arXiv subjects

Frederico Brito

Publications and source records attributed to Frederico Brito.

At least 19 recordsLinked to original sources

arXiv2026

Boltzmann counting in Hilbert space

We introduce a geometric entropy for quantum preparations, defined as the logarithm of the Hilbert-space volume of pure states compatible with a given set of constraints. This construction extends Boltzmann's counting perspective to the quantum setting, where compatible states need not be orthogonal and the relevant notion of "number of states" is naturally replaced by a volume in state space. We analyze three classes of constraints: restriction to a subspace, fixed expectation values, and coarse-grained subsystem descriptions. For representative examples, including subspace projection, spin expectation values, partial trace, and an imperfect detector map, we obtain explicit scaling laws and closed-form expressions for the associated volumes. The resulting framework provides a geometric measure of quantum ignorance at the level of the preparation and complements entropy notions based on density matrices and coarse graining.

quant-ph↗

arXiv2024

Qibocal: an open-source framework for calibration of self-hosted quantum devices

Calibration of quantum devices is fundamental to successfully deploy quantum algorithms on current available quantum hardware. We present Qibocal, an open-source software library to perform calibration and characterization of superconducting quantum devices within the Qibo framework. Qibocal completes the Qibo middleware framework by providing all necessary tools to easily (re)calibrate self-hosted quantum platforms. After presenting the layout and the features of the library, we give an overview on some of the protocols implemented to perform single and two-qubit gates calibration. Finally, we present applications involving recalibration and monitoring of superconducting platforms.

quant-ph↗

arXiv2024

Energy additivity as a requirement for universal quantum thermodynamical frameworks

The quest to develop a general framework for thermodynamics, suitable for the regime of strong coupling and correlations between subsystems of an autonomous quantum "universe," has entailed diverging definitions for basic quantities, including internal energy. While most approaches focus solely on the system of interest, we propose that a universal notion of internal energy should also account for the environment in order to keep consistency with the closed-system energy of the universe. We introduce an abstract framework to describe all effective Hamiltonian-based approaches and address a rigorous definition of energy additivity in this context, in both a weak and a strong forms, discussing the underlying subtleties. As an illustration, we study a particular two-qubit universe model, obtaining the exact master equations for both parties and calculating their effective Hamiltonians and internal energies as given by the recently devised minimal dissipation approach. In this case, we show that internal energies are neither additive nor conservative, which leads to unphysical features.

quant-ph↗

arXiv2024

Experimental Investigation of Geometric Quantum Speed Limits in an Open Quantum System

We studied geometric quantum speed limits (QSL) of a qubit subject to decoherence in an ensemble of chloroform molecules in a Nuclear Magnetic Resonance experiment. The QSL is a fundamental lower bound on the evolution time for quantum systems undergoing general physical processes. To do so, we controlled the system-reservoir interaction and the spin relaxation rates by adding a paramagnetic salt, which allowed us to observe both Markovian and non-Markovian open system dynamics for the qubit. We used two distinguishability measures of quantum states to assess the speed of the qubit evolution: the quantum Fisher information (QFI) and Wigner-Yanase skew information (WY). For non-Markovian dynamics and low salt concentrations, we observed crossovers between QSLs related to the QFI and WY metrics. The WY metric sets the tighter QSL for high concentrations and Markovian dynamics. We also show that QSLs are sensitive even to small fluctuations in spin magnetization.

quant-ph↗

arXiv2023

A constraint on local definitions of quantum internal energy

Recent advances in quantum thermodynamics have been focusing on ever more elementary systems of interest, approaching the limit of a single qubit, with correlations, strong coupling and non-equilibrium environments coming into play. Under such scenarios, it is clear that fundamental physical quantities must be revisited. This article questions whether a universal definition of internal energy for open quantum systems may be devised, setting limits on its possible properties. We argue that, for such a definition to be regarded as local, it should be implemented as a functional of the open system's reduced density operator and its time derivatives. Then we show that it should involve at least up to the second-order derivative, otherwise failing to recover the previously-known internal energy of the "universe". Possible implications of this general result are discussed.

