arXiv subjects

Borhan Ahmadi

Publications and source records attributed to Borhan Ahmadi.

15 recordsLinked to original sources

arXiv2026

Finite-Time Optomechanical Cooling by Multi-Exceptional-Point Braiding

Cooling a mechanical mode in finite time is a routing problem: excitation must reach a lossy mode before thermal noise rebuilds the population. Exceptional points are non-Hermitian degeneracies at which two hybrid modes and their eigenvectors merge, so a loop around them can exchange connected spectral branches. The main obstacle in testing whether this topology improves cooling is causal. Independently optimized enclosing and non-enclosing protocols normally have different waveforms, so topology and waveform geometry change together. We remove this ambiguity in a three-mode optomechanical system with a strongly damped main cavity and an exactly lossless auxiliary cavity. We keep one power modulation and one detuning shape fixed and change only their relative phase. The shift preserves the matched local control resources but changes the loop from non-enclosing to a braid around both exceptional points. Certified dynamics show that the two-exceptional-point protocol lowers the final mechanical occupation by more than 55 percent. The advantage survives the complete admissible phase family, deliberate waveform changes, a finite neighborhood of nearby smooth controls, and the restoration of counter-rotating heating processes. The auxiliary cavity acts as a coherent buffer, while the main cavity remains the only optical loss channel.

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arXiv2026

Finite-Particle Quantum Reduction of Thermodynamic Irreversibility

Thermodynamics is useful because it lets us predict and control complex systems from a few accessible quantities when microscopic reconstruction is impractical. We ask whether this operational reduction can be less costly in a finite quantum system than in a matched classical one. We compare a few-boson Bose--Hubbard chain with its number-conserving classical-field counterpart under the same driving and the same coarse spatial record. The quantum system relaxes more in the observed particle distribution, yet the retained mean energy resolves more of the microscopic structure hidden by that record, leaving less information unusable and smaller entropy generation. We then close the process into a heat-engine cycle whose controls use only the measured mean energy and spatial record, not full state tomography; at matched thermal resources, the smaller entropy generation gives more work and higher efficiency. The advantage fades as the particle number grows toward the classical-field regime.

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arXiv2026

Charging Quantum Batteries with Chiral Squeezing

We propose a quantum-battery charger based on a driven bosonic Kitaev chain (BKC), where chiral squeezing converts passive input fluctuations into ordered, non-passive battery states. While a coherent input pulse exhibits phase-sensitive chiral transport, the charging dynamics is dominated by bidirectionally propagating fluctuations that are amplified and squeezed into orthogonal quadratures at opposite chain ends. In contrast to conventional phase-preserving amplifiers, our scheme stores largely extractable energy and achieves a work-like signal-to-noise ratio (SNR) near unity, even in the presence of thermal noise and moderate symmetry-preserving disorder.

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arXiv2026

Thermodynamic Irreversibility from Inaccessible Endogenous Quantum Histories

Microscopic unitary dynamics preserves all fine information, yet an isolated finite system can show a robust thermodynamic window in which the entropy associated with a restricted record rises. We ask why information hidden from the current record usually fails to rebuild a low-entropy macrostate. For a fixed projective record, every finite step separates exactly into the evolution predicted from the record alone and an exact correction carried by unresolved microscopic structure. The record-only contribution starts only at second order in time, so all instantaneous change of the record comes from hidden currents between macrostates. We derive the exact entropy rate, separate entropy-spreading from return-oriented currents, and resolve those currents into energy-gap amplitudes. Transitions with the same gap add coherently, revealing how the Hamiltonian and the microscopic state organize hidden information for return. In interacting mixing dynamics the current power is spread over many frequencies; free and deliberately commensurate controls progressively concentrate it, and the engineered dynamics reconstructs a low-entropy macrostate. An independent distribution-level test separates hidden dynamical activity from finite-step return, and an exact classical measure-preserving counterpart shows which parts of the construction are not uniquely quantum. Within the specified record and observation window, irreversibility is therefore not loss of microscopic information, but the failure of that information to organize currents that restore macroscopic order.

