arXiv subjects

Sofia Qvarfort

Publications and source records attributed to Sofia Qvarfort.

At least 19 recordsLinked to original sources

arXiv2026

Quantum metasurfaces as probes of vacuum particle content

The quantum vacuum of the electromagnetic field is inherently entangled across distinct spatial sub-regions, resulting in entangled particle content across these sub-regions. However accessing this particle content in a controlled laboratory experiment has remained out of experimental reach. Here, we analyse the feasibility of witnessing such vacuum effects with highly non-perturbative boundary condition changes with a quantum metasurface made from a two-dimensional sub-wavelength atomic array. The array response to light is tunable between transmissive and reflective states by a control atom that is excited to a Rydberg state. We find that vacuum photon content from non-perturbative changes of the boundary conditions and therefore distinct spatial sub-regions of the vacuum causes subtle frequency shifts and analyse the accessibility to sub-wavelength atom array platforms. This novel approach to probing vacuum particle content leverages the unique ability to generate coherent dynamics in superpositions of transmissive and reflective states of the array reflectivity, providing a quantum-enhanced platform for observing vacuum particle creation driven by highly non-perturbative changes in the boundary conditions of the electromagnetic vacuum.

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arXiv2026

Macroscopic Quantum Interference of the Center-of-Mass Motion of Levitated Superconducting Microparticles enabled by Magnetic Higher-Order Traps

We show how magnetostatic higher-order multipole traps can be used to generate macroscopic quantum interference of the motion of levitated superconducting microparticles. An appropriate combination of multipolar magnetic fields offers great versatility in constructing various trap potentials, including anharmonic trap potentials such as Duffing or double-well types. Crucially, the anharmonic trap potentials realize a nonlinearity on the order of hundred times the zero-point motion, i.e., on a length scale below nanometers. These anharmonic potentials allow for the generation of quantum features of the center-of-mass motion of a magnetically levitated superconducting microparticle. Importantly, they can be easily generated with a static arrangement of coils, requiring only that the current running through them is tunable. We propose protocols exploiting the versatility of the magnetic trap landscape to generate non-Gaussian motional states. We solve the dynamics of the center-of-mass motion of the particle in phase space and analyze its parameter dependence. Furthermore, we give a recipe to distinguish classical from quantum behavior in a statistically meaningful way through measurement of the position of the particle. Our results open a path to accessing the quantum regime of the center-of-mass motion of objects with masses larger than picogram, i.e., $10^{13}$ atomic mass units. This will enable fundamental physics experiments for studying the transition between quantum and classical behavior, exploring the intersection between quantum physics and gravity as well as probing of certain types of dark matter.

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arXiv2026

Gravitational wave imprints on spontaneous emission

Despite growing interest, there is a scarcity of known predictions in the regime where both quantum and general relativistic effects become observable. Here, we investigate a combined atom-field system in a curved spacetime, with a specific focus on gravitational-wave backgrounds. We demonstrate that a plane gravitational wave alters spontaneous emission from a single atom, manifesting itself as a direction-dependent change in the emission spectrum. Although the total decay rate remains unchanged, implying that no information about the gravitational wave is stored in the atomic internal state alone, the wave leaves imprints on the evolution of the composite atom-field system. To quantify how well this effect can be measured, we analyze both the classical Fisher information associated with photon number measurements and the quantum Fisher information. Our analysis indicates that the effect could be measured in state-of-the-art cold-atom experiments and points to spontaneous emission as a potential probe of low-frequency gravitational waves.

