arXiv subjects

Kim Fook Lee

Publications and source records attributed to Kim Fook Lee.

14 recordsLinked to original sources

arXiv2026

Classical State Detection Using Quantum State Tomography

We present a model to detect a classical state mixed with an idler photon from a polarization-entangled pair. A weak coherent light with a well-defined polarization, matched in wavelength to the idler photon, is injected into the idler channel. Quantum state tomography is then performed on both the classically mixed idler photon and its entangled signal partner. The reconstructed state is modeled as a combination of an $X-$quantum state and a classical-quantum (CQ) state. In this framework, the weak coherent light acts as a measurement apparatus performing a local polarization measurement on the idler channel, thereby inducing a classical state. The density matrix of the classical state is identified via algorithmic analysis of the diagonal and off-diagonal elements of the reconstructed density matrix. This approach could advance techniques for classical-quantum coexistence in networking applications$\,-\,$such as quantum wrapping$\,-\,$as well as future quantum key distribution protocols based on the coexistence of weak coherent states and entangled photon states.

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arXiv2025

Non-Markovian Dynamics in Fiber Delay-line Buffers

We study the non-Markovian effect on a two-photon polarization entangled state, in which one photon from the pair is stored in a fiber delay-line buffer. We propose a model of a photonic qubit coupled to fiber birefringence and a fiber reservoir representing the environment. We analytically derive a non-Markovian probability function for the buffered photon and its paired photon. To verify the probability function, we perform full quantum state tomography of the photon pairs. The probability function fits well with the experimental data and physical values. Our results indicate that our quantum system operates slightly above the threshold for a non-Markovian transition. We observe a unique polarization dynamic of the buffered photon. We further exploit measures of quantum mutual information to study the quantumness of the photon pairs. Werner's well-known separability criterion occurs at a buffer time of about 0.9$\,$ms. Our results imply that quantum discord can surpass Werner's criterion, and hence, quantum bi-partite correlation can exist for buffer times greater than 0.9$\,$ms.

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arXiv2024

Hong-Ou-Mandel Interference with a Coexisting Clock using Transceivers for Synchronization over Deployed Fiber

Interference between independently generated photons is a key step towards distributing entanglement over long distances, but it requires synchronization between the distantly-located photon sources. Synchronizing the clocks of such photon sources using coexisting two-way classical optical communications over the same fiber that transport the quantum photonic signals is a promising approach for achieving photon-photon interference over long distances, enabling entanglement distribution for quantum networking using the deployed fiber infrastructure. Here, we demonstrate photon-photon interference by observing the Hong-Ou-Mandel dip between two distantly-located sources: a weak coherent state source obtained by attenuating the output of a laser and a heralded single-photon source. We achieve a maximum dip visibility of $0.58 \pm 0.04$ when the two sources are connected via $4.3$ km of deployed fiber. Dip visibilities $>0.5$ are nonclassical and a first step towards achieving teleportation over the deployed fiber infrastructure. In our experiment, the classical optical communication is achieved with $-21$ dBm of optical signal launch power, which is used to synchronize the clocks in the two independent, distantly-located photon sources. The impact of spontaneous Raman scattering from the classical optical signals is mitigated by appropriate choice of the quantum and classical channel wavelengths. All equipment used in our experiment (the photon sources and the synchronization setup) is commercially available. Finally, our experiment represents a scalable approach to enabling practical quantum networking with commercial equipment and coexistence with classical communications in optical fiber.

