arXiv subjects

James Q. Quach

Publications and source records attributed to James Q. Quach.

At least 19 recordsLinked to original sources

arXiv2026

Can a quantum circuit detect the Unruh effect?

The Unruh effect predicts that an accelerating observer perceives the Minkowski vacuum as a thermal bath, yet direct detection remains experimentally inaccessible. Its timelike counterpart, arising from the entanglement of massless fields between the future and past light cones, offers a more feasible route but requires a detector whose transition frequency follows a specific conformal-time scaling. We propose and analyze a practical implementation of such a detector using superconducting fluxonium circuits, which naturally provide two quasi-degenerate ground states and a tunable excited state, forming an effective $Λ$-system. By modulating the excited-state transition frequency in Minkowski time, the detector accumulates a geometric phase associated with the timelike Unruh effect. Open-system simulations predict $\sim 10\%$ shift in the ground-state population within $530$ ns, representing a three-order-of-magnitude sensitivity enhancement over two-level Unruh-DeWitt detectors. These results establish a realistic quantum-circuit platform for experimentally probing the timelike Unruh effect and, more broadly, for testing fundamental nature of quantum fields using engineered quantum systems.

quant-ph↗

arXiv2026

Collective Enhancement of Nuclear Excitation for a Nuclear Quantum Battery

Current implementations of quantum batteries are constrained by limited energy density and short retention times associated with the electronic or molecular excitations. Here we propose a nuclear quantum battery based on collective excitation of the $^{57}$Fe nuclei of density $n$ embedded in a planar hard X-ray waveguide. Using a Green function waveguide-QED description, we study charging via excitation beyond linear response, where saturation and drive back-action reshape the incident pulse. We introduce a self-consistent waveform-engineering protocol that inhibits local radiative decay in the waveguide thus promoting absorption into high-lying collective nuclear excitation manifolds. We show an enhanced excitation cross section of the nuclear ensemble which yields superlinear charging, with maximum studied energy density scaling approximately like $n \sqrt{n}$. Our results provide a route to high-energy-density quantum charging at hard X-ray energies using contemporary X-ray sources and waveguide architectures by identifying nonlinear, collectively enhanced absorption as a key mechanism for nuclear quantum battery operation.

quant-ph↗

arXiv2026

Locally Passive, Globally Charged Quantum Batteries: Coherence-Controlled Work and the Robustness of the Stored Charge

A solvable charger--battery model is introduced in which quantum coherence controls both where a quantum battery's charge is stored and how robustly it survives noise. Charging converts the charger's coherence into charger--battery entanglement and splits the deposited work between a locally extractable part and a correlation-locked part accessible only through joint operations; for a qubit, the split obeys an exact complementarity, and at maximal coherence, the battery is locally passive with the entire charge locked in correlations. Robustness follows local accessibility: the stored energy and locally extractable work are population-based, immune to pure dephasing, and limited only by relaxation, with an energy half-life, whereas the correlation-locked work is fragile to both dephasing and relaxation. Dephasing, global and local depolarization, and amplitude damping are treated through a single gain--loss competition algebra, and the resulting storage lifetimes are made concrete with superconducting-transmon parameters.

quant-ph↗

arXiv2026

Reservoir-independent lossless charging and protected storage of an open quantum battery

