arXiv subjects

Ray-Kuang Lee

Publications and source records attributed to Ray-Kuang Lee.

At least 19 recordsLinked to original sources

arXiv2026

Quantum steering in networks: Measurement-device-independent detection, continuous variables, and practical Gaussian schemes

We consider quantum steering certification in multipartite networks, with a focus on minimal trust scenarios: all-except-one parties are untrusted and treated device-independently. We show that it is always possible to lift steering certification to the measurement-device-independent regime, in which even the (last) trusted party can treat their local hardware as a black-box, except for a set of fiduciary quantum states used as the inputs to the experiment. This holds both for finite-dimensional systems as well as for bosonic continuous-variable systems, for which we provide a full characterization in the bipartite case. Additionally, we introduce measurement-device-independent network steering protocols based entirely on Gaussian operations -- which cannot be used for fully device-independent protocols, and thus become instead a viable option for minimal trust certification as soon as a single trusted input is inserted in the network. Our results present a basis for steering-based applications (such as randomness generation) with minimal trust beyond full nonlocality and with feasible experimental requirements.

quant-ph↗

arXiv2026

Monitoring Beam Splitter Entanglement using Quantumness

We report on an experiment in which two independent squeezed vacuum states get entangled by mixing them with a balanced beam splitter. We follow standard practice and use an inseparability criterion to quantify their entanglement. However, this only allows us to witness the entanglement, but not to determine the deleterious effects of experimental imperfections due to the beam splitter mixing and the associated mode-mismatch and detection imperfections. We therefore introduce an alternative framework suitable for continuous variable systems using the states' quantumness, $\Xi$. We show that, under ideal circumstances, $\Xi$ is a conserved quantity under beam mixing. This allows us to benchmark the experiment's performance by comparing the states' quantumness $\Xi$ after the beam splitter mixing with $\Xi$ before. Such a comparison is not possible with entanglement witnesses, as the input states are unentangled. This highlights the main strength of our approach: its ability to generally quantify the quantumness of multi-mode continuous variable states and use this to probe different stages in an experiment.

quant-ph↗

arXiv2026

Wigner's Phase Space Current for Variable Beam Splitters -- Phase Space Rotations and Newtonian Trajectories

Beam splitters allow us to superpose two continuous single mode quantum systems. To study the behaviour of beam splitters' strongly mode mixing dynamics we consider variable beam splitters acting on Wigner's phase space distribution, W , the evolution of which is governed by the continuity-equation {\partial \tau} W = - {\nabla} J. We derive the form of the corresponding Wigner current, J. J's form allows us to use a classical trajectories-approach to analyze the influence of the two modes on each other. We show that the dynamics for variable beam splitters amounts to a rotation confined within the plane of the two positions together with the same simultaneous rotation confined within the plane of the two momenta. In this way explicit and very transparent expressions for the rotated Wigner distributions and Wigner currents can be given in terms of classical trajectories. This helps us to gain deeper insights and perform geometrical analyses of the mixing of modes at beam splitters.

quant-ph↗

arXiv2026

Demonstration of length control for a filter cavity with coherent control sidebands

For broadband quantum noise reduction of gravitational-wave detectors, a frequency-dependent squeezed vacuum field realized using a filter cavity is the most promising technique and will be implemented in Advanced LIGO and Advanced Virgo in the fourth observing run. To obtain the benefit of frequency-dependent squeezing, the length and alignment of the filter cavity with respect to the squeezed vacuum field must be accurately controlled. To this purpose, a new length and alignment control scheme for a filter cavity, using coherent control sidebands, was suggested [Phys. Rev. D 102, 042003 (2020)]. The coherent control sidebands are already used to control the squeezing angle in squeezed vacuum sources for gravitational-wave detectors. As both the squeezed vacuum field and coherent control sidebands have the same mode-matching conditions and almost the same frequency, the length and alignment of the filter cavity with respect to the squeezed vacuum field can be accurately controlled with this scheme. In this paper, we experimentally demonstrate the new control scheme for a filter cavity with coherent control sidebands. In addition to the conventional filter cavity control with the green field, we succeed in controlling the length of a 300-m filter cavity with coherent control sidebands and reduce the filter cavity length noise (rms) from 6.8 to 2.1 pm.