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arXiv2023

Fiber Loop Quantum Buffer for Photonic Qubits

We report a fiber loop quantum buffer based on a low-loss 2$\times$2 switch and a unit delay made of a fiber delay line. We characterize the device by using a two-photon polarization entangled state in which one photon of the entangled photon pair is stored and retrieved at a repetition rate up to 78$\,\rm{kHz}$. The device, which enables integer multiples of a unit delay, can store the qubit state in a unit of fiber delay line up to 5.4$\,\rm{km}$ and the number of loop round-trips up to 3. Furthermore, we configure the device with other active elements to realize integer multiplies and divider of a unit delay of a qubit. The quantum state tomography is performed on the retrieved photon and its entangled photon. We obtain a state fidelity $>94\%$ with a maximum storage time of 52$\,μ\rm{sec}$. To further characterize the storing and retrieving processes of the device, we perform entanglement-assisted quantum process tomography on the buffered qubit state. The process fidelity of the device is $>$ 0.98. Our result implies that the device preserves the superposition and entanglement of a qubit state from a two-photon polarization-entangled state. This is a significant step towards facilitating applications in optical asynchronous transfer mode (ATM) based quantum networks.

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arXiv2014

Intrinsic-Correlation Quantum Key Generation

A new conceptual key generation scheme is presented by using intrinsic quantum correlations of single photons between Alice and Bob. The intrinsic bi-partite correlation functions allow key bit to be generated through high level communication language i.e. a key bit is directly encoded to shared correlation functions not to the state and detection of a photon at Bob does not mean key bit. These make the scheme robust against intercept-resend attack because Alice and Bob can always check the errors in their measurements and reveal the presence of Eve in their channel without leaking any key bit information. The laser source is located in the middle of Alice and Bob, consists of two modes (x,y), relatively random phase-modulated $(\pm \fracπ{2})$ weak coherent states combined in a perfect 50/50 beam splitter. The scheme is strictly relied on the perfect beam splitter and mean photon number less than 1, where more than one photon in a coherent pulse will introduce more errors in Bob even without the presence of Eve. From the percentage of errors in Bob, we can estimate the amount of information will be leaked to Eve in the photon-number splitting attack. This scheme can preserve the randomness of phase-randomized light source for doubling the communication distance as in original Ekert's protocol and providing the raw key generation rate a factor of 2 higher than weak coherent light protocols.

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arXiv2012

Quantum Key Distribution based on Single Photon Bi-partite Correlation

We present a scheme for key distribution based on bi-partite correlation of single photons. Alice keeps an ancilla photon and sends a signal photon to Bob, where intrinsic bi-partite correlation of these photons is obtained through first order intensity correlation in their detectors. The key bits are distributed through sharing four bi-partite correlation functions and photon counting. The scheme consists of two parts; first, Alice prepares deterministic photon states and Bob measures the photon states based on his random choice on correlation functions. Second, Alice guesses Bob's choice of correlation functions and sets the key bits by sending out photon states. Bob verifies the key bits through the photon states regardless Alice made a right or wrong guess. We called this key distribution scheme as prepare-measure-guess-verify (PMGV) protocol. We discuss the protocol by using a highly attenuated laser light, and then point out the advantages of using a fiber based correlated photon-pair to achieve better performance in security, communication distance and success rate of key distribution.

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arXiv2011

Intrinsic Quantum Correlations of Weak Coherent States for Quantum Communication

Intrinsic quantum correlations of weak coherent states are observed between two parties through a novel detection scheme, which can be used as a supplement to the existence decoy-state BB84 and differential phase-shift quantum key distribution (DPS-QKD) protocols. In a proof-of-principle experiment, we generate bi-partite correlations of weak coherent states using weak local oscillator fields in two spatially separated balanced homodyne detections. We employ nonlinearity of post-measurement method to obtain the bi-partite correlations from two single-field interferences at individual homodyne measurement. This scheme is then used to demonstrate bits correlations between two parties over a distance of 10 km through a transmission fiber. We believe that the scheme can add another physical layer of security to these protocols for quantum key distribution.

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arXiv2011

Optical Phase-Space-Time-Frequency Tomography

We present a new approach for constructing optical phase-space-time-frequency tomography (OPSTFT) of an optical wave field. This tomography can be measured by using a novel four-window optical imaging system based on two local oscillator fields balanced heterodyne detection. The OPSTFT is a Wigner distribution function of two independent Fourier Transform pairs, i.e., phase-space and time-frequency. From its theoretical and experimental aspects, it can provide information of position, momentum, time and frequency of a spatial light field with precision beyond the uncertainty principle. We simulate the OPSTFT for a light field obscured by a wire and a single-line absorption filter. We believe that the four-window system can provide spatial and temporal properties of a wave field for quantum image processing and biophotonics.

