arXiv subjects

Radim Filip

Publications and source records attributed to Radim Filip.

At least 37 records · Page 2Linked to original sources

arXiv2026

Broadcasting quantum nonlinearity in hybrid systems

Linear oscillators contribute to most branches of contemporary quantum science. They have already successfully served as quantum sensors and memories, found applications in quantum communication, and hold promise for cluster-state-based quantum computing. To master universal quantum processing with linear oscillators, an unconditional nonlinear operation is required. We propose such an operation using light-mediated interaction with another system that possesses a nonlinearity equivalent to more than a quadratic potential. Such a potential grants access to a nonlinear operation that can be broadcast to the target linear system. The nonlinear character of the operation can be verified by observing adequate negative values of the target system's Wigner function and the squeezing of the variance of a certain nonlinear combination of the quadratures below the thresholds attainable by Gaussian states. We explicitly evaluate an optically levitated mechanical oscillator as a flexible source of nonlinearity for a proof-of-principle demonstration of the nonlinearity broadcasting to linear systems, for example, mechanical oscillators or macroscopic atomic spin ensembles.

quant-ph↗

arXiv2026

Scaling of multicopy constructive interference of Gaussian states

Quantum technology advances crucially depend on the scaling up of essential quantum resources. Their ideal multiplexing offers more significant gains in applications; however, the scaling of the nonidentical, fragile and varying resources is neither theoretically nor experimentally known. For bosonic systems, multimode interference is an essential tool already widely exploited to develop quantum technology. Here, we analyze, predict and compare essential scaling laws for a constructive interference of multiplexed nonclassical Gaussian states carrying information by displacement with weakly fluctuating squeezing in different multimode interference architectures. The signal-to-noise ratio quantifies the increase in displacement relative to the noise. We introduce the gain-to-instability ratio to numerically estimate the effect of unexplored resource instabilities in a large scale interference scheme. The use of the gain-to-instability ratio to quantify the scaling laws opens steps for extensive theoretical investigation of other bosonic resources and follow-up feasible experimental verification necessary for further development of these platforms.

quant-ph↗

arXiv2026

Qubit-parity interference despite unknown interaction phases

Quantum interference between interacting systems is fundamental to basic science and quantum technology, but it typically requires precise control of the interaction phases of lasers or microwave generators. Can interference be observed if those interaction phases are stable but unknown, usually prohibitive for complex state without active control? Here, we answer this question by experimentally preparing a Schrödinger-cat-like state of an internal qubit and a motional oscillator of a trapped $^{40}$Ca$^{+}$ ion, and its robustness to such uncontrolled phase. By applying alternating red and blue sideband pulses, we enforce a strict qubit-parity correlation and interference inherently insensitive to stable but unknown phases of the driving laser. For this qubit-parity interference, we use a minimal two-pulse interferometric sequence to demonstrate characteristic visibilities of $20\%$ and $40\%$, which approach the theoretical visibility limit, providing a scalable coherence witness without full state tomography for high-dimensional states.

quant-ph↗

arXiv2026

Cavity-QED tools for MBQC with optical binomial-codes

Measurement-based quantum computation (MBQC) offers a promising paradigm for photonic quantum computing, but its implementation requires the generation of specific non-Gaussian resource states. While continuous-variable encodings such as the highly complex (GKP) states have been widely studied, the much simpler binomial codes offer an experimentally accessible alternative, though they demand a distinct set of operational tools. Here, we present a toolkit for MBQC using optical binomial codes, detailing a cavity-QED protocol for conditional generation of cluster states and the implementation of Pauli measurements. Our work proposes the first steps for existing optical atom-cavity architectures to lay the groundwork for their use in quantum computation.

quant-ph↗

arXiv2026

Sub-Planck structure quantification in non-Gaussian probability densities

Sub-Planck structures in non-Gaussian probability densities of phase space variables are pervasive in bosonic quantum systems. They are almost universally present if the bosonic system evolves via nonlinear dynamics or nonlinear measurements. So far, identification and comparison of such structures remains qualitative. Here we provide a universally applicable and experimentally friendly method to identify, quantify and compare sub-Planck structures from directly measurable or estimated probability densities of single phase space variables. We demonstrate the efficacy of this method on experimental high order Fock states of a single-atom mechanical oscillator, showing provably finer sub-Planck structures as the Fock occupation increases despite the accompanying uncertainty increase in the phonon, position, and momentum bases.