A quantum battery charged through a lossy intermediate state faces a structural trade-off between charging speed and dissipation. We show that an exact algebraic cancellation removes it in a driven three-level cell: the radiatively decaying state is fed by a single bright amplitude, and a counterdiabatic field annuls the lone residual source that drives it, holding the lossy state identically empty. Charging is then lossless -- not one photon is emitted through the bridge -- at any one-photon detuning, coupling, linewidth, and speed down to the rotating-wave limit, with no adiabatic elimination, so the charging power is bounded by the drive amplitude (a quantum speed limit) rather than by dissipation. Crucially, this losslessness is independent of the reservoir: because the dark sector never engages the system-bath coupling, the emission vanishes exactly for an arbitrary spectral density, Markovian or not, as an exact damped-pseudomode treatment confirms to machine precision across all memory times. The entire non-Hermitian structure -- a Markovian second-order exceptional point that reservoir memory promotes to a third-order one, and the attendant dissipation phase diagram -- lives in the bright sector, from which the protocol is by construction exempt. This inverts dissipation-engineered charging, where an exceptional point or reservoir memory is a resource; here the lossy sector is never populated at all. The same dark-state structure protects the stored charge, converting fast radiative self-discharge into the slow metastable lifetime, with residuals quadratic in the control error. We detail experimental requirements and representative parameters for neutral alkaline-earth atoms, trapped ions, transmons, and defect centers.

quant-ph↗

arXiv2026

Counterdiabatic Raman Atom Optics for Compact High-Sensitivity Gravimetry

Large-momentum-transfer (LMT) atom interferometry provides a route toward enhanced inertial sensitivity in compact quantum sensors, but its scalability is limited by the accumulation of pulse-transfer errors across long Raman pulse sequences. We investigate theoretically the use of stimulated Raman shortcut-to-adiabatic passage (STIRSAP) for high-fidelity LMT atom optics in a Mach--Zehnder interferometer geometry. The counterdiabatic correction is encoded directly into the Raman pulse envelopes, eliminating the need for auxiliary microwave or radio-frequency control fields. Numerical simulations based on an effective Raman model show that $1~μ\mathrm{s}$ STIRSAP pulses achieve single-pulse transfer fidelities of $F_π= 0.99902$ while maintaining negligible pulse-time overhead even at high momentum order. We analyze the resulting tradeoff between interferometric phase enhancement and compound contrast decay and identify an unconstrained shot-noise optimum near $n\approx270$. The analysis further shows that practical operation at extreme LMT order is constrained by wave-packet separation, vibration noise, Doppler detuning, and accumulated systematic effects rather than by pulse duration itself. These results establish superadiabatic Raman control as a promising approach for scalable high-fidelity atom optics and clarify the physical limitations governing compact high-order atom interferometers.

quant-ph↗

arXiv2026

Towards Real-time Control of a CartPole System on a Quantum Computer

The application of quantum reinforcement learning (QRL) to real-time control systems faces significant challenges regarding hardware latency, noise susceptibility, and learning convergence. This work presents an end-to-end investigation of a minimal hybrid quantum-classical agent applied to the CartPole benchmark, addressing the gap between idealized simulation and execution on a physical superconducting quantum processing unit (QPU). We demonstrate that a single-qubit agent acts as an effective learning model, solving the environment in substantially fewer episodes than a comparable classical actor-critic network even when the training of the hybrid agent is restricted to use parameter-shift for its quantum circuit component. To connect learning to deployment constraints, we map the inference-time trade-off between control-loop rate and measurement shot budget to provide guidance for an eventual real-time control demonstration. The resulting performance matrices show that both inference control frequency and shot count strongly affect balancing stability: higher inference frequencies consistently improve performance, and increasing the shot budget lowers the minimum inference frequency required to achieve near-maximal balancing. These results highlight the importance of finding an optimal medium between shot count and control frequency and developing circuits that are e.g. initial-state invariant. Lastly, we address the critical bottleneck of control latency on NISQ hardware by bypassing the standard high-level software stack and programming the Zurich Instruments readout electronics directly via command tables. These results quantify some of the current boundaries of quantum-assisted control and provide a start for achieving the tens-of-hertz throughput required for real-time closed-loop control feedback.

