanglement generation and harnessing in Waveguide Quantum Electrodynamics platforms. However, the generation of qubit pair BICs through single-photon scattering is hindered by the fact that these states are effectively decoupled from propagating photons. We prove that scattering of a parity-invariant single photon on a qubit pair, combined with a properly engineered time variation of the qubit detuning, is not only feasible, but also more effective than strategies based on the relaxation of the excited states of the qubits when the distance between the qubits gives rise to non-negligible photon delays (non-Markovian regime). The use of tensor network methods to simulate the proposed scheme enables to include such photon delays in collision models, thus opening the possibility to follow the time evolution of the full quantum system, including qubits and field, and to efficiently implement and characterize the dynamics hence identifying optimal working points for the BIC generation.

quant-ph↗

arXiv2024

Exploring the Accuracy of Interferometric Quantum Measurements under Conservation Laws

A (target) quantum system is often measured through observations performed on a second (meter) system to which the target is coupled. In the presence of global conservation laws holding on the joint meter-target system, the Wigner-Araki-Yanase theorem and its generalizations predict a lower-bound on the measurement's error (Ozawa's bound). While practically negligible for macroscopic meters, it becomes relevant for microscopic ones. Here, we propose a simple interferometric setup, arguably within reach of present technology, in which a flying particle (a microscopic quantum meter) is used to measure a qubit by interacting with it in one arm of the interferometer. In this scenario, the globally conserved quantity is the total energy of particle and qubit. We show how the measurement error, $\varepsilon$, is linked to the non-stationary nature of the measured observable and the finite duration of the target-meter interaction while Ozawa's bound, $\varepsilon_{\mathrm B}$, only depends on the momentum uncertainty of the meter's wavepacket. When considering short wavepackets with respect to the evolution time of the qubit, we show that $\varepsilon/\varepsilon_{\mathrm B}$ is strictly tied to the position-momentum uncertainty of the meter's wavepacket and $\varepsilon/\varepsilon_{\mathrm B} \rightarrow 1$ only when employing Gaussian wavepackets. On the contrary, long wavepackets of any shape lead to $\varepsilon/\varepsilon_{\mathrm B} \rightarrow \sqrt{2}$. In addition to their fundamental relevance, our findings have important practical consequences for optimal resource management in quantum technologies.

quant-ph↗

arXiv2024

Thermodynamics of autonomous optical Bloch equations

Optical Bloch Equations (OBEs) are canonical equations describing the dynamics of a classically driven atom coupled to a thermal bath. Their thermodynamics is highly relevant to establish fundamental energetic bounds of key quantum processes. A consistent framework is available in the regime where the drives and baths can be treated classically, i.e. remains insensitive to the coupling with the atom. This regime, however, is not adapted to explore minimal energy costs, nor to measure atom-induced energy variations inside drives and baths -- a key ability to directly measure and optimize work and heat exchanges. This calls for a new framework accounting for atomic back-actions on drives and baths. Here we build such a framework by describing the atom, the drive and the bath as a joint autonomous system, the drive and the bath being parts of the same electromagnetic field. Our approach captures atom-field correlations at fundamental timescales, as well as the atomic back-action on the field, allowing us to define work-like (heat-like) flows as energy flows stemming from effective unitary dynamics induced by one system on the other (non-unitary correlating dynamics). Time-integrated work-like and heat-like flows are directly measurable in the field, as changes of the mean field and fluctuations, respectively. Our approach differs from standard analyses by identifying an additional unitary contribution in the atom's dynamics, the self-drive, and its energetic counterpart, the self-work, yielding a tighter expression of the second law. We relate this tightening to the extra knowledge about the field state, as well as the potential of the interacted driving field to be recycled. Our autonomous framework deepens the current understanding of thermodynamics in the quantum regime and its potential for energy management at quantum scales.

quant-ph↗

arXiv2023

Quantum energetics of a non-commuting measurement

When a measurement observable does not commute with a quantum system's Hamiltonian, the energy of the measured system is typically not conserved during the measurement. Instead, energy can be transferred between the measured system and the meter. In this work, we experimentally investigate the energetics of non-commuting measurements in a circuit quantum electrodynamics system containing a transmon qubit embedded in a 3D microwave cavity. We show through spectral analysis of the cavity photons that a frequency shift is imparted on the probe, in balance with the associated energy changes of the qubit. Our experiment provides new insights into foundations of quantum measurement, as well as a better understanding of the key mechanisms at play in quantum energetics.

quant-ph↗

arXiv2023

Fundamental mechanisms of energy exchanges in autonomous measurements based on dispersive qubit-light interaction

Measuring an observable which does not commute with the Hamiltonian of a quantum system usually modifies the mean energy of this system. In an autonomous measurement scheme, coupling the system to a quantum meter, the system's energy change must be compensated by the meter's energy change. Here, we theoretically study such an autonomous meter-system dynamics: a qubit interacting dispersively with a light pulse propagating in a one-dimensional waveguide. The phase of the light pulse is shifted, conditioned to the qubit's state along the $z$-direction, while the orientation of the qubit Hamiltonian is arbitrary. As the interaction is dispersive, photon number is conserved so that energy balance has to be attained by spectral deformations of the light pulse. Building on analytical and numerical solutions, we reveal the mechanism underlying this spectral deformation and display how it compensates for the qubit's energy change. We explain the formation of a three-peak structure of the output spectrum and we provide the conditions under which this is observable.

quant-ph↗

arXiv2022

Anomalous energy exchanges and Wigner function negativities in a single qubit gate

