arXiv subjects

Donghwa Lee

Publications and source records attributed to Donghwa Lee.

14 recordsLinked to original sources

arXiv2026

Multipartite reference-frame-independent quantum cryptographic communication

Reference frame mismatch among communication parties introduces errors in quantum cryptographic protocols. As the number of participants increases, aligning reference frames becomes increasingly difficult, complicating multipartite quantum cryptographic implementations. Here, we theoretically and experimentally investigate multipartite reference-frame-independent (RFI) quantum cryptographic communication using Greenberger-Horne-Zeilinger (GHZ) states. We generalize the bipartite RFI security parameter $C$ to an $N$-party parameter $C_N$ and derive the asymptotic secret key rate expressed solely in terms of experimentally accessible quantities. We analyze the key rate under global and local depolarizing noise models and find that increasing the number of parties $N$ enhances robustness against global depolarizing noise while increasing vulnerability to local channel noise. We also present a proof-of-principle experimental demonstration of four-party RFI quantum cryptographic communication using four-photon GHZ states, confirming the reference-frame invariance of both the $C_4$ parameter and the secret key rate under various reference frame rotations.

quant-ph↗

arXiv2026

Phase-Sensitive Crystal-Edge Effects in Linear Optical Parametric Oscillators: Why Nominally Identical Squeezers Behave Differently

Efficient and reproducible squeezed-light sources are essential for quantum information processing and precision metrology. Compact linear standing-wave optical parametric oscillators (OPOs) are attractive because they combine low optical loss, low pump-power requirements, and large longitudinal mode spacing. In doubly resonant cavities, however, the nonlinear interaction is not determined solely by bulk phase matching: forward- and backward-generated fields recombine coherently, making the effective gain sensitive to crystal-edge termination, wavelength-dependent coating phases, and the cavity resonance condition. Here, we show that these microscopic phase contributions can produce large threshold variations between nominally similar OPOs. We combine double-pass second-harmonic generation with OPO threshold measurements to extract the relevant crystal-cavity phases and analyse three linear OPO systems. The observed devices exhibit threshold variations of up to nearly six-fold, traced to the phase-dependent nonlinear-gain envelope at accessible doubly resonant operating points. Our results establish a phase-aware framework for compact linear OPOs and provide design guidelines for reproducible low-threshold squeezed-light sources in scalable photonic quantum systems.

quant-ph↗

arXiv2024

Entanglement swapping via lossy channels using photon-number-encoded states

Entanglement shared between distant parties is a key resource in quantum networks. However, photon losses in quantum channels significantly reduce the success probability of entanglement sharing, which scales quadratically with the channel transmission. Quantum repeaters using entanglement swapping can mitigate this effect, but usually require high-performance photonic quantum memories to synchronize photonic qubits. In this work, we theoretically and experimentally investigate an entanglement swapping protocol using photon-number-encoded states that can effectively alleviate quantum channel losses without requiring photonic quantum memories. We demonstrate that the protocol exhibits a success probability scaling linearly with the channel transmission. Furthermore, we show that while unbalanced channel losses can degrade the shared entanglement, this effect can be compensated by optimally adjusting the initial entangled states. Our results highlight the potential of photon-number encoding for realizing robust entanglement distribution in lossy quantum networks.

quant-ph↗

arXiv2024

Photonic variational quantum eigensolver using entanglement measurements

Variational quantum eigensolver (VQE), which combines quantum systems with classical computational power, has been arisen as a promising candidate for near-term quantum computing applications. However, the experimental resources such as the number of measurements to implement VQE rapidly increases as the Hamiltonian problem size grows. Applying entanglement measurements to reduce the number of measurement setups has been proposed to address this issue, however, entanglement measurements themselves can introduce additional resource demands. Here, we apply entanglement measurements to the photonic VQE utilizing polarization and path degrees of freedom of a single-photon. In our photonic VQE, entanglement measurements can be deterministically implemented using linear optics, so it takes full advantage of introducing entanglement measurements without additional experimental demands. Moreover, we show that such a setup can mitigate errors in measurement apparatus for a certain Hamiltonian.