quant-ph↗

arXiv2026

Quenching Speculation in Quantum Markets via Entangled Neural Traders

Speculative trading can drive pronounced market instabilities, yet existing regulatory and macroprudential tools intervene only after such dynamics emerge. Quantum technologies offer a fundamentally new means of shaping economic behavior by introducing non-classical correlations between decision-makers. Here we demonstrate a prototype quantum stock market in which entanglement between traders' valuations mitigates the runaway devaluation characteristic of speculative busts. Using reinforcement-learning agents trading a single commodity, we show that replacing classical valuations with quantum-correlated qubit-encoded valuations stabilizes prices and increases the AI traders' net worth relative to a classical market, where instead agents rapidly converge to liquidation strategies that collapse the asset value. To explain this behavior, we formulate and analyze a quantized version of the $p$-guessing game, a canonical model of speculative dynamics. Quantum entanglement and phase coherence reshape the strategic landscape, eliminating the pathological pure-strategy Nash equilibrium that drives market collapse in the classical game, while mixed-strategy equilibria remain non-degenerate and avoid bust-type outcomes. These results identify quantum correlations as a novel, endogenous mechanism for market stabilization and, more broadly, demonstrate the utility of multi-agent reinforcement learning algorithms for uncovering optimal strategies in complex decision-making frameworks with quantum degrees of freedom.

quant-ph↗

arXiv2025

Magnetic Dipolar Quantum Battery with Spin-Orbit Coupling

We investigate a magnetic dipolar system influenced by the $z$-component of Zeeman splitting, Dzyaloshinsky--Moriya (DM) interaction, and Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) exchange interaction, with emphasis on the role of quantum resources in both closed and open settings. By analyzing the Gibbs thermal state and solving the Lindblad master equation, we study the behavior of quantum coherence, discord, and entanglement under thermal equilibrium and dephasing noise. After exploring these resources, we apply the model to a closed quantum battery (QB). Our results show that while Zeeman splitting degrades quantum resources in noisy and thermal regimes, it enhances QB performance by improving ergotropy, anti-ergotropy, storage capacity, and coherence during cyclic charging. The axial parameter further amplifies performance, leading to coherence saturation and persistent ergotropy growth, in line with the notion of incoherent ergotropy. KSEA interaction and the rhombic term consistently preserve coherence and entanglement under noise, thereby strengthening QB functionality. DM interaction mitigates thermal degradation of resources in the Gibbs state and improves performance, though its effect is limited under Pauli-$X$ dephasing. We reveal diverse behaviors, including increased ergotropy without coherence and the coexistence of coherence with zero extractable work. Finally, we propose Nuclear Magnetic Resonance (NMR) as a feasible platform for experimental implementation.

quant-ph↗

arXiv2025

Superradiant Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) are central to modern display technologies and are promising candidates for low-cost energy-efficient lighting. Their performance is determined by numerous, intricate fabrication parameters, but not least by the number of emissive molecules N, which provide sites for electron-hole recombination and photon generation in the diode host matrix. Counterintuitively, larger concentrations of emitters do not always lead to brighter or more efficient OLEDs due to concentration quenching of luminescence meaning that rates of radiative electron-hole recombination can become severely reduced, negatively impacting charge-to-photon conversion efficiency. In this work we trigger steady-state superradiant light emission from a series of Fabry-Pérot microcavity OLEDs by scaling the operating voltage of each device with emitter concentration. We demonstrate a collective enhancement in the luminance of a microcavity OLED that scales super-extensively when compared to no-cavity controls fabricated in the same run. Triggering quantum correlations between emitters via the confined cavity field allows devices with fewer emitters to match or even exceed the brightness of control OLEDs even when driven by lower voltages. Moreover, our devices show significant narrowing of their emission spectra, offering purer colours at low applied voltages. Leveraging collective effects in microcavity OLEDs provides a new approach to enable brighter, more efficient devices paving the way for next-generation displays and lighting that do not compromise performance for operational efficiency or device lifetime.