Anomalous weak values and Wigner function's negativity are well known witnesses of quantum contextuality. We show that these effects occur when analyzing the energetics of a single qubit gate generated by a resonant coherent field traveling in a waveguide. The buildup of correlations between the qubit and the field is responsible for bounds on the gate fidelity, but also for a nontrivial energy balance recently observed in a superconducting setup. In the experimental scheme, the field is continuously monitored through heterodyne detection and then post-selected over the outcomes of a final qubit's measurement. The post-selected data can be interpreted as field's weak values and can show anomalous values in the variation of the field's energy. We model the joint system dynamics with a collision model, gaining access to the qubit-field entangled state at any time. We find an analytical expression of the quasi-probability distribution of the post-selected heterodyne signal, i.e. the conditional Husimi-Q function. The latter grants access to all the field's weak values: we use it to obtain that of the field's energy change and display its anomalous behaviour. Finally, we derive the field's conditional Wigner function and show that anomalous weak values and Wigner function's negativities arise for the same values of the gate's angle.

quant-ph↗

arXiv2022

Energy-efficient quantum non-demolition measurement with a spin-photon interface

Spin-photon interfaces (SPIs) are key devices of quantum technologies, aimed at coherently transferring quantum information between spin qubits and propagating pulses of polarized light. We study the potential of a SPI for quantum non demolition (QND) measurements of a spin state. After being initialized and scattered by the SPI, the state of a light pulse depends on the spin state. It thus plays the role of a pointer state, information being encoded in the light's temporal and polarization degrees of freedom. Building on the fully Hamiltonian resolution of the spin-light dynamics, we show that quantum superpositions of zero and single photon states outperform coherent pulses of light, producing pointer states which are more distinguishable with the same photon budget. The energetic advantage provided by quantum pulses over coherent ones is maintained when information on the spin state is extracted at the classical level by performing projective measurements on the light pulses. The proposed schemes are robust against imperfections in state of the art semi-conducting devices.

quant-ph↗

arXiv2022

Energetic cost of measurements using quantum, coherent, and thermal light

Quantum measurements are basic operations that play a critical role in the study and application of quantum information. We study how the use of quantum, coherent, and classical thermal states of light in a circuit quantum electrodynamics setup impacts the performance of quantum measurements, by comparing their respective measurement backaction and measurement signal to noise ratio per photon. In the strong dispersive limit, we find that thermal light is capable of performing quantum measurements with comparable efficiency to coherent light, both being outperformed by single-photon light. We then analyze the thermodynamic cost of each measurement scheme. We show that single-photon light shows an advantage in terms of energy cost per information gain, reaching the fundamental thermodynamic cost.

quant-ph↗

arXiv2021

Closed-System Solution of the 1D Atom from Collision Model

Obtaining the total wavefunction evolution of interacting quantum systems provides access to important properties, such as entanglement, shedding light on fundamental aspects, e.g. quantum energetics and thermodynamics, and guiding towards possible application in the fields of quantum computation and communication. We consider a two-level atom (qubit) coupled to the continuum of travelling modes of a field confined in a one-dimensional waveguide. Originally, we treat the light-matter ensemble as a closed, isolated system. We solve its dynamics using a collision model where individual temporal modes of the field locally interact with the qubit in a sequential fashion. This approach allows us to obtain the total wavefunction of the qubit-field system, at any time, when the field starts in a coherent or a single-photon state. Our method is general and can be applied to other initial field states.

quant-ph↗

arXiv2021

Energetics of a Single Qubit Gate

Qubits are physical, a quantum gate thus not only acts on the information carried by the qubit but also on its energy. What is then the corresponding flow of energy between the qubit and the controller that implements the gate? Here we exploit a superconducting platform to answer this question in the case of a quantum gate realized by a resonant drive field. During the gate, the superconducting qubit becomes entangled with the microwave drive pulse so that there is a quantum superposition between energy flows. We measure the energy change in the drive field conditioned on the outcome of a projective qubit measurement. We demonstrate that the drive's energy change associated with the measurement backaction can exceed by far the energy that can be extracted by the qubit. This can be understood by considering the qubit as a weak measurement apparatus of the driving field.

quant-ph↗

arXiv2021

Linking topological features of the Hofstadter model to optical diffraction figures

In two, three and even four spatial dimensions, the transverse responses experienced by a charged particle on a lattice in a uniform magnetic field are fully controlled by topological invariants called Chern numbers, which characterize the energy bands of the underlying Hofstadter Hamiltonian. These remarkable features, solely arising from the magnetic translational symmetry, are captured by Diophantine equations which relate the fraction of occupied states, the magnetic flux and the Chern numbers of the system bands. Here we investigate the close analogy between the topological properties of Hofstadter Hamiltonians and the diffraction figures resulting from optical gratings. In particular, we show that there is a one-to-one relation between the above mentioned Diophantine equation and the Bragg condition determining the far-field positions of the optical diffraction peaks. As an interesting consequence of this mapping, we discuss how the robustness of diffraction figures to structural disorder in the grating is a direct analogue of the robustness of transverse conductance in the Quantum Hall effect.

cond-mat.mes-hall↗

arXiv2021

Probing non-classical light fields with energetic witnesses in Waveguide Quantum Electro-Dynamics

We analyze energy exchanges between a qubit and a resonant field propagating in a waveguide. The joint dynamics is analytically solved within a repeated interaction model. The work received by the qubit is defined as the unitary component of the field-induced energy change. Using the same definition for the field, we show that both work flows compensate each other. Focusing on the charging of a qubit battery by a pulse of light, we evidence that the work provided by a coherent field is an upper bound for the qubit ergotropy, while this bound can be violated by non-classical fields, e.g. a coherent superposition of zero- and single-photon states. Our results provide operational, energy-based witnesses to probe the non-classical nature of a light field.

quant-ph↗