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arXiv2025

Optimizing confidence in negative-partial-transpose-based entanglement criteria

A key requirement of any separable quantum state is that its density matrix has a positive partial transpose. For continuous bipartite quantum states, violation of this condition may be tested via the hierarchy of negative-partial-transpose (NPT) based entanglement criteria introduced by Shchukin and Vogel [Phys. Rev. Lett. 95, 230502 (2005)]. However, a procedure for selecting the optimal NPT-based criterion is currently lacking. Here, we develop a framework to select the optimal criterion by determining the level of confidence of criteria within the Shchukin and Vogel hierarchy for finite measurement number, environmental noise, and the optimal allocation of measurement resources. To demonstrate the utility of our approach, we apply our statistical framework to prominent example Gaussian and non-Gaussian states, including the two-mode squeezed vacuum state, the quanta-subtracted two-mode squeezed vacuum state, and the two-mode Schrödinger-cat state. Beyond bipartite inseparability tests, our framework can be applied to any Hermitian matrix constructed of observable moments and thus can be utilized for a wide variety of other nonclassicality criteria and multi-mode entanglement tests.

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arXiv2025

Metrology of Gravitational Effects with Mechanical Quantum Systems

Mechanical quantum systems, such as resonators and levitated particles, offer unique opportunities for quantum metrology. Particularly, their significant mass and quantum-level control enable applications in measuring gravitational effects. This article highlights key challenges, alongside potential solutions to advance precision sensing and quantum-gravitational research.

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arXiv2024

Sensing force gradients with cavity optomechanics while evading backaction

We study force-gradient sensing with a coherently driven mechanical resonator and phase-sensitive detection of motion through the two-tone backaction evading measurement of cavity optomechanics. The response of the optomechanical system, solved by numerical integration of the classical equations of motion, shows an extended region which is monotonic to changes in force gradient. We use Floquet theory to model the fluctuations, which rise only slightly above that of the usual backaction evading measurement in the presence of the mechanical drive. The monotonic response and minimal backaction are advantageous for applications such as atomic force microscopy.

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arXiv2024

Massive quantum systems as interfaces of quantum mechanics and gravity

The traditional view from particle physics is that quantum gravity effects should only become detectable at extremely high energies and small length scales. Due to the significant technological challenges involved, there has been limited progress in identifying experimentally detectable effects that can be accessed in the foreseeable future. However, in recent decades, the size and mass of quantum systems that can be controlled in the laboratory have reached unprecedented scales, enabled by advances in ground-state cooling and quantum-control techniques. Preparations of massive systems in quantum states pave the way for the explorations of a low-energy regime in which gravity can be both sourced and probed by quantum systems. Such approaches constitute an increasingly viable alternative to accelerator-based, laser-interferometric, torsion-balance, and cosmological tests of gravity. In this review, we provide an overview of proposals where massive quantum systems act as interfaces between quantum mechanics and gravity. We discuss conceptual difficulties in the theoretical description of quantum systems in the presence of gravity, review tools for modeling massive quantum systems in the laboratory, and provide an overview of the current state-of-the-art experimental landscape. Proposals covered in this review include, among others, precision tests of gravity, tests of gravitationally-induced wavefunction collapse and decoherence, as well as gravitymediated entanglement. We conclude the review with an outlook and summary of the key questions raised.

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arXiv2023

Exceptional points and exponential sensitivity for periodically driven Lindblad equations

In this contribution to the memorial issue of Göran Lindblad, we investigate the periodically driven Lindblad equation for a two-level system. We analyze the system using both adiabatic diagonalization and numerical simulations of the time-evolution, as well as Floquet theory. Adiabatic diagonalization reveals the presence of exceptional points in the system, which depend on the system parameters. We show how the presence of these exceptional points affects the system evolution, leading to a rapid dephasing at these points and a staircase-like loss of coherence. This phenomenon can be experimentally observed by measuring, for example, the population inversion. We also observe that the presence of exceptional points seems to be related to which underlying Lie algebra the system supports. In the Floquet analysis, we map the time-dependent Liouvillian to a non-Hermitian Floquet Hamiltonian and analyze its spectrum. For weak decay rates, we find a Wannier-Stark ladder spectrum accompanied by corresponding Stark-localized eigenstates. For larger decay rates, the ladders begin to dissolve, and new, less localized states emerge. Additionally, their eigenvalues are exponentially sensitive to perturbations, similar to the skin effect found in certain non-Hermitian Hamiltonians.