quant-ph↗

arXiv2025

Experimental demonstration of a scalable room-temperature quantum battery

Harnessing quantum phenomena in energy storage systems offers an opportunity to introduce a new generation of batteries with quantum-enhanced performance. Until now, the quantum battery has largely remained a theoretical concept, with little progress towards experimental realisation, due to the challenges in quantum coherent control. Here, we experimentally demonstrate a scalable room-temperature quantum battery with a multi-layered organic-microcavity design. We show that it exhibits superextensive charging, metastabilisation of stored energy, and generates superextensive electrical power, the latter an unpredicted phenomenon. The combination of these properties in a single device is the first demonstration of the full cycle of a quantum battery, laying the framework for future designs.

quant-ph↗

arXiv2024

Ergotropy and capacity optimization in Heisenberg spin-chain quantum batteries

This study examines the performance of finite spin quantum batteries (QBs) using Heisenberg spin models with Dzyaloshinsky-Moriya (DM) and Kaplan--Shekhtman--Entin-Wohlman--Aharony (KSEA) interactions. The QBs are modeled as interacting quantum spins in local inhomogeneous magnetic fields, inducing variable Zeeman splitting. We derive analytical expressions for the maximal extractable work, ergotropy and the capacity of QBs, as recently examined by Yang et al. [Phys. Rev. Lett. 131, 030402 (2023)]. These quantities are analytically linked through certain quantum correlations, as posited in the aforementioned study. Different Heisenberg spin chain models exhibit distinct behaviors under varying conditions, emphasizing the importance of model selection for optimizing QB performance. In antiferromagnetic (AFM) systems, maximum ergotropy occurs with a Zeeman splitting field applied to either spin, while ferromagnetic (FM) systems benefit from a uniform Zeeman field. Temperature significantly impacts QB performance, with ergotropy in the AFM case being generally more robust against temperature increases compared to the FM case. Incorporating DM and KSEA couplings can significantly enhance the capacity and ergotropy extraction of QBs. However, there exists a threshold beyond which additional increases in these interactions cause a sharp decline in capacity and ergotropy. This behavior is influenced by temperature and quantum coherence, which signal the occurrence of a sudden phase transition. The resource theory of quantum coherence proposed by Baumgratz et al. [Phys. Rev. Lett. 113, 140401 (2014)] plays a crucial role in enhancing ergotropy and capacity. However, ergotropy is limited by both the system's capacity and the amount of coherence. These findings support the theoretical framework of spin-based QBs and may benefit future research on quantum energy storage devices.

quant-ph↗

arXiv2024

Quantum deformation of cubic string field theory

In this paper, we will analyze a quantum deformation of cubic string field theory. This will be done by first constructing a quantum deformation of string theory, in a covariant gauge, and then using the quantum deformed stringy theory to construct a quantum deformation of string field theory. This quantum deformed string field will then be used to contract a quantum deformed version of cubic string field theory. We will explicitly demonstrate that the axioms of cubic string field theory hold even after quantum deformation. Finally, we will analyze the effect of the quantum deformation of string field theory on the string vertices.

hep-th↗

arXiv2024

Extending the self-discharge time of Dicke quantum batteries using molecular triplets

Quantum batteries, quantum systems for energy storage, have gained interest due to their potential scalable charging power density. A quantum battery proposal based on the Dicke model has been explored using organic microcavities, which enable a cavity-enhanced energy transfer process called superabsorption. However, energy storage lifetime in these devices is limited by fast radiative emission losses, worsened by superradiance. Here, we demonstrate a promising approach to extend the energy storage lifetime of Dicke quantum batteries using molecular triplet states. We examine a type of multi-layer microcavities where an active absorption layer transfers energy to the molecular triplets of a storage layer, identifying two regimes based on exciton-polariton resonances. We tested one of these mechanisms by fabricating and characterising five devices across a triplet-polariton resonance. We conclude by discussing potential optimisation outlooks for this class of devices.

quant-ph↗

arXiv2023

Quantum batteries -- The future of energy storage?