quant-ph↗

arXiv2023

Quantum amplitude damping for solving homogeneous linear differential equations: A noninterferometric algorithm

In contexts where relevant problems can easily attain configuration spaces of enormous sizes, solving Linear Differential Equations (LDEs) can become a hard achievement for classical computers; on the other hand, the rise of quantum hardware can conceptually enable such high-dimensional problems to be solved with a foreseeable number of qubits, whilst also yielding quantum advantage in terms of time complexity. Nevertheless, in order to bridge towards experimental realizations with several qubits and harvest such potential in a short-term basis, one must dispose of efficient quantum algorithms that are compatible with near-term projections of state-of-the-art hardware, in terms of both techniques and limitations. As the conception of such algorithms is no trivial task, insights on new heuristics are welcomed. This work proposes a novel approach by using the Quantum Amplitude Damping operation as a resource, in order to construct an efficient quantum algorithm for solving homogeneous LDEs. As the intended implementation involves performing Amplitude Damping exclusively via a simple equivalent quantum circuit, our algorithm shall be given by a gate-level quantum circuit (predominantly composed of elementary 2-qubit gates) and is particularly nonrestrictive in terms of connectivity within and between some of its main quantum registers. We show that such an open quantum system-inspired circuitry allows for constructing the real exponential terms in the solution in a non-interferometric way; we also provide a guideline for guaranteeing a lower bound on the probability of success for each realization, by exploring the decay properties of the underlying quantum operation.

quant-ph↗

arXiv2022

A Schmidt decomposition approach to quantum thermodynamics

The development of a self-consistent thermodynamic theory of quantum systems is of fundamental importance for modern physics. Still, despite its essential role in quantum science and technology, there is no unifying formalism for characterizing the thermodynamics within general autonomous quantum systems, and many fundamental open questions remain unanswered. Along these lines, most current efforts and approaches restrict the analysis to particular scenarios of approximative descriptions and semi-classical regimes. Here we propose a novel approach to describe the thermodynamics of arbitrary bipartite autonomous quantum systems based on the well-known Schmidt decomposition. This formalism provides a simple, exact and symmetrical framework for expressing the energetics between interacting systems, including scenarios beyond the standard description regimes, such as strong coupling. We show that this procedure allows a straightforward identification of local effective operators suitable for characterizing the physical local internal energies. We also demonstrate that these quantities naturally satisfy the usual thermodynamic notion of energy additivity.

quant-ph↗

arXiv2022

Emergence of energy-avoiding and energy-seeking behaviours in nonequilibrium dissipative quantum systems

A longstanding challenge in nonequilibrium thermodynamics is to predict the emergence of self-organized behaviours and functionalities typical of living matter. Despite the progress with classical complex systems, it remains far from obvious how to extrapolate these results down to the quantum scale. Here, we employ the paradigmatic master equation framework to establish that some lifelike behaviours and functionalities can indeed emerge in elementary dissipative quantum systems driven out of equilibrium. Specifically, we find both energy-avoiding (low steady dissipation) and energy-seeking behaviours (high steady dissipation), as well as self-adaptive shifts between these modes, in generic few-level systems. We also find emergent functionalities, namely, a self-organized thermal gradient in the system's environment (in the energy-seeking mode) and an active equilibration against thermal gradients (in the energy-avoiding mode). Finally, we discuss the possibility that our results could be related to the concept of dissipative adaptation.

quant-ph↗

arXiv2021

Quantum dissipative adaptation

Dissipative adaptation is a general thermodynamic mechanism that explains self-organization in a broad class of driven classical many-body systems. It establishes how the most likely (adapted) states of a system subjected to a given drive tend to be those following trajectories of highest work absorption, followed by dissipated heat to the reservoir. Here, we extend the dissipative adaptation phenomenon to the quantum realm. We employ a fully-quantized exactly solvable model, where the source of work on a three-level system is a single-photon pulse added to a zero-temperature infinite environment, a scenario that cannot be treated by the classical framework. We find a set of equalities relating adaptation likelihood, absorbed work, heat dissipation and variation of the informational entropy of the environment. Our proof of principle provides the starting point towards a quantum thermodynamics of driven self-organization.