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arXiv2009

Direct Measurement of Kirkwood-Rihaczek distribution for spatial properties of coherent light beam

We present direct measurement of Kirkwood-Rihaczek (KR) distribution for spatial properties of coherent light beam in terms of position and momentum (angle) coordinates. We employ a two-local oscillator (LO) balanced heterodyne detection (BHD) to simultaneously extract distribution of transverse position and momentum of a light beam. The two-LO BHD could measure KR distribution for any complex wave field (including quantum mechanical wave function) without applying tomography methods (inverse Radon transformation). Transformation of KR distribution to Wigner, Glauber Sudarshan P- and Husimi or Q- distributions in spatial coordinates are illustrated through experimental data. The direct measurement of KR distribution could provide local information of wave field, which is suitable for studying particle properties of a quantum system. While Wigner function is suitable for studying wave properties such as interference, and hence provides nonlocal information of the wave field. The method developed here can be used for exploring spatial quantum state for quantum mapping and computing, optical phase space imaging for biomedical applications.

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arXiv2008

Optical Implementation of Non-locality with Coherent Light Fields for Quantum Communication

Polarization correlations of two distant observers are observed by using coherent light fields based on Stapp's formulation of nonlocality. Using a 50/50 beam splitter transformation, a vertically polarized coherent light field is found to be entangled with a horizontally polarized coherent noise field. The superposed light fields at each output port of the beam splitter are sent to two distant observers, where the fields are interfered and manipulated at each observer by using a quarter wave plate and an analyzer. The interference signal contains information of the projection angle of the analyzer, which is hidden by the phase noises. The nonlocal correlations between the projection angles of two distant observers are established by analyzing their data through analog signal multiplication without any post-selection technique. This scheme can be used to implement Ekert's protocol for quantum key distribution.

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arXiv2008

Demonstration of a Quantum Controlled-NOT Gate in the Telecom Band

We present the first quantum controlled-NOT (CNOT) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 $μ$m telecommunications band. Using this free-space CNOT gate, all four Bell states are produced and fully characterized by performing quantum state tomography, demonstrating the gate's unambiguous entangling capability and high fidelity. Telecom-band operation makes this CNOT gate particularly suitable for quantum information processing tasks that are at the interface of quantum communication and linear optical quantum computing.

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arXiv2008

Telecom-Band Entanglement Generation for Chipscale Quantum Processing

We demonstrate polarization-entanglement for non-degenerate and degenerate photon-pairs generated through Kerr-nonlinearity in a nano-scale silicon-on-insulator(SOI) waveguide. We use a compact counter propagating configuration to create two-photon polarization-entangled state, |H>|H> + |V>|V>. We observe two-photon interference with visibility > 91% and > 80% for non-degenerate and degenerate photon-pairs, respectively. The experimental structure can be implemented on optical chips as an integrated source of entangled photons for future quantum computer and communication applications.

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arXiv2007

Quantum theory of degenerate $χ^{(3)}$ two-photon state

We developed a quantum theory for degenerate $χ^{(3)}$ two-photon state generated from optical fiber, and compared the theory predictions with an experimental result which exhibits a Hong-Ou-Mandel dip visibility of around 94%. Excellent agreement between theory and experiment has been achieved, and we attribute the missing 6% visibility mainly to spatial mode mismatch between signal and idler photons at the beamsplitter.

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arXiv2006

An integrable optical-fiber source of polarization entangled photon-pairs in the telecom band

We demonstrate an optical-fiber based source of polarization entangled photon-pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550 nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersion-shifted fiber with two orthogonally-polarized pump pulses, one propagating in the clockwise and the other in the counter-clockwise direction. We characterize this source by investigating two-photon interference between the generated signal-idler photon-pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon-pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal/idler bands and ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal/idler bands. We obtain two-photon interference with visibility $>90$% without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or the idler photons.

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