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arXiv2023

Enhanced optomechanical nonlinearity through non-Markovian mechanical noise

Cavity optomechanical systems in the quantum regime consist of a cavity mode and mechanical element coupled together through radiation pressure. In the nonlinear optomechanical regime, open-system dynamics is generally challenging to treat analytically, since the noise terms do not commute with the optomechanical interaction term. Specifically, a general treatment of both Markovian and non-Markovian mechanical noise in the nonlinear optomechanical regime is still outstanding. Here we address this question by solving the full dynamics of an optomechanical system in the nonlinear regime where the mechanical element interacts with a bath of harmonic oscillators, representing full quantum Brownian motion. The solutions, which are exact and analytic, allow us to consider the strength of the optomechanical nonlinearity in the presence of both Markovian (Ohmic) and non-Markovian (sub-Ohmic and super-Ohmic) baths. We show that that while the strength of the nonlinearity is generally reduced by a Markovian bath spectrum, it can be enhanced by constructing a bath with a highly non-Markovian structure. The results have potential implications for future optomechanical experiments which seek to achieve a strong optomechanical nonlinearity.

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arXiv2023

Optimal quantum parametric feedback cooling

We propose an optimal protocol using phase-preserving quantum measurements and phase-dependent modulations of the trapping potential at parametric resonance to cool a quantum oscillator to an occupation number of less than one quantum. We derive the optimal phase relationship and duration for the parametric modulations, and compute the lowest-possible occupation number in the steady state. The protocol is robust against moderate amounts of dissipation and phase errors in the feedback loop. Our work has implications for the cooling of levitated mechanical resonators in the quantum regime.

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arXiv2022

Fast Optomechanical Photon Blockade

The photon blockade effect is commonly exploited in the development of single-photon sources. While the photon blockade effect could be used to prepare high-fidelity single-photon states in idealized regimes, practical implementations in optomechanical systems suffer from an interplay of competing processes. Here we derive a control scheme that exploits destructive interference of Fock state amplitudes of more than one photon. The resulting preparation time for photon-blockaded quantum states is limited only by the optomechanical interaction strength and can thus be orders of magnitude shorter than in existing schemes that achieve photon-blockaded state in the steady state.

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arXiv2022

Solving quantum dynamics with a Lie algebra decoupling method

At the heart of quantum technology development is the control of quantum systems at the level of individual quanta. Mathematically, this is realised through the study of Hamiltonians and the use of methods to solve the dynamics of quantum systems in various regimes. Here, we present a pedagogical introduction to solving the dynamics of quantum systems by the use of a Lie algebra decoupling theorem. As background, we include an overview of Lie groups and Lie algebras aimed at a general physicist audience. We then prove the theorem and apply it to three well-known examples of linear and quadratic Hamiltonian that frequently appear in quantum optics and related fields. The result is a set of differential equations that describe the most Gaussian dynamics for all linear and quadratic single-mode Hamiltonian with generic time-dependent interaction terms. We also discuss the use of the decoupling theorem beyond quadratic Hamiltonians and for solving open-system dynamics.

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arXiv2022

Two-mode Schrödinger-cat states with nonlinear optomechanics: generation and verification of non-Gaussian mechanical entanglement

Cavity quantum optomechanics has emerged as a new platform for quantum science and technology with applications ranging from quantum-information processing to tests of the foundations of physics. Of crucial importance for optomechanics is the generation and verification of non-Gaussian states of motion and a key outstanding challenge is the observation of a canonical two-mode Schrödinger-cat state in the displacement of two mechanical oscillators. In this work, we introduce a pulsed approach that utilizes the nonlinearity of the radiation-pressure interaction combined with photon-counting measurements to generate this entangled non-Gaussian mechanical state, and, importantly, describe a protocol using subsequent pulsed interactions to verify the non-Gaussian entanglement generated. Our pulsed verification protocol allows quadrature moments of the two mechanical oscillators to be measured up to any finite order providing a toolset for experimental characterisation of bipartite mechanical quantum states and allowing a broad range of inseparability criteria to be evaluated. Key experimental factors, such as optical loss and open-system dynamics, are carefully analyzed and we show that the scheme is feasible with only minor improvements to current experiments that operate outside the resolved-sideband regime. Our scheme provides a new avenue for quantum experiments with entangled mechanical oscillators and offers significant potential for further research and development that utilizes such non-Gaussian states for quantum-information and sensing applications, and for studying the quantum-to-classical transition.