quant-ph↗

arXiv2021

Non-linear equation of motion for Page-Wootters mechanism with interaction and quasi-ideal clocks

Among the many proposals to approach the concept of time in quantum theory, the Page-Wootters mechanism has attracted much attention in the last few years. Originally, such a mechanism explored a stationary bipartite non-interacting global system, i.e., a system of interest together with an ancillary clock, to determine how the evolution in time can emerge as an equation of motion for a quantum particle conditioned to the measurement of the state of the clock. After the seminal proposal, many variations of it were considered, and different aspects of the mechanism were elucidated. Our contribution to these investigations is that we analyze such a timeless approach to quantum theory but deriving an equation of motion for a mixed state system that evolves according to its gravitationally induced interaction with a non-ideal quantum clock. The interaction considered is known to describe the gravitational decoherence mechanism, and the clock model is the recently proposed quasi-ideal clock, i.e., one constructed to approximate the time-energy canonical commutation relation. As a result of our considerations, we obtained an equation of motion that is non-linear in nature, dependent on the system's initial conditions.

quant-ph↗

arXiv2019

Probing the Unruh effect with an accelerated extended system

It has been proved in the context of quantum fields in Minkowski spacetime that the vacuum state is a thermal state according to uniformly accelerated observers -- a seminal result known as the Unruh effect. Recent claims, however, have challenged the validity of this result for extended systems, thus casting doubts on its physical reality. Here, we study the dynamics of an extended system, uniformly accelerated in the vacuum. We show that its reduced density matrix evolves to a Gibbs thermal state with local temperature given by the Unruh temperature. This proves that the vacuum state does induce thermalization of an accelerated extended system -- which is all one can expect of a legitimate thermal reservoir.

gr-qc↗

arXiv2019

Non-thermal quantum engine in transmon qubits

The design and implementation of quantum technologies necessitates the understanding of thermodynamic processes in the quantum domain. In stark contrast to macroscopic thermodynamics, at the quantum scale processes generically operate far from equilibrium and are governed by fluctuations. Thus, experimental insight and empirical findings are indispensable in developing a comprehensive framework. To this end, we theoretically propose an experimentally realistic quantum engine that uses transmon qubits as working substance. We solve the dynamics analytically and calculate its efficiency.

quant-ph↗

arXiv2018

Validation of Quantum Adiabaticity through Non-Inertial Frames and Its Trapped-Ion Realization

Validity conditions for the adiabatic approximation are useful tools to understand and predict the quantum dynamics. Remarkably, the resonance phenomenon in oscillating quantum systems has challenged the adiabatic theorem. In this scenario, inconsistencies in the application of quantitative adiabatic conditions have led to a sequence of new approaches for adiabaticity. Here, by adopting a different strategy, we introduce a validation mechanism for the adiabatic approximation by driving the quantum system to a non-inertial reference frame. More specifically, we begin by considering several relevant adiabatic approximation conditions previously derived and show that all of them fail by introducing a suitable oscillating Hamiltonian for a single quantum bit (qubit). Then, by evaluating the adiabatic condition in a rotated non-inertial frame, we show that all of these conditions, including the standard adiabatic condition, can correctly describe the adiabatic dynamics in the original frame, either far from resonance or at a resonant point. Moreover, we prove that this validation mechanism can be extended for general multi-particle quantum systems, establishing the conditions for the equivalence of the adiabatic behavior as described in inertial or non-inertial frames. In order to experimentally investigate our method, we consider a hyperfine qubit through a single trapped Ytterbium ion $^{171}$Yb$^{+}$, where the ion hyperfine energy levels are used as degrees of freedom of a two-level system. By monitoring the quantum evolution, we explicitly show the consistency of the adiabatic conditions in the non-inertial frame.