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arXiv2026

Reservoir-Engineered Low-Threshold Quantum Energy Storage

Fast charging of quantum batteries requires amplification of the energy transferred to a storage mode without uncontrolled gain or phenomenological non-Hermitian dynamics. Inspired by broken/unbroken dynamical regimes, we introduce a reservoir-engineered quantum battery in which a two-photon-driven charger and a battery mode are coupled through a lossy dissipative mediator. Eliminating the fast mediator yields a reduced two-mode Lindblad model with a complex dissipative coupling and renormalized damping rates. Its drift matrix has a pump-induced stability threshold: below threshold the seeded response is bounded, whereas above threshold a weak seed excites a growing mode and the battery occupation increases exponentially. Compared with a coherent beam-splitter charger--battery benchmark at equal effective coupling, the dissipative architecture reaches this broken regime at a lower pump amplitude. For the parameters studied here, this corresponds to about $61\%$ less critical pump power and opens a pump-power window in which dissipative charging is exponential while the coherent benchmark remains below threshold. In the broken dissipative regime, the growth is dominated by a seed-selected coherent battery displacement rather than incoherent fluctuation buildup, so a large fraction of the stored energy is directly extractable by a displacement operation. The broken-regime boundary is a dynamical stability threshold, not generally an exceptional point, and the full three-mode Lindblad model confirms the reduced description in the fast-mediator regime. Our results give a completely positive route to pump-efficient, low-threshold, and coherently addressable quantum energy storage using engineered reservoirs.

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arXiv2026

Quantum Batteries as Work Sources for Phase-Locked Parametric Amplification

Quantum batteries have been proposed as locally precharged work sources for superconducting quantum technologies, suggesting a route to reduce continuously supplied microwave drives. Here we ask whether the pump tone of a quantum-limited parametric amplifier can be replaced, or strongly duty-cycled, by a finite bosonic quantum battery. Quantizing the pump of a nondegenerate parametric amplifier exposes a resource distinction hidden in the classical description: stored pump energy can generate signal-idler photons, but pump phase coherence is required to generate a phase-locked amplifier field. In a closed trilinear model, coherent and phase-randomized coherent pumps with the same photon-number distribution produce comparable pair numbers, yet only the coherent pump produces anomalous two-mode coherence and an EPR-squeezed interference dip. Including leakage, we collect the emitted fields into cascaded temporal modes. At matched collector bandwidth, the coherent pump gives $I_{\min}^{(f)}=0.553$, whereas the phase-randomized pump gives $I_{\min}^{(f)}=1.94$ at nearly identical collected energy. Weak amplitude squeezing slightly improves the dip by reducing finite-pump number fluctuations while preserving the coherent displacement. Thus battery-powered parametric amplification requires phase-coherent stored energy, possibly assisted by number-noise reduction, rather than stored energy alone.

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arXiv2026

Quantum-Battery-Powered Geometric Landau-Zener Interferometry

Classical microwave drives are usually treated as ideal phase-coherent work sources for superconducting-qubit control. What if such a drive is replaced by a finite quantum battery. As a demanding benchmark, we consider echo-refocused geometric Landau--Zener interferometry powered by a single quantized bosonic mode. The qubit--battery dynamics are described by a Jaynes--Cummings Hamiltonian, while the echo pulse is retained as a qubit-only refocusing operation that cancels the dynamical phase. In the macroscopic coherent-state limit, the usual classical geometric interferometer is recovered. At finite mean photon number, however, the Jaynes--Cummings coupling generates photon-number-resolved avoided crossings with gaps $Ω_n=2g\sqrt{n}$. The qubit-only echo redistributes amplitudes between neighboring excitation sectors, so the finite-battery protocol is not a single classical interferometer but a coherent sector-resolved quantum evolution. This produces contrast loss, interferogram distortions, and measurable battery back-action. We further show that reducing photon-number fluctuations alone is not sufficient: geometric control requires a first-order phase reference. Geometric Landau--Zener interferometry therefore provides a practical benchmark for certifying phase-coherent quantum-battery energy.