quant-ph↗

arXiv2025

Optimal Phase-Insensitive Force Sensing with Non-Gaussian States

Quantum metrology enables sensitivity to approach the limits set by fundamental physical laws. Even a single continuous mode offers enhanced precision, with the improvement scaling with its occupation number. Due to their high information capacity, continuous modes allow for the engineering of quantum non-Gaussian states, which not only improve metrological performance but can also be tailored to specific experimental platforms and conditions. Recent advancements in control over continuous platforms operating in the quantum regime have renewed interest in sensing weak forces, also coupling to massive macroscopic objects. In this work, we investigate a force-sensing scheme where a physical process completely randomizes the direction of the induced phase-space displacement, and the unknown force strength is inferred through excitation-number-resolving measurements. We find that $N$-spaced states, where only every $N^{\text{th}}$ Fock state occupation is nonzero, approach the achievable sensing bound. Additionally, non-Gaussian states are shown to be more resilient against decoherence than their Gaussian counterparts with the same occupation number. While Fock states typically offer the best protection against decoherence, we uncover a transition in the metrological landscape -- revealed through a tailored decoherence-aware Fisher-information-based reward functional -- where experimental constraints favor a family of number-squeezed Schrödinger cat states. Specifically, by implementing quantum optimal control in a minimal spin-boson system, we identify these states as maximizing force sensitivity under lossy dynamics and finite system controllability. Our results provide a pathway for enhancing force sensing in a variety of continuous quantum systems, ranging from massive systems like mechanical oscillators to massless systems such as quantum light and microwave resonators.

quant-ph↗

arXiv2025

Nullifiers of non-Gaussian cluster states through homodyne measurement

In continuous variable optical platforms, large-scale Gaussian cluster states have already been demonstrated, but non-Gaussian resources are essential to achieve universality and fault tolerance in measurement-based quantum computation. However, characterizing and certifying non-Gaussian cluster states remains an outstanding challenge. Here, we introduce a general framework for the characterization of non-Gaussian cluster states based on non-Gaussian nullifiers, extending the widely used Gaussian nullifier concept. We show that these nullifiers can be directly evaluated from homodyne measurement data, making them experimentally accessible. As an illustration, we derive and experimentally demonstrate non-Gaussian nullifiers for photon-subtracted squeezed states. Our results provide a practical and operational tool for certifying quantum non-Gaussianity in large-scale optical cluster states.

quant-ph↗

arXiv2025

Nonlinear stochastic and quantum motion from Coulomb forces

Controllable nonlinear quantum interactions are a much sought after target for modern quantum technologies. They are typically difficult and costly to engineer for bespoke purposes. However controllable nonlinearities may have always been in reach via the natural and fundamental forces between quantum particles. The Coulomb interaction between charged particles is the simplest example. We show that after eliminating the harmonic part of the Coulomb force by an auxiliary linear force, the remaining reciprocal nonlinear part results in a directly observable non-reciprocal nonlinear effect: increase of the signal-to-noise ratio (SNR) of the coherent displacement of one particle, driven by the position noise, or uncertainty in quantum regime, in another particle. This essential evidence of nonlinear forces is present across large ranges of trap frequency and mass scales, as well as visible in both stochastic and quantum regimes.

quant-ph↗

arXiv2025

Coherence of quantum non-Gaussian states via nonlinear absorption of quanta

The linear and phase insensitive absorption of a single quanta via coherent interactions with a saturable system, even a single ground state qubit, is sufficient to deterministically generate quantum non-Gaussian states in an oscillator, even stimulated merely by increasing thermal oscillator energy. However, the resultant states only approach Fock states and therefore do not exhibit quantum coherence. Here we overcome this limitation using a minimal step: a nonlinear phase-insensitive absorption process added to the linear one. The coherent addition of such individually passive processes allows coherence to emerge and increase in phase space without an external drive and with minimal interaction requirements. The coherence of quantum non-Gaussian states emerges because the linear and nonlinear absorption processes are not mutually passive. In the simplest case rotationally symmetric Wigner functions of the oscillator Fock states convert their many negative regions to an extremely complex asymmetric structure in sharp contrast to the rotational symmetry of those obtained by the individual interactions. We extend this case to include an unsaturable absorber (oscillator) and analyse switching between linear and nonlinear absorptions, suitable for broad classes of experiments.