quant-ph↗

arXiv2018

Spin-phase-space-entropy production

Quantifying the degree of irreversibility of an open system dynamics represents a problem of both fundamental and applied relevance. Even though a well-known framework exists for thermal baths, the results give diverging results in the limit of zero temperature and are also not readily extended to nonequilibrium reservoirs, such as dephasing baths. Aimed at filling this gap, in this paper we introduce a phase-space-entropy production framework for quantifying the irreversibility of spin systems undergoing Lindblad dynamics. The theory is based on the spin Husimi-Q function and its corresponding phase-space entropy, known as Wehrl entropy. Unlike the von Neumann entropy production rate, we show that in our framework, the Wehrl entropy roduction rate remains valid at any temperature and is also readily extended to arbitrary nonequilibrium baths. As an application, we discuss the irreversibility associated with the interaction of a two-level system with a single-photon pulse, a problem which cannot be treated using the conventional approach.

quant-ph↗

arXiv2017

Reliability of digitized quantum annealing and the decay of entanglement

We performed a banged-digital-analog simulation of a quantum annealing protocol in a two-qubit Nuclear Magnetic Resonance (NMR) quantum computer. Our experimental simulation employed up to 235 Trotter steps, with more than 2000 gates (pulses), and we obtained a protocol success above 80%. Given the exquisite control of the NMR quantum computer, we performed the simulation with different noise levels. We thus analyzed the reliability of the quantum annealing process, and related it to the level of entanglement produced during the protocol. Although the presence of entanglement is not a sufficient signature for a better-than-classical simulation, the level of entanglement achieved relates to the fidelity of the protocol.

quant-ph↗

arXiv2017

On the adiabatic quantum dynamics of fabricated Ising chains

Physical implementations of quantum computation must be scrutinized about their reliability under real conditions, in order to be considered as viable candidates. Among the proposed models, those based on adiabatic quantum dynamics have shown great potential for solving specific tasks and have already been successfully implemented using superconducting devices. In this context, we address the issue of how the fabrication variations are expected to affect on average the computation results, when only dynamical effects occur. By simulating the dynamics of small-scale systems, it is found a considerable robustness for the computation when analyzing results obtained from ensembles of such machines. In addition, it is also addressed whether conditions for adiabaticity could be taken as quantitative measures of it. From the analysis of four known conditions, it is obtained that none could have such an use.

quant-ph↗

arXiv2016

The most elementary heat to work conversion: work extraction from a completely passive state by a single photon

The concepts of work and heat in the quantum domain, as well as their interconversion principles, are still an open debate. We have found theoretical evidence that a single photon packet is capable of extracting work from a single two-level system (TLS). More importantly, this effect is found for a photon as it is spontaneously emitted, therefore carrying only heat away from the emitter. This is the most elementary process in which heat is converted into work, requiring not more than two off-resonance atoms for that purpose. From a more practical point of view it is found that, surprisingly, work can be extracted from a TLS in a completely passive initial state. The process is cyclic in the sense that the TLS initial and final states are equal. The state of the TLS remains passive throughout the interaction time with the single-photon packet. The physical meaning of the work performed by the TLS is found to be a dynamical change in the color of the photon during the reemission process. All our predictions are, in principle, measurable within state-of-the-art experiments.

quant-ph↗

arXiv2015

Testing Time Reversal Symmetry in Artificial Atoms

Over the past several decades, a rich series of experiments has repeatedly verified the quantum nature of superconducting devices, leading some of these systems to be regarded as artificial atoms. In addition to their application in quantum information processing, these `atoms' provide a test bed for studying quantum mechanics in macroscopic limits. Regarding the last point, we present here a feasible protocol for directly testing time reversal symmetry through the verification of the microreversibility principle in a superconducting artificial atom. Time reversal symmetry is a fundamental property of quantum mechanics and is expected to hold if the dynamics of the artificial atom strictly follow the Schrödinger equation. However, this property has yet to be tested in any macroscopic quantum system. In the end, as an application of this work, we outline how the successful implementation of the protocol would provide the first verification of the quantum work fluctuation theorems with superconducting systems.

quant-ph↗