physics.ins-det↗

arXiv2025

A Sensitive Nonclassicality Certification Functional for Continuous-Variable Systems

If the phase space-based Glauber-Sudarshan distribution, $P_{\varrho}$, has negative values the quantum state, $\varrho$, it describes is nonclassical. Due to $P$'s singular behaviour this simple criterion is impractical to use. Recent work [Bohmann and Agudelo, Phys. Rev. Lett. 124, 133601 (2020)] presented a general, sensitive, and noise-tolerant certification functional, $\xi[P]$, for the detection of non\-classical behaviour of quantum states $P_{\varrho}$. There, it was shown that when this functional takes on negative values somewhere in phase space, $\xi[P](x,p) < 0$, this is \emph{sufficient} to certify the nonclassicality of a state. Here we give examples where this certification fails. We investigate states which are known to be nonclassical but the certification function is non-negative, $\xi(x,p) \geq 0$, everywhere in phase space. We generalize $\xi$, giving it an appealing form, ${\cal S}$, which allows for slight improvements in certification, but ${\cal S}$ also fails for mixed very weakly nonclassical states. More important than a slight improvement in sensitivity of ${\cal S}$ over $\xi$ is that we showed how very sensitive $\xi$ and ${\cal S}$ are, and more important still is our simple derivation of ${\cal S}$ allowing us to generalize $\xi$ and ${\cal S}$ to multiple modes.

quant-ph↗

arXiv2025

Triple measurements uncertainty and the distinguishment between the separable and entangled states

Uncertainty and entanglement are both profound and key concepts in quantum theory. For three observables, the tightest uncertainty constants for both product and summation forms are revealed. In this work, we give an alternative proof for three observables, also with a physical interpretation of the uncertainty constants. Our results show that such constants are intimately connected with the distinguishment between separable and entangled states.

quant-ph↗

arXiv2025

An edge-based and subspace reduction encoding scheme to solve the traveling salesman problem in quantum computers

This paper introduces a novel edge-based encoding technique for solving the Traveling Salesman Problem (TSP) on a quantum computer, reducing the required number of qubits. For implementation in real quantum devices, we applied the subspace reduction encoding to further reduce the dimension of the TSP solution space. We attack the TSP for 4-, 5-, and 6-city instances in both simulators and real quantum computers across different encoding frameworks. Optimal solutions of the 4-city TSP instance are obtained on state-of-the art IQM quantum computer. Our study presents a comparative analysis between edge-based encoding scheme and the node-based encoding methodology in the literature. Our findings indicate that the proposed encoding scheme outperforms conventional methods in terms of statistical measures, quantum resource utilization, and computational efficiency when applied to smaller TSP instances.

quant-ph↗

arXiv2025

Fast and length-independent transport time supported by topological edge states in finite-size Su-Schrieffer-Heeger chains

In order to transport information with topological protection, we explore experimentally the fast transport time using edge states in one-dimensional Su-Schrieffer-Heeger (SSH) chains. The transport time is investigated in both one- and two-dimensional models with topological non-trivial band structures. The fast transport is inherited with the wavefunction localization, giving a stronger effective coupling strength between the mode and the measurement leads. Also the transport time in one-dimension is independent of the system size. To verify the asertion, we implement a chain of split-ring resonators and their complementary ones with controllable hopping strengths. By performing the measurements on the group delay of non-trivially topological edge states with pulse excitations, the transport time between two edge states is directly observed with the chain length up to $20$. Along the route to harness topology to protect optical information, our experimental demonstrations provide a crucial guideline for utilizing photonic topological devices.

physics.optics↗

arXiv2025

The adiabatic theorem for non-Hermitian quantum systems with real eigenvalues and the complex geometric phase

The adiabatic theorem is one of the most interesting and significant theorems in quantum mechanics. However, the adiabatic theorem can fail for general non-Hermitian quantum systems. In this paper, by utilizing the complex geometric phase, the functional calculus for biorthogonal systems and the Gr\"{o}nwall inequality, we prove rigorously that the adiabatic theorem is still valid for diagonalizable non-Hermitian systems with real eigenvalues. The proof also justifies the definition of a complex Berry phase for non-Hermitian systems, in both Abelian and non-Abelian cases.