quant-ph↗

arXiv2025

Local quantum coherence with intersource interactions at nonzero temperature

Local quantum coherence in a two-level system (TLS) is typically generated via time-dependent driving. However, it can also emerge autonomously from symmetry-breaking interactions between the TLS and its surrounding environment at a low temperature. Although such environments often consist of interacting atoms or spins, the role of interactions within the environment in generating the autonomous local coherence has remained unexplored. Here, we address this gap by analyzing an exactly solvable model, which comprises a target TLS coupled to $N$ interacting source TLSs that represent the environment, with the whole system being in thermal equilibrium. We show that the local coherence not only persists but can be enhanced at finite temperatures of the environment compared to the case of no inter-source interactions. The temperature dependence of the coherence bears signatures of a quantum phase transition, and our analytical results suggest strategies for its optimization. Our findings reveal generic properties of the autonomously generated quantum coherence and point to viable routes for observing the coherence at nonzero temperatures.

quant-ph↗

arXiv2025

Witnessing quantum non-Gaussianity from intensity moments

Direct measurement of quantum non-Gaussianity requires some variant of a discrete photon-resolving detection, which is feasible only for low mean photon numbers. For a large mean photon number, intensity detection by linear photodiodes provides a continuous signal; therefore, the Fock probabilities of the unknown input state are not directly available. On the other hand, intensity moments can be measured directly, and photon-number moments can be estimated. Therefore, we derive and analyze a quantum non-Gaussianity witness based solely on the photon number mean and variance (or alternatively, the second-order correlation $g^{(2)}$) of an unknown state. Due to the simplicity of the used photon-number moments, the measurement results are easy to correct for losses and additive noise. We provide examples of simple amplification-based measurement schemes where our witness can be applied directly, thereby opening pathways to proof-of-principle tests and applications.

quant-ph↗

arXiv2025

Nonlinear Phase Gates as Airy Transforms of the Wigner Function

Low-order nonlinear phase gates allow the construction of versatile higher-order nonlinearities for bosonic systems and grant access to continuous variable quantum simulations of many unexplored aspects of nonlinear quantum dynamics. The resulting nonlinear transformations produce, even with small strength, multiple regions of negativity in the Wigner function and thus show an immediate departure from classical phase space. Towards the development of realistic, bounded versions of these gates we show that the action of a quartic-bounded cubic gate on an arbitrary multimode quantum state in phase space can be understood as an Airy transform of the Wigner function. This toolbox generalises the symplectic transformations associated with Gaussian operations and allows for the practical calculation, analysis and interpretation of explicit Wigner functions and the quantum non-Gaussian phenomena resulting from bounded nonlinear potentials.

quant-ph↗

arXiv2025

Certifying non-classicality and non-Gaussianity through optical parametric amplification

Non-Gaussian states of light are essential for numerous quantum information protocols; thus, certifying non-Gaussianity is crucial. Full quantum state tomography, commonly used for this purpose, is a complicated procedure and yields inconclusive results for strongly mixed states. Certifying non-Gaussianity through directly measurable parameters is a simpler alternative, typically achieved by measuring photon-number probabilities - either directly, using photon-number resolving detectors, or through Hanbury Brown--Twiss type measurements with single-photon detectors. Here, we demonstrate theoretically and experimentally that optical parametric amplification combined with conventional intensity detectors can effectively replace this approach without the need for photon-number resolution. In our method, we measure the mean photon number and the second-order correlation function for the amplified state. Using it, we successfully certify the non-Gaussianity of a heralded quasi-single-photon state. Since optical parametric amplification is a broadband and multimode process, our method provides a foundation for developing high-dimensional quantum technologies utilizing broadband multimode non-Gaussian states.

quant-ph↗

arXiv2025

Optical states with higher stellar rank

Quantum non-Gaussian states of traveling light fields are crucial components of quantum information processing protocols; however, their preparation is experimentally challenging. In this paper, we discuss the minimal requirements imposed on the quantum efficiency of photon number resolving detectors and the quality of the squeezing operation in an experimental realization of certifiable quantum non-Gaussian states of individual photonic states with three, four, and five photons.

quant-ph↗

arXiv2025

Fermat's Spiral-Based Characterization of Squeezed Nonlinear Motional States of Levitated Nanoparticle