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arXiv2022

Constraining modified gravity with quantum optomechanics

We derive the best possible bounds that can be placed on Yukawa- and chameleon-like modifications to the Newtonian gravitational potential with a cavity optomechanical quantum sensor. By modelling the effects on an oscillating source-sphere on the optomechanical system from first-principles, we derive the fundamental sensitivity with which these modifications can be detected in the absence of environmental noise. In particular, we take into account the large size of the optomechanical probe compared with the range of the fifth forces that we wish to probe and quantify the resulting screening effect when both the source and probe are spherical. Our results show that optomechanical systems in high vacuum could, in principle, further constrain the parameters of chameleon-like modifications to Newtonian gravity.

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arXiv2021

Master-equation treatment of nonlinear optomechanical systems with optical loss

Open-system dynamics play a key role in the experimental and theoretical study of cavity optomechanical systems. In many cases, the quantum Langevin equations have enabled excellent models for optical decoherence, yet a master-equation approach to the fully nonlinear optomechanical Hamiltonian has thus far proven more elusive. To address this outstanding question and broaden the mathematical tool set available, we derive a solution to the Lindblad master equation that models optical decoherence for a system evolving with the nonlinear optomechanical Hamiltonian. The method combines a Lie-algebra solution to the unitary dynamics with a vectorization of the Lindblad equation, and we demonstrate its applicability by considering the preparation of optical cat states via the optomechanical nonlinearity in the presence of optical loss. Our results provide a direct way of analytically assessing the impact of optical decoherence on the optomechanical intracavity state.

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arXiv2021

Optimal estimation of time-dependent gravitational fields with quantum optomechanical systems

We study the fundamental sensitivity that can be achieved with an ideal optomechanical system in the nonlinear regime for measurements of time-dependent gravitational fields. Using recently developed methods to solve the dynamics of a nonlinear optomechanical system with a time-dependent Hamiltonian, we compute the quantum Fisher information for linear displacements of the mechanical element due to gravity. We demonstrate that the sensitivity can not only be further enhanced by injecting squeezed states of the cavity field, but also by modulating the light--matter coupling of the optomechanical system. We specifically apply our results to the measurement of gravitational fields from small oscillating masses, where we show that, in principle, the gravitational field of an oscillating nano-gram mass can be detected based on experimental parameters that will likely be accessible in the near-term future. Finally, we identify the experimental parameter regime necessary for gravitational wave detection with a quantum optomechanical sensor.

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arXiv2020

Mesoscopic entanglement through central potential interactions

The generation and detection of entanglement between mesoscopic systems would have major fundamental and applicative implications. In this work, we demonstrate the utility of continuous variable tools to evaluate the Gaussian entanglement arising between two homogeneous levitated nanobeads interacting through a central potential. We compute the entanglement for the steady state and determine the measurement precision required to detect the entanglement in the laboratory.

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arXiv2020

Hydrogenic entanglement

Is there any entanglement in the simplest ubiquitous bound system? We study the solutions to the time-independent Schrödinger equation for a Hydrogenic system and devise two entanglement tests for free and localised states. For free Hydrogenic systems, we compute the Schmidt basis diagonalisation for general energy eigenstates, and for a Hydrogenic system localised to a three-dimensional Gaussian wavepacket, we demonstrate that measuring its second moments is sufficient for detecting entanglement. Our results apply to any system that exhibits Hydrogenic structure.

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