quant-ph↗

arXiv2025

Machine Learning for Quantum State Tomography: Robust Covariance Matrix Estimation for Squeezed Vacuum States with Thermal Noise

We present a supervised machine learning-based method using convolutional neural networks to estimate the covariance matrix of Gaussian quantum states in the presence of thermal noise. Unlike computationally intensive density matrix reconstructions, our machine learning-based method allows for the reconstruction of impure squeezed vacuum states using sparse measurements of quadrature sequences based on a model employing a two-component state mixed together from thermal and squeezed thermal states. The method achieves high fidelity and precision, notably also at high squeezing levels, while offering an effective characterization of physical quantities and accurately estimating the covariance matrix. We benchmark our machine against experimental data of single-mode squeezed vacuum states, demonstrating its accuracy and capability to quantify experimental degradation to squeezing and purity. We experimentally verify that our covariance matrix estimation exhibits robustness to state degradation induced by thermal state admixtures. We provide a method for lightweight, compact, and complete representation of lab-generated Gaussian states and lay the foundation for extending real-time quantum state tomography for thermal multi-component Gaussian states to multi-mode systems.

quant-ph↗

arXiv2025

Circuit Knitting for Continuous-Variable Quantum States

In finite-dimensional systems, circuit knitting can be used to simulate non-classical quantum operations using a limited set of resources. In this work, we extend circuit knitting techniques to infinite-dimensional quantum systems. We develop a general theoretical framework for simulating non-Gaussian states from the given set of available states. Also, we establish fundamental constraints with the no-go theorem on the circuit knitting of multi-mode Gaussian operations, by showing that the exact knitting with separable operations requires infinite sampling overhead. We further explore several applications of our theory, including simulation of approximate Fock states, GKP state generation, and cat-state amplification.

quant-ph↗

arXiv2025

Quantum sensor network metrology with bright solitons

We consider multiparameter quantum metrology problem with bright soliton networks in the presence of weak losses. We introduce General Heisenberg Limit (GHL) $\sigma_{\boldsymbol{\chi}}=1/N^k$ that characterizes fundamental limitations for unknown parameter measurement and estimation accuracy $\sigma_{\boldsymbol{\chi}}$ within linear ($k=1$) and nonlinear ($k=3$) quantum metrology approaches to solitons. We examine multipartite $N00N$ states specially prepared for the improvement of multiparameter estimation protocols. As a particular example of producing such states, we propose the three-mode soliton Josephson junction (TMSJJ) system as a three mode extension for the soliton Josephson junction (SJJ) bosonic model, which we previously proposed. The energy spectrum of the TMSJJ exhibits sharp phase transition peculiarities for the TMSJJ ground state. The transition occurs from a Gaussian-like (coherent) state to the superposition of entangled Fock states, which rapidly approach the three-mode $N00N$ state. We show that in the presence of weak losses, the TMSJJ enables saturate scaling relevant to the optimal state limit close to the GHL. Our findings open new prospects for quantum network sensorics with atomtronic circuits.

quant-ph↗

arXiv2025

Generalized cross-resonance scheme for maximally-entangling two-qutrit gates

To utilize higher-dimensional quantum systems, in this Letter, we derive a generalized cross-resonance (GCR) scheme for realizing maximally entangling two-qutrit gates on fixed-frequency transmons beyond the 0-1 subspace. Our two-qutrit gates, namely, $U_{CR}^{01}$ and $U_{CR}^{12}$, acting on the $0{\text -}1$ and $1{\text -}2$ energy transitions of transmons, respectively, directly allow for entanglement on the $1{\text -}2$ levels. Unlike the known works, our gate is parametric in nature, enabling us to construct multiple entangling gates of interest. By performing simulations in Qiskit, we demonstrate two-qutrit generalized controlled-$X$ ($U_{CX}^{01}$ and $U_{CX}^{12}$) and controlled-$H$ ($U_{CH}^{01}$ and $U_{CH}^{12}$) gates, which are instances of the proposed $U_{CR}$ gates, with reported gate fidelities of $86.14\%~(99.73\%),~84.6\%~(97.88\%),~92.35\%~(99.39\%)$, and $91.99\%~(98.99\%)$, respectively with (and without) noise. We also reveal a two-qutrit Bell state with a fidelity of $99.06 \pm 0.01\%$, with a complete Bell state preparation in a $\sim514$ ns pulse sequence, which is less than the gate time of the known scheme by cross-Kerr-based entangling gates.