Controlling the state of motion of optically levitated nanoparticles is crucial for the advancement of precision sensing, fundamental tests of physics, and the development of hybrid classical-quantum technologies. Experimentally, such control can be achieved by pulsed modifications of the optical potential confining the nanoparticle. Most frequently, the applied potential pulses are parabolic in nanoparticle position, and they expand/squeeze or displace the initial Gaussian state of motion to a modified Gaussian state. The time-dependent mean values and covariance matrix of the phase-space variables can fully characterize such a state. However, quasi-parabolic optical potentials with added weak Duffing-type nonlinearity, encountered in real-world experiments, can generally distort the state of motion to a non-Gaussian one, for which the description based solely on the mean values and covariance matrix fails. Here, we introduce a nonlinear transformation of the phase-space coordinates using the concept of Fermat's spiral, which effectively removes the state distortion induced by the Duffing-type nonlinearity and enables characterization of the state of motion by the standard Gaussian-state metrics. Comparisons of the experimental data with theoretical models show that the proposed coordinate transformation can recover the ideal behavior of a harmonic oscillator even after extended evolution of the system in the nonlinear potential. The presented scheme enables the separation of the effects of the applied state manipulation, the system's gradual thermalization, and the nonlinearity of the confinement on the experimentally observed dynamics of the system, thereby facilitating the design of advanced protocols for levitated optomechanics.

physics.optics↗

arXiv2025

Quantum non-Gaussian high Fock states of light pulses and their superpositions

The generation of high Fock states of light pulses and their superpositions with provable quantum non-Gaussian features is still very challenging, although the power of conditional methods to herald the approximate state from the available Gaussian states is growing. The atom-light interaction in the high-Q cavity has been considered a viable alternative to the heralded Fock states of the light pulses from nonlinear optics limited to three-photon Fock states for the last decade. Here, by optimizing the realistic protocol combining it with available optical delay elements we conclusively predict filtering of Fock states up to ten photons with a high success rate of $20\%$ using a hierarchy of quantum non-Gaussian criteria. Moreover, the filtering protocol enables the preparation of superposition of Fock states and we analyse this emerging case up to two photons with with provable quantum non-Gaussian coherence and high success rate $50\%$. To demonstrate their quality for applications, we evaluate the robustness of such features, the bunching capability in a linear network, and the sensing capability to estimate the magnitude of unknown force, noise and phase, These assessments outline the essential conditions and application criteria for realistic optical cavity QED interaction on light pulses to outperform photon detection methods.

quant-ph↗

arXiv2025

Nanomechanical State Amplifier Based on Optical Inverted Pendulum

A contactless control of mean values and fluctuations of position and velocity of a nanoobject belongs among the key methods needed for ultra-precise nanotechnology and the upcoming quantum technology of macroscopic systems. An analysis of experimental implementations of such a control, including assessments of linearity and the effects of added noise, is required. Here, we present a protocol of linear amplification of mean values and fluctuations along an arbitrary phase space variable and squeezing along the complementary one, referred to as a nanomechanical state amplifier. It utilizes the experimental platform of a single optically levitating nanoparticle and the three-step protocol combines a controlled fast switching of the parabolic trapping potential to an inverted parabolic potential and back to the parabolic potential. The protocol can be sequentially repeated or extended to shape the nanomechanical state appropriately. Experimentally, we achieve amplification of position with a gain of $|G| \simeq 2$ and a classical squeezing coefficient above 4 dB in as short a timestep as one period of nanoparticle oscillations ($7.6\,μ$s). Amplification in velocity, with the same parameters, squeezes the input noise and enhances force sensing.

quant-ph↗

arXiv2025

Hierarchical Verification of Non-Gaussian Coherence in Bosonic Quantum States

Non-Gaussianity, a distinctive characteristic of bosonic quantum states, is pivotal in advancing quantum networks, fault-tolerant quantum computing, and high-precision metrology. Verifying the quantum nature of a state, particularly its non-Gaussian features, is essential for ensuring the reliability and performance of these technologies. However, the specific properties required for each application demand tailored validation thresholds. Here, we introduce a hierarchical framework comprising absolute, relative, and qubit-specific thresholds to assess the non-Gaussianity of local coherences. We illustrate this framework using heralded optical non-Gaussian states with the highest purities available in optical platforms. This comprehensive framework presents the first detailed evaluation of number state coherences and can be extended to a wide range of bosonic states.

quant-ph↗