quant-ph↗

arXiv2024

Quantum enhanced distributed phase sensing with a truncated SU(1,1) interferometer

In recent years, distributed quantum sensing has gained interest for a range of applications requiring networks of sensors, from global-scale clock synchronization to high energy physics. In particular, a network of entangled sensors can improve not only the sensitivity beyond the shot noise limit, but also enable a Heisenberg scaling with the number of sensors. Here, using bright entangled twin beams, we theoretically and experimentally demonstrate the detection of a linear combination of two distributed phases beyond the shot noise limit with a truncated SU(1,1) interferometer. We experimentally demonstrate a quantum noise reduction of 1.7 dB and a classical 3 dB signal-to-noise ratio improvement over the separable sensing approach involving two truncated SU(1,1) interferometers. Additionally, we theoretically extend the use of a truncated SU(1,1) interferometer to a multi-phase-distributed sensing scheme that leverages entanglement as a resource to achieve a quantum improvement in the scaling with the number of sensors in the network. Our results pave the way for developing quantum enhanced sensor networks that can achieve an entanglement-enhanced sensitivity.

quant-ph↗

arXiv2022

Aluminum nitride waveguide beam splitters for integrated quantum photonic circuits

We demonstrate integrated photonic circuits for quantum devices using sputtered polycrystalline aluminum nitride (AlN) on insulator. The on-chip AlN waveguide directional couplers, which are one of the most important components in quantum photonics, are fabricated and show the output power splitting ratios from 50:50 to 99:1. The polarization beam splitters with an extinction ratio of more than 10 dB are also realized from the AlN directional couplers. Using the fabricated AlN waveguide beam splitters, we observe the Hong-Ou-Mandel interference with a visibility of 91.7 +(-) 5.66 %.

physics.optics↗

arXiv2022

Entangling three identical particles via spatial overlap

Quantum correlations between identical particles are at the heart of quantum technologies. Several studies with two identical particles have shown that the spatial overlap and indistinguishability between the particles are necessary for generating bipartite entanglement. On the other hand, researches on the extension to more than two-particle systems are limited by the practical difficulty to control multiple identical particles in laboratories. In this work, we propose schemes to generate two fundamental classes of genuine tripartite entanglement, i.e., GHZ and W classes, which are experimentally demonstrated with three identical photons. We also show that the tripartite entanglement class decays from the genuine entanglement to the full separability as the particles become more distinguishable from each other. Our results support the prediction that particle indistinguishability is a fundamental element for entangling identical particles.

quant-ph↗

arXiv2021

Error-mitigated photonic variational quantum eigensolver using a single-photon ququart

We report the experimental resource-efficient implementation of the variational quantum eigensolver (VQE) using four-dimensional photonic quantum states of single-photons. The four-dimensional quantum states are implemented by utilizing polarization and path degrees of freedom of a single-photon. Our photonic VQE is equipped with the quantum error mitigation (QEM) protocol that efficiently reduces the effects of Pauli noise in the quantum processing unit. We apply our photonic VQE to estimate the ground state energy of He--H$^{+}$ cation. The simulation and experimental results demonstrate that our resource-efficient photonic VQE can accurately estimate the bond dissociation curve, even in the presence of large noise in the quantum processing unit.

quant-ph↗

arXiv2021

Implementation of a 3 x 3 directionally-unbiased linear optical multiport

Linear optical multiports are widely used in photonic quantum information processing. Naturally, these devices are directionally-biased since photons always propagate from the input ports toward the output ports. Recently, the concept of directionally-unbiased linear optical multiports was proposed. These directionally-unbiased multiports allow photons to propagate along a reverse direction, which can greatly reduce the number of required linear optical elements for complicated linear optical quantum networks. Here, we report an experimental demonstration of a 3 x 3 directionally-unbiased linear optical fiber multiport using an optical tritter and mirrors. Compared to the previous demonstration using bulk optical elements which works only with light sources with a long coherence length, our experimental directionally-unbiased 3 x 3 optical multiport does not require a long coherence length since it provides negligible optical path length differences among all possible optical trajectories. It can be a useful building block for implementing large-scale quantum walks on complex graph networks.