According to the International Energy Agency, each human uses more than 80 GJ of energy per year; this is equivalent to leaving a washing machine continuously running for one year for every person on Earth. This consumption is expected to increase by 28% by 2040 (from 2015 levels). The majority (86%) of this energy comes from fossil fuels. This dependence on fossil fuels comes with major environmental costs, with climate change arguably being the greatest challenge facing our era. Renewable energy offers a possible solution. However, renewable energy sources, like solar and wind are not continuous sources, and therefore energy storage technology or batteries, remain an urgent challenge for further worldwide adoption of renewable energy. Alongside the need for efficient batteries to store renewable energy, the portability of batteries makes them an essential component in mobile technologies, including electric vehicles. Current batteries operate on the basis of well-understood electrochemical principles which were developed two centuries ago. While there is an ongoing intense effort aimed at improving their performance through optimization of the materials and the device architecture, it is worth exploring completely novel and disruptive approaches towards energy storage. Quantum batteries are energy storage devices that utilise quantum mechanics to enhance performance or functionality. While they are still in their infancy with only proof-of-principle demonstrations achieved, their radically innovative design principles offer a potential solution to future energy challenges.

quant-ph↗

arXiv2023

Colloquium: Quantum Batteries

Recent years have witnessed an explosion of interest in quantum devices for the production, storage, and transfer of energy. In this Colloquium, we concentrate on the field of quantum energy storage by reviewing recent theoretical and experimental progress in quantum batteries. We first provide a theoretical background discussing the advantages that quantum batteries offer with respect to their classical analogues. We then review the existing quantum many-body battery models and present a thorough discussion of important issues related to their open nature. We finally conclude by discussing promising experimental implementations, preliminary results available in the literature, and perspectives.

quant-ph↗

arXiv2022

The Berry phase from the entanglement of future and past light cones: detecting the timelike Unruh effect

The Unruh effect can not only arise out of the entanglement between modes of left and right Rindler wedges, but also between modes of future and past light cones. We explore the geometric phase resulting from this timelike entanglement between the future and past, showing that it can be captured in a simple $Λ$-system. This provides an alternative paradigm to the Unruh-deWitt detector. The Unruh effect has not been experimentally verified because the accelerations needed to excite a response from Unruh-deWitt detectors are prohibitively large. We demonstrate that a stationary but time-dependent $Λ$-system detects the timelike Unruh effect with current technology.

gr-qc↗

arXiv2021

A Quantum Informational Approach to the Problem of Time

Several novel approaches have been proposed to resolve the problem of time by relating it to change. We argue using quantum information theory that the Hamiltonian constraint in quantum gravity cannot probe change, so it cannot be used to obtain a meaningful notion of time. This is due to the absence of quantum Fisher information with respect to the quantum Hamiltonian of a time-reparametization invariant system. We also observe that the inability of this Hamiltonian to probe change can be related to its inability to discriminate between states of such a system. However, if the time-reparametization symmetry is spontaneously broken due to the formation of quantum cosmological time crystals, these problems can be resolved, and it is possible for time to emerge in quantum gravity.

gr-qc↗

arXiv2021

Time Fisher Information associated with Fluctuations in Quantum Geometry

As time is not an observable, we use Fisher information (FI) to address the problem of time. We show that the Hamiltonian constraint operator cannot be used to analyze any quantum process for quantum geometries that are associated with time-reparametrization invariant classical geometries. This is because the Hamiltonian constraint does not contain FI about time. We demonstrate that although the Hamiltonian operator is the generator of time, the Hamiltonian constraint operator can not observe the change that arises through the passage of time. This means that the problem of time is inescapably problematic in the associated quantum gravitational theories. Although we explicitly derive these results on the world-sheet of bosonic strings, we argue that it holds in general. We also identify an operator on the world-sheet which contains FI about time in a string theoretical processes. Motivated by this observation, we propose that a criteria for a meaningful operator of any quantum gravitational process, is that it should contain non-vanishing FI about time.

gr-qc↗