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arXiv2025

Charge-Preserving Operations in Quantum Batteries

Ergotropy provides a fundamental measure of the extractable work from a quantum system and, consequently, of the maximal useful energy, or charge, stored within it. Understanding how this quantity can be manipulated and transformed efficiently is crucial for advancing quantum energy management technologies. Here, we introduce and formalize the concepts of isoergotropic states and ergotropy-preserving operations, which reorganize the internal structure of ergotropy while keeping its total value unchanged. These ideas are illustrated for both discrete (two-level systems) and continuous-variable systems (single-mode Gaussian states). In each case, we show how ergotropy-preserving operations redistribute the respective coherent-incoherent and displacement-squeezing components. We further examine the thermodynamic exchanges accompanying ergotropy-preserving operations, including variations in energy and entropy, and demonstrate that these transformations can be dynamically implemented through standard beam-splitter-type interactions with an auxiliary system. Finally, we discuss the practical implications of isoergotropic states and operations in optimizing charging protocols and mitigating charge loss in open quantum batteries.

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arXiv2025

Harnessing Environmental Noise for Quantum Energy Storage

Quantum hardware increasingly relies on energy reserves that can later be converted into useful work; yet, most battery-like proposals demand coherent drives or engineered non-equilibrium resources, limiting practicality in noisy settings. We develop an autonomous charging paradigm in which an ensemble of identical two-level units, collectively coupled to a thermal environment, acquires work capacity without any external control. The common bath mediates interference between emission and absorption pathways, steering the many-body state away from passivity and into a steady regime with nonzero extractable work. The full charging dynamics and closed-form expressions are obtained for the steady-state, showing favorable scaling with the number of cells that approach the many-body optimum. We show that the mechanism is robust to local noise: under a convex mixture of collective and local dissipation, non-zero steady-state ergotropy persists, exhibits counterintuitive finite-temperature optima, and remains operative when the collective channel is comparable to or stronger than the local one. We show that environmental fluctuations can be harnessed to realize drive-free, scalable quantum batteries compatible with circuit- and cavity-QED platforms. Used as local work buffers, such batteries could potentially enable rapid ancilla reset, bias dissipative stabilizer pumps, and reduce syndrome-extraction overhead in fault-tolerant quantum computing.

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arXiv2025

Two-time weak measurement protocol for ergotropy protection in open quantum batteries

Quantum batteries are emerging as highly efficient energy storage devices that can exceed classical performance limits. Although there have been significant advancements in controlling these systems, challenges remain in stabilizing stored energy and minimizing losses due to inevitable environmental interaction. In this paper, we propose a protocol that employs selective weak measurements to protect quantum states from such influence and mitigate battery discharging, that is feasible in state-of-the-art technologies. We establish thermodynamic constraints that allow this method to be implemented without disrupting the overall energy and ergotropy balance of the system, i.e., with no extra net recharging. Our findings demonstrate that appropriately chosen measurement intensity can reduce unwanted discharging effects, thereby preserving ergotropy and improving the stability of quantum batteries. We illustrate the protocol with single and two-qubit systems and establish the generalization for $N$-cell batteries. Additionally, we explore how weak measurements influence the coherent and incoherent components of ergotropy, providing new insights into the practical application of quantum coherence in energy storage technologies.

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arXiv2024

Super-Optimal Charging of Quantum Batteries via Reservoir Engineering

Energy dissipation, typically considered an undesirable process, has recently been shown to be harnessed as a resource to optimize the performance of a quantum battery. Following this perspective, we introduce a novel technique of charging in which coherent charger-battery interaction is replaced by a dissipative interaction via an engineered shared reservoir. We demonstrate that exploiting collective effects of the engineered shared reservoir allows for extra optimization giving rise to optimal redistribution of energy, which leads to a significant enhancement in the efficiency of the charging process. The article unveils the intricacies of built-in detuning within the context of a shared environment, offering a deeper understanding of the charging mechanisms involved. These findings apply naturally to quantum circuit battery architectures, suggesting the feasibility of efficient energy storage in these systems. Moreover, the super-optimal charging offers a practical justification for charger-battery configurations.