quant-ph↗

arXiv2021

Cloud-Based Content Cooperation System to Assist Collaborative Learning Environment

Online educational systems running on smart devices have the advantage of allowing users to learn online regardless of the location of the users. In particular, data synchronization enables users to cooperate on contents in real time anywhere by sharing their files via cloud storage. However, users cannot collaborate by simultaneously modifying files that are shared with each other. In this paper, we propose a content collaboration method and a history management technique that are based on distributed system structure and can synchronize data shared in the cloud for multiple users and multiple devices.

cs.DC↗

arXiv2020

Characterizing Bell state analyzer using weak coherent pulses

Bell state analyzer (BSA) is one of the most crucial apparatus in photonic quantum information processing. While linear optics provide a practical way to implement BSA, it provides unavoidable errors when inputs are not ideal single-photon states. Here, we propose a simple method to deduce the BSA for single-photon inputs using weak coherent pulses. By applying the method to Reference-Frame-Independent Measurement-Device-Independent Quantum Key Distribution, we experimentally verify the feasibility and effectiveness of the method.

quant-ph↗

arXiv2020

Reference-Frame-Independent, Measurement-Device-Independent quantum key distribution using fewer quantum states

Reference-Frame-Independent Quantum Key Distribution (RFI-QKD) provides a practical way to generate secret keys between two remote parties without sharing common reference frames. On the other hand, Measurement-Device-Independent QKD (MDI-QKD) offers high level of security as it immunes against all the quantum hacking attempts to the measurement devices. The combination of these two QKD protocols, i.e., RFI-MDI-QKD, is one of the most fascinating QKD protocols since it holds both advantages of practicality and security. For further practicality of RFI-MDI-QKD, it is beneficial to reduce the implementation complexity. Here, we have shown that RFI-MDI-QKD can be implemented using fewer quantum states than those of its original proposal. We found that, in principle, the number of quantum states for one of the parties can be reduced from six to three without compromising security. Comparing to the conventional RFI-MDI-QKD where both parties should transmit six quantum states, it significantly simplifies the implementation of the QKD protocol. We also verify the feasibility of the scheme with the proof-of-principle experiment.

quant-ph↗

arXiv2014

Magnetic fluctuation from oxygen deficiency centers on the SiO$_2$ surface

The magnetic stability of oxygen deficiency centers on the surface of α-quartz is investigated with first- principles calculations to understand their role in contributing to magnetic flux noise in superconducting qubits (SQs) and superconducting quantum interference devices (SQUIDs) fabricated on amorphous silica substrates. Magnetic defects on the substrate are likely responsible for some of the 1/f noise that plagues these systems. Dangling-bonds associated with three-coordinated Si atoms allow electron density transfer between spin up and down channels, resulting in low energy magnetic states. Such under-coordinated Si defects are common in both stoichiometric and oxygen deficient silica and quartz and are a probable source of magnetic flux fluctuations in SQs and SQUIDs.

cond-mat.mtrl-sci↗

arXiv2013

Investigation of electron beam induced migration of hydrogen in Mg-doped GaN using Eu as a probe

We demonstrate the use of hydrogen induced changes in the emission of isoelectric Eu ions, in Mg-doped p-type GaN, as a powerful probe to study the dynamics of hydrogen movement under electron beam irradiation. We identify, experimentally, a two-step process in the dissociation of Mg-H complexes and propose, based on density functional theory, that the presence of minority carriers and resulting charge states of the hydrogen drives this process.

cond-mat.mtrl-sci↗