quant-ph↗

arXiv2025

Machine Learning Enhanced Quantum State Tomography on FPGA

Machine learning techniques have opened new avenues for real-time quantum state tomography (QST). In this work, we demonstrate the deployment of machine learning-based QST onto edge devices, specifically utilizing field programmable gate arrays (FPGAs). This implementation is realized using the {\it Vitis AI Integrated Development Environment} provided by AMD\textsuperscript \textregistered~Inc. Compared to the Graphics Processing Unit (GPU)-based machine learning QST, our FPGA-based one reduces the average inference time by an order of magnitude, from 38 ms to 2.94 ms, but only sacrifices the average fidelity about $1\% $ reduction (from 0.99 to 0.98). The FPGA-based QST offers a highly efficient and precise tool for diagnosing quantum states, marking a significant advancement in the practical applications for quantum information processing and quantum sensing.

quant-ph↗

arXiv2024

Adding or Subtracting a single Photon is the same for Pure Squeezed Vacuum States

The addition of a single photon to a light field can lead to exactly the same \emph{outcome} as the subtraction of a single photon. We prove that this \cterm is true for pure squeezed vacuum states of light, and in some sense only for those. We show that mixed states can show this \cterm for addition or subtraction of a photon if they are generated from incoherent sums of pure squeezed vacuum states with the same squeezing. We point out that our results give a reinterpretation to the fact that pure squeezed vacuum states, with squeezing $e^{-z}$, are formally annihilated by Bogoliubov-transformed annihilation operators: $\hat a_z = \hat a \cosh(z) - \hat a^\dagger \sinh(z) $.

quant-ph↗

arXiv2024

Neural Network Enhanced Single-Photon Fock State Tomography

Even though heralded single-photon sources have been generated routinely through the spontaneous parametric down conversion, vacuum and multiple photon states are unavoidably involved. With machine-learning, we report the experimental implementation of single-photon quantum state tomography by directly estimating target parameters. Compared to the Hanbury Brown and Twiss (HBT) measurements only with clicked events recorded, our neural network enhanced quantum state tomography characterizes the photon number distribution for all possible photon number states from the balanced homodyne detectors. By using the histogram-based architecture, a direct parameter estimation on the negativity in Wigner's quasi-probability phase space is demonstrated. Such a fast, robust, and precise quantum state tomography provides us a crucial diagnostic toolbox for the applications with single-photon Fock states and other non-Gaussisan quantum states.

quant-ph↗

arXiv2023

A Sensitive Quantumness Measure for One-Dimensional Continuous-Variable Systems

For one-dimensional continuous-variable quantum systems such as single-mode quantum optical systems, we give a quantification of the quantumness of such a system's state, {\rho}, by introducing the measure of quantumness, {\Xi}, which works for all states, pure or mixed. {\Xi} is a measure which is universal, sensitive, monotonic, and unbounded. {\Xi}[{\rho}] yields a single positive value to quantify how nonclassical {\rho} is. {\Xi} employs phase space distributions to represent {\rho} and is a fixed function, {\Xi}[.], independent of the system, its environment or the type of state.

quant-ph↗

arXiv2023

Wigner's Phase Space Current for Variable Beam Splitters -Seeing Beam Splitters in a New Light-

Beam splitters allow us to superpose two continuous single mode quantum systems. To study the behaviour of their strongly mode mixing dynamics we consider variable beam splitters and their dynamics using Wigner's phase space distribution, W, the evolution of which is governed by the continuity-equation $ \frac{\partial}{\partial \tau} W = - {\nabla } \cdot {J}$. We derive the form of the corresponding Wigner current, J, of each outgoing mode after tracing out the other. The influence of the modes on each other is analyzed and visualized using their respective Wigner distributions and Wigner currents. This allows us to perform geometrical analyses of the mode interactions, casting new light on beam splitter behaviour. Several of the presented results should be immediately testable in experiments.

quant-ph↗