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Our laboratory explores the frontiers of quantum optics and quantum information through advanced experimental studies using entangled states generated via spontaneous parametric down-conversion (SPDC). Our research spans the full spectrum of quantum science, from fundamental principles and enabling technologies to cutting-edge applications in quantum computing, quantum communication, and quantum sensing.
Our laboratory explores the frontiers of quantum optics and quantum information through advanced experimental studies using entangled states generated via spontaneous parametric down-conversion (SPDC). Our research spans the full spectrum of quantum science, from fundamental principles and enabling technologies to cutting-edge applications in quantum computing, quantum communication, and quantum sensing.
- Foundation of Quantum Physics & Quantum Information
Unlike classical measurements, quantum measurements inevitably disturb the state being measured—the more information we obtain, the greater the disturbance. Strong (projective) measurements completely erase the initial state, whereas weak measurements allow, with some probability, the state to be restored. However, as the information gain increases, this reversibility decreases.
Unlike classical measurements, quantum measurements inevitably disturb the state being measured—the more information we obtain, the greater the disturbance. Strong (projective) measurements completely erase the initial state, whereas weak measurements allow, with some probability, the state to be restored. However, as the information gain increases, this reversibility decreases.
We quantify these aspects as information gain (G), disturbance (D = 1 – F), and reversibility (R). In 2014, we experimentally demonstrated the trade-offs between G and F, and between G and R, introducing the concepts of minimum-disturbance and maximum-reversibility measurements [1]. In 2022, we established and experimentally verified the first comprehensive inequality linking G, F, and R [2]. This general framework defines optimal measurements that satisfy the G-F-R relationship, allowing complete characterization of quantum measurement without loss of information.
We quantify these aspects as information gain (G), disturbance (D = 1 – F), and reversibility (R). In 2014, we experimentally demonstrated the trade-offs between G and F, and between G and R, introducing the concepts of minimum-disturbance and maximum-reversibility measurements [1]. In 2022, we established and experimentally verified the first comprehensive inequality linking G, F, and R [2]. This general framework defines optimal measurements that satisfy the G-F-R relationship, allowing complete characterization of quantum measurement without loss of information.
This fundamental result applies broadly to quantum information processing, including quantum teleportation and quantum error correction, where state recovery after measurement is essential. Our 2022 PRL paper was highlighted as a cover article and an Editor’s Suggestion [2].
This fundamental result applies broadly to quantum information processing, including quantum teleportation and quantum error correction, where state recovery after measurement is essential. Our 2022 PRL paper was highlighted as a cover article and an Editor’s Suggestion [2].
[1] H.-T. Lim, Y.-S. Ra, K.-H. Hong, S.-W. Lee, and Y.-H. Kim, Fundamental Bounds in Measurements for Estimating Quantum States, Phys. Rev. Lett. 113, 020504 (2014)
[2] S. Hong, Y.-S. Kim, Y.-W. Cho, J. Kim, S.-W. Lee*, and H.-T. Lim*, Demonstration of Complete Information Trade-Off in Quantum Measurement, Phys. Rev. Lett. 128, 050401 (2022)
2. Photonic Quantum Simulation: Qudit-based VQE for Quantum Chemistry
2. Photonic Quantum Simulation: Qudit-based VQE for Quantum Chemistry
In the NISQ era, quantum computing faces resource limitations and noise, yet aims to demonstrate practical advantages over classical methods. One promising application is quantum chemistry, where the Variational Quantum Eigensolver (VQE) is widely used to compute molecular ground-state energies. While conventional qubit-based VQE suffers from scalability issues—due to the exponential growth of CNOT gates and their limited fidelity—I proposed an alternative approach using photonic qudits instead of qubits.
In the NISQ era, quantum computing faces resource limitations and noise, yet aims to demonstrate practical advantages over classical methods. One promising application is quantum chemistry, where the Variational Quantum Eigensolver (VQE) is widely used to compute molecular ground-state energies. While conventional qubit-based VQE suffers from scalability issues—due to the exponential growth of CNOT gates and their limited fidelity—I proposed an alternative approach using photonic qudits instead of qubits.
Qudits, with d-dimensional states, allow for richer encoding of quantum information. In this study, we employed single photons with orbital angular momentum (OAM) as qudits, exploiting their naturally high-dimensional structure. For the first time, we successfully performed VQE-based quantum chemical simulations of H₂ (4D) and LiH (16D) molecules using single-photon OAM states, achieving chemical accuracy (1.6 mHartree) without error mitigation [1].
Qudits, with d-dimensional states, allow for richer encoding of quantum information. In this study, we employed single photons with orbital angular momentum (OAM) as qudits, exploiting their naturally high-dimensional structure. For the first time, we successfully performed VQE-based quantum chemical simulations of H₂ (4D) and LiH (16D) molecules using single-photon OAM states, achieving chemical accuracy (1.6 mHartree) without error mitigation [1].
This approach significantly reduces resource overhead and extends the computational space, paving the way for simulations of more complex molecules. In the long term, high-dimensional photonic quantum simulation may enable breakthroughs in materials science, catalysis, and drug discovery, outperforming classical methods in solving intractable problems.
This approach significantly reduces resource overhead and extends the computational space, paving the way for simulations of more complex molecules. In the long term, high-dimensional photonic quantum simulation may enable breakthroughs in materials science, catalysis, and drug discovery, outperforming classical methods in solving intractable problems.