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arXiv2024

Unveiling Detuning Effects for Heat-Current Control in Quantum Thermal Devices

Navigating the intricacies of thermal management at the quantum scale is a challenge in the pursuit of advanced nanoscale technologies. To this extent, theoretical frameworks introducing minimal models mirroring the functionality of electronic current amplifiers and transistors, for instance, have been proposed. Different architectures of the subsystems composing a quantum thermal device can be considered, tacitly bringing drawbacks or advantages if properly engineered. This paper extends the prior research on thermotronics, studying a strongly coupled three-subsystem thermal device with a specific emphasis on a third excited level in the control subsystem. Our setup can be employed as a multipurpose device conditioned on the specific choice of internal parameters: heat switch, rectifier, stabilizer, and amplifier. The exploration of the detuned levels unveils a key role in the performance and working regime of the device. We observe a stable and strong amplification effect persisting over broad ranges of temperature. We conclude that considering a three-level system, as the one directly in contact with the control temperature, boosts output currents and the ability to operate our devices as a switch at various temperatures.

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arXiv2024

Nonreciprocal Quantum Batteries

Nonreciprocity, arising from the breaking of time-reversal symmetry, has become a fundamental tool in diverse quantum technology applications. It enables directional flow of signals and efficient noise suppression, constituting a key element in the architecture of current quantum information and computing systems. Here we explore its potential in optimizing the charging dynamics of a quantum battery. By introducing nonreciprocity through reservoir engineering during the charging process, we induce a directed energy flow from the quantum charger to the battery, resulting in a substantial increase in energy accumulation. Despite local dissipation, the nonreciprocal approach demonstrates a fourfold increase in battery energy compared to conventional charger-battery systems. We demonstrate that employing a shared reservoir can establish an optimal condition where nonreciprocity enhances charging efficiency and elevates energy storage in the battery. This effect is observed in the stationary limit and remains applicable even in overdamped coupling regimes, eliminating the need for precise temporal control over evolution parameters. Our result can be extended to a chiral network of quantum nodes, serving as a multi-cell quantum battery system to enhance storage capacity. The proposed approach is straightforward to implement using current state-of-the-art quantum circuits, both in photonics and superconducting quantum systems. In a broader context, the concept of nonreciprocal charging has significant implications for sensing, energy capture, and storage technologies or studying quantum thermodynamics.

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arXiv2023

An approximation scheme and non-Hermitian re-normalization for description of atom-field system evolution

Interactions between a source of light and atoms are ubiquitous in nature. The study of them is interesting on the fundamental level as well as for applications. They are in the core of Quantum Information Processing tasks and in Quantum Thermodynamics protocols. However, even for two-level atom interacting with field in rotating wave approximation there exists no exact solution. This touches as basic problem in quantum field theory, where we can only calculate the transitions in the time asymptotic limits (i.e. minus and plus infinity), while we are not able to trace the evolution. In this paper we want to get more insight into the time evolution of a total system of a two-level atom and a continuous-mode quantum field. We propose an approximation, which we are able to apply systematically to each order of Dyson expansion, resulting in greatly simplified formula for the evolution of the combined system at any time. Our tools include a proposed novel, {\it non-Hermitian} re-normalization method. As a sanity check, by applying our framework, we derive the known optical Bloch equations.

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arXiv2023

Catalysis in Charging Quantum Batteries

We propose a novel approach for optimization of charging of harmonic oscillators (quantum batteries) coupled to a harmonic oscillator (charger), driven by laser field. We demonstrate that energy transfer limitations can be significantly mitigated in the presence of catalyst systems, mediating between the charger and quantum batteries. We show that these catalyst systems, either qubits or harmonic oscillators, enhance the amount of energy transferred to quantum batteries, while they themselves store almost no energy. It eliminates the need for optimizing frequency of the charging laser field, whose optimal value in the bare setting depends on coupling strengths between the charger and the batteries.

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