[1] B. Kim, K. Hu, M. Sohn, Y. Kim, Y.-S. Kim, S.-W. Lee, and H.-T. Lim, Qudit-based variational quantum eigensolver using photonic orbital angular momentum states, Sci. Adv. 10, eado3472 (2024)
3. Photonic Quantum Sensing: Quantum Multiparameter Estimation & Distributed Quantum Sensing
3. Photonic Quantum Sensing: Quantum Multiparameter Estimation & Distributed Quantum Sensing
Quantum sensing surpasses classical precision limits by leveraging quantum entanglement, enabling measurements at the Heisenberg Limit (HL) rather than the Standard Quantum Limit. While conventional approaches focus on single-parameter estimation, we developed techniques for simultaneous multi-parameter estimation and distributed quantum sensing (DQS).
Quantum sensing surpasses classical precision limits by leveraging quantum entanglement, enabling measurements at the Heisenberg Limit (HL) rather than the Standard Quantum Limit. While conventional approaches focus on single-parameter estimation, we developed techniques for simultaneous multi-parameter estimation and distributed quantum sensing (DQS).
We experimentally realized a four-mode N00N state, enabling the simultaneous estimation of three phase parameters with near-Heisenberg precision—the first such demonstration [1-6]. Building on this, we implemented a distributed quantum sensing network, showing that just two entangled photons can measure four spatially separated phase parameters over 3 km of optical fiber, breaking the previous scaling constraint between parameters and photon number [7, 8].
We experimentally realized a four-mode N00N state, enabling the simultaneous estimation of three phase parameters with near-Heisenberg precision—the first such demonstration [1-6]. Building on this, we implemented a distributed quantum sensing network, showing that just two entangled photons can measure four spatially separated phase parameters over 3 km of optical fiber, breaking the previous scaling constraint between parameters and photon number [7, 8].
Our work provides a scalable framework for quantum sensor networks, with future efforts focused on deploying entangled-state distribution over commercial fiber infrastructure, paving the way for high-precision sensing across large-scale networks.
Our work provides a scalable framework for quantum sensor networks, with future efforts focused on deploying entangled-state distribution over commercial fiber infrastructure, paving the way for high-precision sensing across large-scale networks.
[1] S. Hong, J. ur Rehman, Y.-S. Kim, Y.-W. Cho, S.-W. Lee, H. Jung, M. Sung, S.-W. Han, and H.-T. Lim, Quantum enhanced multiple-phase estimation with multi-mode N00N states, Nat. Commun. 12, 5211 (2021)
[2] S. Hong, J. ur Rehman, Y.-S. Kim, Y.-W. Cho, S.-W. Lee, S.-Y. Lee, and H.-T. Lim, Practical Sensitivity Bound for Multiple Phase Estimation with Multi-Mode N00N States, Laser & Photonics Rev. 16, 2100682 (2022)
[3] J. ur Rehman, S. Hong, Y.-S. Kim, Y.-W. Cho, S.-W. Lee, H. Jung, M. Sung, H. Shin, S.-W. Han, and H.-T. Lim, Optimal strategy for multiple-phase estimation under practical measurement with multimode NOON states, Phys. Rev. A 106, 032612 (2022)
[4] M. Namkung, D.-H. Kim, S. Hong, Y.-S. Kim, C. Lee*, and H.-T. Lim*, Optimal multiple-phase estimation with multi-mode NOON states against photon loss, New J. Phys. 26, 073028 (2024)
[5] M. Namkung, D.-H. Kim, S. Hong, C. Lee*, and H.-T. Lim*, Persistent quantum advantage with definite photon-number states in lossy multiple-phase estimation, Phys. Rev. A 112, 013714 (2025)
[6] M. Namkung, C. Lee*, and H.-T. Lim*, Unified Cramér-Rao bound for quantum multi-parameter estimation: Invertible and non-invertible Fisher information matrix, arXiv:2412.01117
[7] D.-H. Kim, S. Hong, Y.-S. Kim, Y. Kim, S.-W. Lee, R. C. Pooser, K. Oh, S.-Y. Lee, C. Lee, and H.-T. Lim, Distributed quantum sensing of multiple phases with fewer photons, Nat. Commun. 15, 266 (2024)
[8] D.-H. Kim, S. Hong, Y.-S. Kim, K. Oh, S.-Y. Lee, C. Lee, and H.-T. Lim, Distributed Quantum Sensing with Multimode N00N States, Phys. Rev. Lett. 135, 050802 (2025)
4. Entanglement-based Quantum Network: Telecom-band Entangled Photons
4. Entanglement-based Quantum Network: Telecom-band Entangled Photons
Implementing quantum network protocols—such as distributed quantum computing, sensing, and multi-party communication—requires sharing entangled states across distant nodes. For scalability, fiber-optic quantum communication using existing telecom infrastructure (1.5 µm wavelength) is essential due to minimal fiber loss.
Implementing quantum network protocols—such as distributed quantum computing, sensing, and multi-party communication—requires sharing entangled states across distant nodes. For scalability, fiber-optic quantum communication using existing telecom infrastructure (1.5 µm wavelength) is essential due to minimal fiber loss.
At KIST, we developed a polarization-entangled photon source at 1560 nm using a 780 nm pump laser. Building on this, we are creating four-photon GHZ states in the telecom band, which are crucial for advanced quantum networking protocols.
At KIST, we developed a polarization-entangled photon source at 1560 nm using a 780 nm pump laser. Building on this, we are creating four-photon GHZ states in the telecom band, which are crucial for advanced quantum networking protocols.
Our long-term goal is to integrate this system with commercial fiber networks, enabling real-world demonstrations of quantum communication and networking with external partners.
Our long-term goal is to integrate this system with commercial fiber networks, enabling real-world demonstrations of quantum communication and networking with external partners.
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